/src/postgres/src/backend/access/transam/multixact.c

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/*-------------------------------------------------------------------------
 *
 * multixact.c
 *    PostgreSQL multi-transaction-log manager
 *
 * The pg_multixact manager is a pg_xact-like manager that stores an array of
 * MultiXactMember for each MultiXactId.  It is a fundamental part of the
 * shared-row-lock implementation.  Each MultiXactMember is comprised of a
 * TransactionId and a set of flag bits.  The name is a bit historical:
 * originally, a MultiXactId consisted of more than one TransactionId (except
 * in rare corner cases), hence "multi".  Nowadays, however, it's perfectly
 * legitimate to have MultiXactIds that only include a single Xid.
 *
 * The meaning of the flag bits is opaque to this module, but they are mostly
 * used in heapam.c to identify lock modes that each of the member transactions
 * is holding on any given tuple.  This module just contains support to store
 * and retrieve the arrays.
 *
 * We use two SLRU areas, one for storing the offsets at which the data
 * starts for each MultiXactId in the other one.  This trick allows us to
 * store variable length arrays of TransactionIds.  (We could alternatively
 * use one area containing counts and TransactionIds, with valid MultiXactId
 * values pointing at slots containing counts; but that way seems less robust
 * since it would get completely confused if someone inquired about a bogus
 * MultiXactId that pointed to an intermediate slot containing an XID.)
 *
 * XLOG interactions: this module generates a record whenever a new OFFSETs or
 * MEMBERs page is initialized to zeroes, as well as an
 * XLOG_MULTIXACT_CREATE_ID record whenever a new MultiXactId is defined.
 * This module ignores the WAL rule "write xlog before data," because it
 * suffices that actions recording a MultiXactId in a heap xmax do follow that
 * rule.  The only way for the MXID to be referenced from any data page is for
 * heap_lock_tuple() or heap_update() to have put it there, and each generates
 * an XLOG record that must follow ours.  The normal LSN interlock between the
 * data page and that XLOG record will ensure that our XLOG record reaches
 * disk first.  If the SLRU members/offsets data reaches disk sooner than the
 * XLOG records, we do not care; after recovery, no xmax will refer to it.  On
 * the flip side, to ensure that all referenced entries _do_ reach disk, this
 * module's XLOG records completely rebuild the data entered since the last
 * checkpoint.  We flush and sync all dirty OFFSETs and MEMBERs pages to disk
 * before each checkpoint is considered complete.
 *
 * Like clog.c, and unlike subtrans.c, we have to preserve state across
 * crashes and ensure that MXID and offset numbering increases monotonically
 * across a crash.  We do this in the same way as it's done for transaction
 * IDs: the WAL record is guaranteed to contain evidence of every MXID we
 * could need to worry about, and we just make sure that at the end of
 * replay, the next-MXID and next-offset counters are at least as large as
 * anything we saw during replay.
 *
 * We are able to remove segments no longer necessary by carefully tracking
 * each table's used values: during vacuum, any multixact older than a certain
 * value is removed; the cutoff value is stored in pg_class.  The minimum value
 * across all tables in each database is stored in pg_database, and the global
 * minimum across all databases is part of pg_control and is kept in shared
 * memory.  Whenever that minimum is advanced, the SLRUs are truncated.
 *
 * When new multixactid values are to be created, care is taken that the
 * counter does not fall within the wraparound horizon considering the global
 * minimum value.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/multixact.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/multixact.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "commands/dbcommands.h"
#include "funcapi.h"
#include "lib/ilist.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/fmgrprotos.h"
#include "utils/guc_hooks.h"
#include "utils/injection_point.h"
#include "utils/memutils.h"


/*
 * Defines for MultiXactOffset page sizes.  A page is the same BLCKSZ as is
 * used everywhere else in Postgres.
 *
 * Note: because MultiXactOffsets are 32 bits and wrap around at 0xFFFFFFFF,
 * MultiXact page numbering also wraps around at
 * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE, and segment numbering at
 * 0xFFFFFFFF/MULTIXACT_OFFSETS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need
 * take no explicit notice of that fact in this module, except when comparing
 * segment and page numbers in TruncateMultiXact (see
 * MultiXactOffsetPagePrecedes).
 */

/* We need four bytes per offset */
#define MULTIXACT_OFFSETS_PER_PAGE (BLCKSZ / sizeof(MultiXactOffset))

static inline int64
MultiXactIdToOffsetPage(MultiXactId multi)
{
  return multi / MULTIXACT_OFFSETS_PER_PAGE;
}

static inline int
MultiXactIdToOffsetEntry(MultiXactId multi)
{
  return multi % MULTIXACT_OFFSETS_PER_PAGE;
}

static inline int64
MultiXactIdToOffsetSegment(MultiXactId multi)
{
  return MultiXactIdToOffsetPage(multi) / SLRU_PAGES_PER_SEGMENT;
}

/*
 * The situation for members is a bit more complex: we store one byte of
 * additional flag bits for each TransactionId.  To do this without getting
 * into alignment issues, we store four bytes of flags, and then the
 * corresponding 4 Xids.  Each such 5-word (20-byte) set we call a "group", and
 * are stored as a whole in pages.  Thus, with 8kB BLCKSZ, we keep 409 groups
 * per page.  This wastes 12 bytes per page, but that's OK -- simplicity (and
 * performance) trumps space efficiency here.
 *
 * Note that the "offset" macros work with byte offset, not array indexes, so
 * arithmetic must be done using "char *" pointers.
 */
/* We need eight bits per xact, so one xact fits in a byte */
#define MXACT_MEMBER_BITS_PER_XACT      8
#define MXACT_MEMBER_FLAGS_PER_BYTE     1
#define MXACT_MEMBER_XACT_BITMASK ((1 << MXACT_MEMBER_BITS_PER_XACT) - 1)

/* how many full bytes of flags are there in a group? */
#define MULTIXACT_FLAGBYTES_PER_GROUP   4
#define MULTIXACT_MEMBERS_PER_MEMBERGROUP \
  (MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE)
/* size in bytes of a complete group */
#define MULTIXACT_MEMBERGROUP_SIZE \
  (sizeof(TransactionId) * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP)
#define MULTIXACT_MEMBERGROUPS_PER_PAGE (BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
#define MULTIXACT_MEMBERS_PER_PAGE  \
  (MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)

/*
 * Because the number of items per page is not a divisor of the last item
 * number (member 0xFFFFFFFF), the last segment does not use the maximum number
 * of pages, and moreover the last used page therein does not use the same
 * number of items as previous pages.  (Another way to say it is that the
 * 0xFFFFFFFF member is somewhere in the middle of the last page, so the page
 * has some empty space after that item.)
 *
 * This constant is the number of members in the last page of the last segment.
 */
#define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
    ((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))

/* page in which a member is to be found */
static inline int64
MXOffsetToMemberPage(MultiXactOffset offset)
{
  return offset / MULTIXACT_MEMBERS_PER_PAGE;
}

static inline int64
MXOffsetToMemberSegment(MultiXactOffset offset)
{
  return MXOffsetToMemberPage(offset) / SLRU_PAGES_PER_SEGMENT;
}

/* Location (byte offset within page) of flag word for a given member */
static inline int
MXOffsetToFlagsOffset(MultiXactOffset offset)
{
  MultiXactOffset group = offset / MULTIXACT_MEMBERS_PER_MEMBERGROUP;
  int     grouponpg = group % MULTIXACT_MEMBERGROUPS_PER_PAGE;
  int     byteoff = grouponpg * MULTIXACT_MEMBERGROUP_SIZE;

  return byteoff;
}

static inline int
MXOffsetToFlagsBitShift(MultiXactOffset offset)
{
  int     member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;
  int     bshift = member_in_group * MXACT_MEMBER_BITS_PER_XACT;

  return bshift;
}

/* Location (byte offset within page) of TransactionId of given member */
static inline int
MXOffsetToMemberOffset(MultiXactOffset offset)
{
  int     member_in_group = offset % MULTIXACT_MEMBERS_PER_MEMBERGROUP;

  return MXOffsetToFlagsOffset(offset) +
    MULTIXACT_FLAGBYTES_PER_GROUP +
    member_in_group * sizeof(TransactionId);
}

/* Multixact members wraparound thresholds. */
#define MULTIXACT_MEMBER_SAFE_THRESHOLD   (MaxMultiXactOffset / 2)
#define MULTIXACT_MEMBER_DANGER_THRESHOLD \
  (MaxMultiXactOffset - MaxMultiXactOffset / 4)

static inline MultiXactId
PreviousMultiXactId(MultiXactId multi)
{
  return multi == FirstMultiXactId ? MaxMultiXactId : multi - 1;
}

/*
 * Links to shared-memory data structures for MultiXact control
 */
static SlruCtlData MultiXactOffsetCtlData;
static SlruCtlData MultiXactMemberCtlData;

#define MultiXactOffsetCtl  (&MultiXactOffsetCtlData)
#define MultiXactMemberCtl  (&MultiXactMemberCtlData)

/*
 * MultiXact state shared across all backends.  All this state is protected
 * by MultiXactGenLock.  (We also use SLRU bank's lock of MultiXactOffset and
 * MultiXactMember to guard accesses to the two sets of SLRU buffers.  For
 * concurrency's sake, we avoid holding more than one of these locks at a
 * time.)
 */
typedef struct MultiXactStateData
{
  /* next-to-be-assigned MultiXactId */
  MultiXactId nextMXact;

  /* next-to-be-assigned offset */
  MultiXactOffset nextOffset;

  /* Have we completed multixact startup? */
  bool    finishedStartup;

  /*
   * Oldest multixact that is still potentially referenced by a relation.
   * Anything older than this should not be consulted.  These values are
   * updated by vacuum.
   */
  MultiXactId oldestMultiXactId;
  Oid     oldestMultiXactDB;

  /*
   * Oldest multixact offset that is potentially referenced by a multixact
   * referenced by a relation.  We don't always know this value, so there's
   * a flag here to indicate whether or not we currently do.
   */
  MultiXactOffset oldestOffset;
  bool    oldestOffsetKnown;

  /* support for anti-wraparound measures */
  MultiXactId multiVacLimit;
  MultiXactId multiWarnLimit;
  MultiXactId multiStopLimit;
  MultiXactId multiWrapLimit;

  /* support for members anti-wraparound measures */
  MultiXactOffset offsetStopLimit;  /* known if oldestOffsetKnown */

  /*
   * This is used to sleep until a multixact offset is written when we want
   * to create the next one.
   */
  ConditionVariable nextoff_cv;

  /*
   * Per-backend data starts here.  We have two arrays stored in the area
   * immediately following the MultiXactStateData struct. Each is indexed by
   * ProcNumber.
   *
   * In both arrays, there's a slot for all normal backends
   * (0..MaxBackends-1) followed by a slot for max_prepared_xacts prepared
   * transactions.
   *
   * OldestMemberMXactId[k] is the oldest MultiXactId each backend's current
   * transaction(s) could possibly be a member of, or InvalidMultiXactId
   * when the backend has no live transaction that could possibly be a
   * member of a MultiXact.  Each backend sets its entry to the current
   * nextMXact counter just before first acquiring a shared lock in a given
   * transaction, and clears it at transaction end. (This works because only
   * during or after acquiring a shared lock could an XID possibly become a
   * member of a MultiXact, and that MultiXact would have to be created
   * during or after the lock acquisition.)
   *
   * OldestVisibleMXactId[k] is the oldest MultiXactId each backend's
   * current transaction(s) think is potentially live, or InvalidMultiXactId
   * when not in a transaction or not in a transaction that's paid any
   * attention to MultiXacts yet.  This is computed when first needed in a
   * given transaction, and cleared at transaction end.  We can compute it
   * as the minimum of the valid OldestMemberMXactId[] entries at the time
   * we compute it (using nextMXact if none are valid).  Each backend is
   * required not to attempt to access any SLRU data for MultiXactIds older
   * than its own OldestVisibleMXactId[] setting; this is necessary because
   * the relevant SLRU data can be concurrently truncated away.
   *
   * The oldest valid value among all of the OldestMemberMXactId[] and
   * OldestVisibleMXactId[] entries is considered by vacuum as the earliest
   * possible value still having any live member transaction -- OldestMxact.
   * Any value older than that is typically removed from tuple headers, or
   * "frozen" via being replaced with a new xmax.  VACUUM can sometimes even
   * remove an individual MultiXact xmax whose value is >= its OldestMxact
   * cutoff, though typically only when no individual member XID is still
   * running.  See FreezeMultiXactId for full details.
   *
   * Whenever VACUUM advances relminmxid, then either its OldestMxact cutoff
   * or the oldest extant Multi remaining in the table is used as the new
   * pg_class.relminmxid value (whichever is earlier).  The minimum of all
   * relminmxid values in each database is stored in pg_database.datminmxid.
   * In turn, the minimum of all of those values is stored in pg_control.
   * This is used as the truncation point for pg_multixact when unneeded
   * segments get removed by vac_truncate_clog() during vacuuming.
   */
  MultiXactId perBackendXactIds[FLEXIBLE_ARRAY_MEMBER];
} MultiXactStateData;

/*
 * Size of OldestMemberMXactId and OldestVisibleMXactId arrays.
 */
#define MaxOldestSlot (MaxBackends + max_prepared_xacts)

/* Pointers to the state data in shared memory */
static MultiXactStateData *MultiXactState;
static MultiXactId *OldestMemberMXactId;
static MultiXactId *OldestVisibleMXactId;


/*
 * Definitions for the backend-local MultiXactId cache.
 *
 * We use this cache to store known MultiXacts, so we don't need to go to
 * SLRU areas every time.
 *
 * The cache lasts for the duration of a single transaction, the rationale
 * for this being that most entries will contain our own TransactionId and
 * so they will be uninteresting by the time our next transaction starts.
 * (XXX not clear that this is correct --- other members of the MultiXact
 * could hang around longer than we did.  However, it's not clear what a
 * better policy for flushing old cache entries would be.)  FIXME actually
 * this is plain wrong now that multixact's may contain update Xids.
 *
 * We allocate the cache entries in a memory context that is deleted at
 * transaction end, so we don't need to do retail freeing of entries.
 */
typedef struct mXactCacheEnt
{
  MultiXactId multi;
  int     nmembers;
  dlist_node  node;
  MultiXactMember members[FLEXIBLE_ARRAY_MEMBER];
} mXactCacheEnt;

#define MAX_CACHE_ENTRIES 256
static dclist_head MXactCache = DCLIST_STATIC_INIT(MXactCache);
static MemoryContext MXactContext = NULL;

#ifdef MULTIXACT_DEBUG
#define debug_elog2(a,b) elog(a,b)
#define debug_elog3(a,b,c) elog(a,b,c)
#define debug_elog4(a,b,c,d) elog(a,b,c,d)
#define debug_elog5(a,b,c,d,e) elog(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f) elog(a,b,c,d,e,f)
#else
#define debug_elog2(a,b)
#define debug_elog3(a,b,c)
#define debug_elog4(a,b,c,d)
#define debug_elog5(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f)
#endif

/* internal MultiXactId management */
static void MultiXactIdSetOldestVisible(void);
static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
                 int nmembers, MultiXactMember *members);
static MultiXactId GetNewMultiXactId(int nmembers, MultiXactOffset *offset);

/* MultiXact cache management */
static int  mxactMemberComparator(const void *arg1, const void *arg2);
static MultiXactId mXactCacheGetBySet(int nmembers, MultiXactMember *members);
static int  mXactCacheGetById(MultiXactId multi, MultiXactMember **members);
static void mXactCachePut(MultiXactId multi, int nmembers,
              MultiXactMember *members);

static char *mxstatus_to_string(MultiXactStatus status);

/* management of SLRU infrastructure */
static int  ZeroMultiXactOffsetPage(int64 pageno, bool writeXlog);
static int  ZeroMultiXactMemberPage(int64 pageno, bool writeXlog);
static bool MultiXactOffsetPagePrecedes(int64 page1, int64 page2);
static bool MultiXactMemberPagePrecedes(int64 page1, int64 page2);
static bool MultiXactOffsetPrecedes(MultiXactOffset offset1,
                  MultiXactOffset offset2);
static void ExtendMultiXactOffset(MultiXactId multi);
static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
static bool MultiXactOffsetWouldWrap(MultiXactOffset boundary,
                   MultiXactOffset start, uint32 distance);
static bool SetOffsetVacuumLimit(bool is_startup);
static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
static void WriteMZeroPageXlogRec(int64 pageno, uint8 info);
static void WriteMTruncateXlogRec(Oid oldestMultiDB,
                  MultiXactId startTruncOff,
                  MultiXactId endTruncOff,
                  MultiXactOffset startTruncMemb,
                  MultiXactOffset endTruncMemb);


/*
 * MultiXactIdCreate
 *    Construct a MultiXactId representing two TransactionIds.
 *
 * The two XIDs must be different, or be requesting different statuses.
 *
 * NB - we don't worry about our local MultiXactId cache here, because that
 * is handled by the lower-level routines.
 */
MultiXactId
MultiXactIdCreate(TransactionId xid1, MultiXactStatus status1,
          TransactionId xid2, MultiXactStatus status2)
{
  MultiXactId newMulti;
  MultiXactMember members[2];

  Assert(TransactionIdIsValid(xid1));
  Assert(TransactionIdIsValid(xid2));

  Assert(!TransactionIdEquals(xid1, xid2) || (status1 != status2));

  /* MultiXactIdSetOldestMember() must have been called already. */
  Assert(MultiXactIdIsValid(OldestMemberMXactId[MyProcNumber]));

  /*
   * Note: unlike MultiXactIdExpand, we don't bother to check that both XIDs
   * are still running.  In typical usage, xid2 will be our own XID and the
   * caller just did a check on xid1, so it'd be wasted effort.
   */

  members[0].xid = xid1;
  members[0].status = status1;
  members[1].xid = xid2;
  members[1].status = status2;

  newMulti = MultiXactIdCreateFromMembers(2, members);

  debug_elog3(DEBUG2, "Create: %s",
        mxid_to_string(newMulti, 2, members));

  return newMulti;
}

/*
 * MultiXactIdExpand
 *    Add a TransactionId to a pre-existing MultiXactId.
 *
 * If the TransactionId is already a member of the passed MultiXactId with the
 * same status, just return it as-is.
 *
 * Note that we do NOT actually modify the membership of a pre-existing
 * MultiXactId; instead we create a new one.  This is necessary to avoid
 * a race condition against code trying to wait for one MultiXactId to finish;
 * see notes in heapam.c.
 *
 * NB - we don't worry about our local MultiXactId cache here, because that
 * is handled by the lower-level routines.
 *
 * Note: It is critical that MultiXactIds that come from an old cluster (i.e.
 * one upgraded by pg_upgrade from a cluster older than this feature) are not
 * passed in.
 */
MultiXactId
MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
{
  MultiXactId newMulti;
  MultiXactMember *members;
  MultiXactMember *newMembers;
  int     nmembers;
  int     i;
  int     j;

  Assert(MultiXactIdIsValid(multi));
  Assert(TransactionIdIsValid(xid));

  /* MultiXactIdSetOldestMember() must have been called already. */
  Assert(MultiXactIdIsValid(OldestMemberMXactId[MyProcNumber]));

  debug_elog5(DEBUG2, "Expand: received multi %u, xid %u status %s",
        multi, xid, mxstatus_to_string(status));

  /*
   * Note: we don't allow for old multis here.  The reason is that the only
   * caller of this function does a check that the multixact is no longer
   * running.
   */
  nmembers = GetMultiXactIdMembers(multi, &members, false, false);

  if (nmembers < 0)
  {
    MultiXactMember member;

    /*
     * The MultiXactId is obsolete.  This can only happen if all the
     * MultiXactId members stop running between the caller checking and
     * passing it to us.  It would be better to return that fact to the
     * caller, but it would complicate the API and it's unlikely to happen
     * too often, so just deal with it by creating a singleton MultiXact.
     */
    member.xid = xid;
    member.status = status;
    newMulti = MultiXactIdCreateFromMembers(1, &member);

    debug_elog4(DEBUG2, "Expand: %u has no members, create singleton %u",
          multi, newMulti);
    return newMulti;
  }

  /*
   * If the TransactionId is already a member of the MultiXactId with the
   * same status, just return the existing MultiXactId.
   */
  for (i = 0; i < nmembers; i++)
  {
    if (TransactionIdEquals(members[i].xid, xid) &&
      (members[i].status == status))
    {
      debug_elog4(DEBUG2, "Expand: %u is already a member of %u",
            xid, multi);
      pfree(members);
      return multi;
    }
  }

  /*
   * Determine which of the members of the MultiXactId are still of
   * interest. This is any running transaction, and also any transaction
   * that grabbed something stronger than just a lock and was committed. (An
   * update that aborted is of no interest here; and having more than one
   * update Xid in a multixact would cause errors elsewhere.)
   *
   * Removing dead members is not just an optimization: freezing of tuples
   * whose Xmax are multis depends on this behavior.
   *
   * Note we have the same race condition here as above: j could be 0 at the
   * end of the loop.
   */
  newMembers = (MultiXactMember *)
    palloc(sizeof(MultiXactMember) * (nmembers + 1));

  for (i = 0, j = 0; i < nmembers; i++)
  {
    if (TransactionIdIsInProgress(members[i].xid) ||
      (ISUPDATE_from_mxstatus(members[i].status) &&
       TransactionIdDidCommit(members[i].xid)))
    {
      newMembers[j].xid = members[i].xid;
      newMembers[j++].status = members[i].status;
    }
  }

  newMembers[j].xid = xid;
  newMembers[j++].status = status;
  newMulti = MultiXactIdCreateFromMembers(j, newMembers);

  pfree(members);
  pfree(newMembers);

  debug_elog3(DEBUG2, "Expand: returning new multi %u", newMulti);

  return newMulti;
}

/*
 * MultiXactIdIsRunning
 *    Returns whether a MultiXactId is "running".
 *
 * We return true if at least one member of the given MultiXactId is still
 * running.  Note that a "false" result is certain not to change,
 * because it is not legal to add members to an existing MultiXactId.
 *
 * Caller is expected to have verified that the multixact does not come from
 * a pg_upgraded share-locked tuple.
 */
bool
MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly)
{
  MultiXactMember *members;
  int     nmembers;
  int     i;

  debug_elog3(DEBUG2, "IsRunning %u?", multi);

  /*
   * "false" here means we assume our callers have checked that the given
   * multi cannot possibly come from a pg_upgraded database.
   */
  nmembers = GetMultiXactIdMembers(multi, &members, false, isLockOnly);

  if (nmembers <= 0)
  {
    debug_elog2(DEBUG2, "IsRunning: no members");
    return false;
  }

  /*
   * Checking for myself is cheap compared to looking in shared memory;
   * return true if any live subtransaction of the current top-level
   * transaction is a member.
   *
   * This is not needed for correctness, it's just a fast path.
   */
  for (i = 0; i < nmembers; i++)
  {
    if (TransactionIdIsCurrentTransactionId(members[i].xid))
    {
      debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
      pfree(members);
      return true;
    }
  }

  /*
   * This could be made faster by having another entry point in procarray.c,
   * walking the PGPROC array only once for all the members.  But in most
   * cases nmembers should be small enough that it doesn't much matter.
   */
  for (i = 0; i < nmembers; i++)
  {
    if (TransactionIdIsInProgress(members[i].xid))
    {
      debug_elog4(DEBUG2, "IsRunning: member %d (%u) is running",
            i, members[i].xid);
      pfree(members);
      return true;
    }
  }

  pfree(members);

  debug_elog3(DEBUG2, "IsRunning: %u is not running", multi);

  return false;
}

/*
 * MultiXactIdSetOldestMember
 *    Save the oldest MultiXactId this transaction could be a member of.
 *
 * We set the OldestMemberMXactId for a given transaction the first time it's
 * going to do some operation that might require a MultiXactId (tuple lock,
 * update or delete).  We need to do this even if we end up using a
 * TransactionId instead of a MultiXactId, because there is a chance that
 * another transaction would add our XID to a MultiXactId.
 *
 * The value to set is the next-to-be-assigned MultiXactId, so this is meant to
 * be called just before doing any such possibly-MultiXactId-able operation.
 */
void
MultiXactIdSetOldestMember(void)
{
  if (!MultiXactIdIsValid(OldestMemberMXactId[MyProcNumber]))
  {
    MultiXactId nextMXact;

    /*
     * You might think we don't need to acquire a lock here, since
     * fetching and storing of TransactionIds is probably atomic, but in
     * fact we do: suppose we pick up nextMXact and then lose the CPU for
     * a long time.  Someone else could advance nextMXact, and then
     * another someone else could compute an OldestVisibleMXactId that
     * would be after the value we are going to store when we get control
     * back.  Which would be wrong.
     *
     * Note that a shared lock is sufficient, because it's enough to stop
     * someone from advancing nextMXact; and nobody else could be trying
     * to write to our OldestMember entry, only reading (and we assume
     * storing it is atomic.)
     */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);

    /*
     * We have to beware of the possibility that nextMXact is in the
     * wrapped-around state.  We don't fix the counter itself here, but we
     * must be sure to store a valid value in our array entry.
     */
    nextMXact = MultiXactState->nextMXact;
    if (nextMXact < FirstMultiXactId)
      nextMXact = FirstMultiXactId;

    OldestMemberMXactId[MyProcNumber] = nextMXact;

    LWLockRelease(MultiXactGenLock);

    debug_elog4(DEBUG2, "MultiXact: setting OldestMember[%d] = %u",
          MyProcNumber, nextMXact);
  }
}

/*
 * MultiXactIdSetOldestVisible
 *    Save the oldest MultiXactId this transaction considers possibly live.
 *
 * We set the OldestVisibleMXactId for a given transaction the first time
 * it's going to inspect any MultiXactId.  Once we have set this, we are
 * guaranteed that SLRU data for MultiXactIds >= our own OldestVisibleMXactId
 * won't be truncated away.
 *
 * The value to set is the oldest of nextMXact and all the valid per-backend
 * OldestMemberMXactId[] entries.  Because of the locking we do, we can be
 * certain that no subsequent call to MultiXactIdSetOldestMember can set
 * an OldestMemberMXactId[] entry older than what we compute here.  Therefore
 * there is no live transaction, now or later, that can be a member of any
 * MultiXactId older than the OldestVisibleMXactId we compute here.
 */
static void
MultiXactIdSetOldestVisible(void)
{
  if (!MultiXactIdIsValid(OldestVisibleMXactId[MyProcNumber]))
  {
    MultiXactId oldestMXact;
    int     i;

    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);

    /*
     * We have to beware of the possibility that nextMXact is in the
     * wrapped-around state.  We don't fix the counter itself here, but we
     * must be sure to store a valid value in our array entry.
     */
    oldestMXact = MultiXactState->nextMXact;
    if (oldestMXact < FirstMultiXactId)
      oldestMXact = FirstMultiXactId;

    for (i = 0; i < MaxOldestSlot; i++)
    {
      MultiXactId thisoldest = OldestMemberMXactId[i];

      if (MultiXactIdIsValid(thisoldest) &&
        MultiXactIdPrecedes(thisoldest, oldestMXact))
        oldestMXact = thisoldest;
    }

    OldestVisibleMXactId[MyProcNumber] = oldestMXact;

    LWLockRelease(MultiXactGenLock);

    debug_elog4(DEBUG2, "MultiXact: setting OldestVisible[%d] = %u",
          MyProcNumber, oldestMXact);
  }
}

/*
 * ReadNextMultiXactId
 *    Return the next MultiXactId to be assigned, but don't allocate it
 */
MultiXactId
ReadNextMultiXactId(void)
{
  MultiXactId mxid;

  /* XXX we could presumably do this without a lock. */
  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  mxid = MultiXactState->nextMXact;
  LWLockRelease(MultiXactGenLock);

  if (mxid < FirstMultiXactId)
    mxid = FirstMultiXactId;

  return mxid;
}

/*
 * ReadMultiXactIdRange
 *    Get the range of IDs that may still be referenced by a relation.
 */
void
ReadMultiXactIdRange(MultiXactId *oldest, MultiXactId *next)
{
  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  *oldest = MultiXactState->oldestMultiXactId;
  *next = MultiXactState->nextMXact;
  LWLockRelease(MultiXactGenLock);

  if (*oldest < FirstMultiXactId)
    *oldest = FirstMultiXactId;
  if (*next < FirstMultiXactId)
    *next = FirstMultiXactId;
}


/*
 * MultiXactIdCreateFromMembers
 *    Make a new MultiXactId from the specified set of members
 *
 * Make XLOG, SLRU and cache entries for a new MultiXactId, recording the
 * given TransactionIds as members.  Returns the newly created MultiXactId.
 *
 * NB: the passed members[] array will be sorted in-place.
 */
MultiXactId
MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
{
  MultiXactId multi;
  MultiXactOffset offset;
  xl_multixact_create xlrec;

  debug_elog3(DEBUG2, "Create: %s",
        mxid_to_string(InvalidMultiXactId, nmembers, members));

  /*
   * See if the same set of members already exists in our cache; if so, just
   * re-use that MultiXactId.  (Note: it might seem that looking in our
   * cache is insufficient, and we ought to search disk to see if a
   * duplicate definition already exists.  But since we only ever create
   * MultiXacts containing our own XID, in most cases any such MultiXacts
   * were in fact created by us, and so will be in our cache.  There are
   * corner cases where someone else added us to a MultiXact without our
   * knowledge, but it's not worth checking for.)
   */
  multi = mXactCacheGetBySet(nmembers, members);
  if (MultiXactIdIsValid(multi))
  {
    debug_elog2(DEBUG2, "Create: in cache!");
    return multi;
  }

  /* Verify that there is a single update Xid among the given members. */
  {
    int     i;
    bool    has_update = false;

    for (i = 0; i < nmembers; i++)
    {
      if (ISUPDATE_from_mxstatus(members[i].status))
      {
        if (has_update)
          elog(ERROR, "new multixact has more than one updating member: %s",
             mxid_to_string(InvalidMultiXactId, nmembers, members));
        has_update = true;
      }
    }
  }

  /* Load the injection point before entering the critical section */
  INJECTION_POINT_LOAD("multixact-create-from-members");

  /*
   * Assign the MXID and offsets range to use, and make sure there is space
   * in the OFFSETs and MEMBERs files.  NB: this routine does
   * START_CRIT_SECTION().
   *
   * Note: unlike MultiXactIdCreate and MultiXactIdExpand, we do not check
   * that we've called MultiXactIdSetOldestMember here.  This is because
   * this routine is used in some places to create new MultiXactIds of which
   * the current backend is not a member, notably during freezing of multis
   * in vacuum.  During vacuum, in particular, it would be unacceptable to
   * keep OldestMulti set, in case it runs for long.
   */
  multi = GetNewMultiXactId(nmembers, &offset);

  INJECTION_POINT_CACHED("multixact-create-from-members", NULL);

  /* Make an XLOG entry describing the new MXID. */
  xlrec.mid = multi;
  xlrec.moff = offset;
  xlrec.nmembers = nmembers;

  /*
   * XXX Note: there's a lot of padding space in MultiXactMember.  We could
   * find a more compact representation of this Xlog record -- perhaps all
   * the status flags in one XLogRecData, then all the xids in another one?
   * Not clear that it's worth the trouble though.
   */
  XLogBeginInsert();
  XLogRegisterData(&xlrec, SizeOfMultiXactCreate);
  XLogRegisterData(members, nmembers * sizeof(MultiXactMember));

  (void) XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_CREATE_ID);

  /* Now enter the information into the OFFSETs and MEMBERs logs */
  RecordNewMultiXact(multi, offset, nmembers, members);

  /* Done with critical section */
  END_CRIT_SECTION();

  /* Store the new MultiXactId in the local cache, too */
  mXactCachePut(multi, nmembers, members);

  debug_elog2(DEBUG2, "Create: all done");

  return multi;
}

/*
 * RecordNewMultiXact
 *    Write info about a new multixact into the offsets and members files
 *
 * This is broken out of MultiXactIdCreateFromMembers so that xlog replay can
 * use it.
 */
static void
RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
           int nmembers, MultiXactMember *members)
{
  int64   pageno;
  int64   prev_pageno;
  int     entryno;
  int     slotno;
  MultiXactOffset *offptr;
  int     i;
  LWLock     *lock;
  LWLock     *prevlock = NULL;

  pageno = MultiXactIdToOffsetPage(multi);
  entryno = MultiXactIdToOffsetEntry(multi);

  lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
  LWLockAcquire(lock, LW_EXCLUSIVE);

  /*
   * Note: we pass the MultiXactId to SimpleLruReadPage as the "transaction"
   * to complain about if there's any I/O error.  This is kinda bogus, but
   * since the errors will always give the full pathname, it should be clear
   * enough that a MultiXactId is really involved.  Perhaps someday we'll
   * take the trouble to generalize the slru.c error reporting code.
   */
  slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
  offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
  offptr += entryno;

  *offptr = offset;

  MultiXactOffsetCtl->shared->page_dirty[slotno] = true;

  /* Release MultiXactOffset SLRU lock. */
  LWLockRelease(lock);

  /*
   * If anybody was waiting to know the offset of this multixact ID we just
   * wrote, they can read it now, so wake them up.
   */
  ConditionVariableBroadcast(&MultiXactState->nextoff_cv);

  prev_pageno = -1;

  for (i = 0; i < nmembers; i++, offset++)
  {
    TransactionId *memberptr;
    uint32     *flagsptr;
    uint32    flagsval;
    int     bshift;
    int     flagsoff;
    int     memberoff;

    Assert(members[i].status <= MultiXactStatusUpdate);

    pageno = MXOffsetToMemberPage(offset);
    memberoff = MXOffsetToMemberOffset(offset);
    flagsoff = MXOffsetToFlagsOffset(offset);
    bshift = MXOffsetToFlagsBitShift(offset);

    if (pageno != prev_pageno)
    {
      /*
       * MultiXactMember SLRU page is changed so check if this new page
       * fall into the different SLRU bank then release the old bank's
       * lock and acquire lock on the new bank.
       */
      lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
      if (lock != prevlock)
      {
        if (prevlock != NULL)
          LWLockRelease(prevlock);

        LWLockAcquire(lock, LW_EXCLUSIVE);
        prevlock = lock;
      }
      slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
      prev_pageno = pageno;
    }

    memberptr = (TransactionId *)
      (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);

    *memberptr = members[i].xid;

    flagsptr = (uint32 *)
      (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);

    flagsval = *flagsptr;
    flagsval &= ~(((1 << MXACT_MEMBER_BITS_PER_XACT) - 1) << bshift);
    flagsval |= (members[i].status << bshift);
    *flagsptr = flagsval;

    MultiXactMemberCtl->shared->page_dirty[slotno] = true;
  }

  if (prevlock != NULL)
    LWLockRelease(prevlock);
}

/*
 * GetNewMultiXactId
 *    Get the next MultiXactId.
 *
 * Also, reserve the needed amount of space in the "members" area.  The
 * starting offset of the reserved space is returned in *offset.
 *
 * This may generate XLOG records for expansion of the offsets and/or members
 * files.  Unfortunately, we have to do that while holding MultiXactGenLock
 * to avoid race conditions --- the XLOG record for zeroing a page must appear
 * before any backend can possibly try to store data in that page!
 *
 * We start a critical section before advancing the shared counters.  The
 * caller must end the critical section after writing SLRU data.
 */
static MultiXactId
GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
{
  MultiXactId result;
  MultiXactOffset nextOffset;

  debug_elog3(DEBUG2, "GetNew: for %d xids", nmembers);

  /* safety check, we should never get this far in a HS standby */
  if (RecoveryInProgress())
    elog(ERROR, "cannot assign MultiXactIds during recovery");

  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);

  /* Handle wraparound of the nextMXact counter */
  if (MultiXactState->nextMXact < FirstMultiXactId)
    MultiXactState->nextMXact = FirstMultiXactId;

  /* Assign the MXID */
  result = MultiXactState->nextMXact;

  /*----------
   * Check to see if it's safe to assign another MultiXactId.  This protects
   * against catastrophic data loss due to multixact wraparound.  The basic
   * rules are:
   *
   * If we're past multiVacLimit or the safe threshold for member storage
   * space, or we don't know what the safe threshold for member storage is,
   * start trying to force autovacuum cycles.
   * If we're past multiWarnLimit, start issuing warnings.
   * If we're past multiStopLimit, refuse to create new MultiXactIds.
   *
   * Note these are pretty much the same protections in GetNewTransactionId.
   *----------
   */
  if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
  {
    /*
     * For safety's sake, we release MultiXactGenLock while sending
     * signals, warnings, etc.  This is not so much because we care about
     * preserving concurrency in this situation, as to avoid any
     * possibility of deadlock while doing get_database_name(). First,
     * copy all the shared values we'll need in this path.
     */
    MultiXactId multiWarnLimit = MultiXactState->multiWarnLimit;
    MultiXactId multiStopLimit = MultiXactState->multiStopLimit;
    MultiXactId multiWrapLimit = MultiXactState->multiWrapLimit;
    Oid     oldest_datoid = MultiXactState->oldestMultiXactDB;

    LWLockRelease(MultiXactGenLock);

    if (IsUnderPostmaster &&
      !MultiXactIdPrecedes(result, multiStopLimit))
    {
      char     *oldest_datname = get_database_name(oldest_datoid);

      /*
       * Immediately kick autovacuum into action as we're already in
       * ERROR territory.
       */
      SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

      /* complain even if that DB has disappeared */
      if (oldest_datname)
        ereport(ERROR,
            (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
             errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database \"%s\"",
                oldest_datname),
             errhint("Execute a database-wide VACUUM in that database.\n"
                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
      else
        ereport(ERROR,
            (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
             errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database with OID %u",
                oldest_datoid),
             errhint("Execute a database-wide VACUUM in that database.\n"
                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    }

    /*
     * To avoid swamping the postmaster with signals, we issue the autovac
     * request only once per 64K multis generated.  This still gives
     * plenty of chances before we get into real trouble.
     */
    if (IsUnderPostmaster && (result % 65536) == 0)
      SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

    if (!MultiXactIdPrecedes(result, multiWarnLimit))
    {
      char     *oldest_datname = get_database_name(oldest_datoid);

      /* complain even if that DB has disappeared */
      if (oldest_datname)
        ereport(WARNING,
            (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
                     "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
                     multiWrapLimit - result,
                     oldest_datname,
                     multiWrapLimit - result),
             errhint("Execute a database-wide VACUUM in that database.\n"
                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
      else
        ereport(WARNING,
            (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
                     "database with OID %u must be vacuumed before %u more MultiXactIds are used",
                     multiWrapLimit - result,
                     oldest_datoid,
                     multiWrapLimit - result),
             errhint("Execute a database-wide VACUUM in that database.\n"
                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    }

    /* Re-acquire lock and start over */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    result = MultiXactState->nextMXact;
    if (result < FirstMultiXactId)
      result = FirstMultiXactId;
  }

  /* Make sure there is room for the MXID in the file.  */
  ExtendMultiXactOffset(result);

  /*
   * Reserve the members space, similarly to above.  Also, be careful not to
   * return zero as the starting offset for any multixact. See
   * GetMultiXactIdMembers() for motivation.
   */
  nextOffset = MultiXactState->nextOffset;
  if (nextOffset == 0)
  {
    *offset = 1;
    nmembers++;       /* allocate member slot 0 too */
  }
  else
    *offset = nextOffset;

  /*----------
   * Protect against overrun of the members space as well, with the
   * following rules:
   *
   * If we're past offsetStopLimit, refuse to generate more multis.
   * If we're close to offsetStopLimit, emit a warning.
   *
   * Arbitrarily, we start emitting warnings when we're 20 segments or less
   * from offsetStopLimit.
   *
   * Note we haven't updated the shared state yet, so if we fail at this
   * point, the multixact ID we grabbed can still be used by the next guy.
   *
   * Note that there is no point in forcing autovacuum runs here: the
   * multixact freeze settings would have to be reduced for that to have any
   * effect.
   *----------
   */
#define OFFSET_WARN_SEGMENTS  20
  if (MultiXactState->oldestOffsetKnown &&
    MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
                 nmembers))
  {
    /* see comment in the corresponding offsets wraparound case */
    SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

    ereport(ERROR,
        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
         errmsg("multixact \"members\" limit exceeded"),
         errdetail_plural("This command would create a multixact with %u members, but the remaining space is only enough for %u member.",
                  "This command would create a multixact with %u members, but the remaining space is only enough for %u members.",
                  MultiXactState->offsetStopLimit - nextOffset - 1,
                  nmembers,
                  MultiXactState->offsetStopLimit - nextOffset - 1),
         errhint("Execute a database-wide VACUUM in database with OID %u with reduced \"vacuum_multixact_freeze_min_age\" and \"vacuum_multixact_freeze_table_age\" settings.",
             MultiXactState->oldestMultiXactDB)));
  }

  /*
   * Check whether we should kick autovacuum into action, to prevent members
   * wraparound. NB we use a much larger window to trigger autovacuum than
   * just the warning limit. The warning is just a measure of last resort -
   * this is in line with GetNewTransactionId's behaviour.
   */
  if (!MultiXactState->oldestOffsetKnown ||
    (MultiXactState->nextOffset - MultiXactState->oldestOffset
     > MULTIXACT_MEMBER_SAFE_THRESHOLD))
  {
    /*
     * To avoid swamping the postmaster with signals, we issue the autovac
     * request only when crossing a segment boundary. With default
     * compilation settings that's roughly after 50k members.  This still
     * gives plenty of chances before we get into real trouble.
     */
    if ((MXOffsetToMemberPage(nextOffset) / SLRU_PAGES_PER_SEGMENT) !=
      (MXOffsetToMemberPage(nextOffset + nmembers) / SLRU_PAGES_PER_SEGMENT))
      SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
  }

  if (MultiXactState->oldestOffsetKnown &&
    MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
                 nextOffset,
                 nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
    ereport(WARNING,
        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
         errmsg_plural("database with OID %u must be vacuumed before %d more multixact member is used",
                 "database with OID %u must be vacuumed before %d more multixact members are used",
                 MultiXactState->offsetStopLimit - nextOffset + nmembers,
                 MultiXactState->oldestMultiXactDB,
                 MultiXactState->offsetStopLimit - nextOffset + nmembers),
         errhint("Execute a database-wide VACUUM in that database with reduced \"vacuum_multixact_freeze_min_age\" and \"vacuum_multixact_freeze_table_age\" settings.")));

  ExtendMultiXactMember(nextOffset, nmembers);

  /*
   * Critical section from here until caller has written the data into the
   * just-reserved SLRU space; we don't want to error out with a partly
   * written MultiXact structure.  (In particular, failing to write our
   * start offset after advancing nextMXact would effectively corrupt the
   * previous MultiXact.)
   */
  START_CRIT_SECTION();

  /*
   * Advance counters.  As in GetNewTransactionId(), this must not happen
   * until after file extension has succeeded!
   *
   * We don't care about MultiXactId wraparound here; it will be handled by
   * the next iteration.  But note that nextMXact may be InvalidMultiXactId
   * or the first value on a segment-beginning page after this routine
   * exits, so anyone else looking at the variable must be prepared to deal
   * with either case.  Similarly, nextOffset may be zero, but we won't use
   * that as the actual start offset of the next multixact.
   */
  (MultiXactState->nextMXact)++;

  MultiXactState->nextOffset += nmembers;

  LWLockRelease(MultiXactGenLock);

  debug_elog4(DEBUG2, "GetNew: returning %u offset %u", result, *offset);
  return result;
}

/*
 * GetMultiXactIdMembers
 *    Return the set of MultiXactMembers that make up a MultiXactId
 *
 * Return value is the number of members found, or -1 if there are none,
 * and *members is set to a newly palloc'ed array of members.  It's the
 * caller's responsibility to free it when done with it.
 *
 * from_pgupgrade must be passed as true if and only if only the multixact
 * corresponds to a value from a tuple that was locked in a 9.2-or-older
 * installation and later pg_upgrade'd (that is, the infomask is
 * HEAP_LOCKED_UPGRADED).  In this case, we know for certain that no members
 * can still be running, so we return -1 just like for an empty multixact
 * without any further checking.  It would be wrong to try to resolve such a
 * multixact: either the multixact is within the current valid multixact
 * range, in which case the returned result would be bogus, or outside that
 * range, in which case an error would be raised.
 *
 * In all other cases, the passed multixact must be within the known valid
 * range, that is, greater than or equal to oldestMultiXactId, and less than
 * nextMXact.  Otherwise, an error is raised.
 *
 * isLockOnly must be set to true if caller is certain that the given multi
 * is used only to lock tuples; can be false without loss of correctness,
 * but passing a true means we can return quickly without checking for
 * old updates.
 */
int
GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
            bool from_pgupgrade, bool isLockOnly)
{
  int64   pageno;
  int64   prev_pageno;
  int     entryno;
  int     slotno;
  MultiXactOffset *offptr;
  MultiXactOffset offset;
  int     length;
  int     truelength;
  MultiXactId oldestMXact;
  MultiXactId nextMXact;
  MultiXactId tmpMXact;
  MultiXactOffset nextOffset;
  MultiXactMember *ptr;
  LWLock     *lock;
  bool    slept = false;

  debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);

  if (!MultiXactIdIsValid(multi) || from_pgupgrade)
  {
    *members = NULL;
    return -1;
  }

  /* See if the MultiXactId is in the local cache */
  length = mXactCacheGetById(multi, members);
  if (length >= 0)
  {
    debug_elog3(DEBUG2, "GetMembers: found %s in the cache",
          mxid_to_string(multi, length, *members));
    return length;
  }

  /* Set our OldestVisibleMXactId[] entry if we didn't already */
  MultiXactIdSetOldestVisible();

  /*
   * If we know the multi is used only for locking and not for updates, then
   * we can skip checking if the value is older than our oldest visible
   * multi.  It cannot possibly still be running.
   */
  if (isLockOnly &&
    MultiXactIdPrecedes(multi, OldestVisibleMXactId[MyProcNumber]))
  {
    debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
    *members = NULL;
    return -1;
  }

  /*
   * We check known limits on MultiXact before resorting to the SLRU area.
   *
   * An ID older than MultiXactState->oldestMultiXactId cannot possibly be
   * useful; it has already been removed, or will be removed shortly, by
   * truncation.  If one is passed, an error is raised.
   *
   * Also, an ID >= nextMXact shouldn't ever be seen here; if it is seen, it
   * implies undetected ID wraparound has occurred.  This raises a hard
   * error.
   *
   * Shared lock is enough here since we aren't modifying any global state.
   * Acquire it just long enough to grab the current counter values.  We may
   * need both nextMXact and nextOffset; see below.
   */
  LWLockAcquire(MultiXactGenLock, LW_SHARED);

  oldestMXact = MultiXactState->oldestMultiXactId;
  nextMXact = MultiXactState->nextMXact;
  nextOffset = MultiXactState->nextOffset;

  LWLockRelease(MultiXactGenLock);

  if (MultiXactIdPrecedes(multi, oldestMXact))
    ereport(ERROR,
        (errcode(ERRCODE_INTERNAL_ERROR),
         errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
            multi)));

  if (!MultiXactIdPrecedes(multi, nextMXact))
    ereport(ERROR,
        (errcode(ERRCODE_INTERNAL_ERROR),
         errmsg("MultiXactId %u has not been created yet -- apparent wraparound",
            multi)));

  /*
   * Find out the offset at which we need to start reading MultiXactMembers
   * and the number of members in the multixact.  We determine the latter as
   * the difference between this multixact's starting offset and the next
   * one's.  However, there are some corner cases to worry about:
   *
   * 1. This multixact may be the latest one created, in which case there is
   * no next one to look at.  In this case the nextOffset value we just
   * saved is the correct endpoint.
   *
   * 2. The next multixact may still be in process of being filled in: that
   * is, another process may have done GetNewMultiXactId but not yet written
   * the offset entry for that ID.  In that scenario, it is guaranteed that
   * the offset entry for that multixact exists (because GetNewMultiXactId
   * won't release MultiXactGenLock until it does) but contains zero
   * (because we are careful to pre-zero offset pages). Because
   * GetNewMultiXactId will never return zero as the starting offset for a
   * multixact, when we read zero as the next multixact's offset, we know we
   * have this case.  We handle this by sleeping on the condition variable
   * we have just for this; the process in charge will signal the CV as soon
   * as it has finished writing the multixact offset.
   *
   * 3. Because GetNewMultiXactId increments offset zero to offset one to
   * handle case #2, there is an ambiguity near the point of offset
   * wraparound.  If we see next multixact's offset is one, is that our
   * multixact's actual endpoint, or did it end at zero with a subsequent
   * increment?  We handle this using the knowledge that if the zero'th
   * member slot wasn't filled, it'll contain zero, and zero isn't a valid
   * transaction ID so it can't be a multixact member.  Therefore, if we
   * read a zero from the members array, just ignore it.
   *
   * This is all pretty messy, but the mess occurs only in infrequent corner
   * cases, so it seems better than holding the MultiXactGenLock for a long
   * time on every multixact creation.
   */
retry:
  pageno = MultiXactIdToOffsetPage(multi);
  entryno = MultiXactIdToOffsetEntry(multi);

  /* Acquire the bank lock for the page we need. */
  lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
  LWLockAcquire(lock, LW_EXCLUSIVE);

  slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, multi);
  offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
  offptr += entryno;
  offset = *offptr;

  Assert(offset != 0);

  /*
   * Use the same increment rule as GetNewMultiXactId(), that is, don't
   * handle wraparound explicitly until needed.
   */
  tmpMXact = multi + 1;

  if (nextMXact == tmpMXact)
  {
    /* Corner case 1: there is no next multixact */
    length = nextOffset - offset;
  }
  else
  {
    MultiXactOffset nextMXOffset;

    /* handle wraparound if needed */
    if (tmpMXact < FirstMultiXactId)
      tmpMXact = FirstMultiXactId;

    prev_pageno = pageno;

    pageno = MultiXactIdToOffsetPage(tmpMXact);
    entryno = MultiXactIdToOffsetEntry(tmpMXact);

    if (pageno != prev_pageno)
    {
      LWLock     *newlock;

      /*
       * Since we're going to access a different SLRU page, if this page
       * falls under a different bank, release the old bank's lock and
       * acquire the lock of the new bank.
       */
      newlock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
      if (newlock != lock)
      {
        LWLockRelease(lock);
        LWLockAcquire(newlock, LW_EXCLUSIVE);
        lock = newlock;
      }
      slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, tmpMXact);
    }

    offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
    offptr += entryno;
    nextMXOffset = *offptr;

    if (nextMXOffset == 0)
    {
      /* Corner case 2: next multixact is still being filled in */
      LWLockRelease(lock);
      CHECK_FOR_INTERRUPTS();

      INJECTION_POINT("multixact-get-members-cv-sleep", NULL);

      ConditionVariableSleep(&MultiXactState->nextoff_cv,
                   WAIT_EVENT_MULTIXACT_CREATION);
      slept = true;
      goto retry;
    }

    length = nextMXOffset - offset;
  }

  LWLockRelease(lock);
  lock = NULL;

  /*
   * If we slept above, clean up state; it's no longer needed.
   */
  if (slept)
    ConditionVariableCancelSleep();

  ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));

  truelength = 0;
  prev_pageno = -1;
  for (int i = 0; i < length; i++, offset++)
  {
    TransactionId *xactptr;
    uint32     *flagsptr;
    int     flagsoff;
    int     bshift;
    int     memberoff;

    pageno = MXOffsetToMemberPage(offset);
    memberoff = MXOffsetToMemberOffset(offset);

    if (pageno != prev_pageno)
    {
      LWLock     *newlock;

      /*
       * Since we're going to access a different SLRU page, if this page
       * falls under a different bank, release the old bank's lock and
       * acquire the lock of the new bank.
       */
      newlock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
      if (newlock != lock)
      {
        if (lock)
          LWLockRelease(lock);
        LWLockAcquire(newlock, LW_EXCLUSIVE);
        lock = newlock;
      }

      slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, multi);
      prev_pageno = pageno;
    }

    xactptr = (TransactionId *)
      (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);

    if (!TransactionIdIsValid(*xactptr))
    {
      /* Corner case 3: we must be looking at unused slot zero */
      Assert(offset == 0);
      continue;
    }

    flagsoff = MXOffsetToFlagsOffset(offset);
    bshift = MXOffsetToFlagsBitShift(offset);
    flagsptr = (uint32 *) (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);

    ptr[truelength].xid = *xactptr;
    ptr[truelength].status = (*flagsptr >> bshift) & MXACT_MEMBER_XACT_BITMASK;
    truelength++;
  }

  LWLockRelease(lock);

  /* A multixid with zero members should not happen */
  Assert(truelength > 0);

  /*
   * Copy the result into the local cache.
   */
  mXactCachePut(multi, truelength, ptr);

  debug_elog3(DEBUG2, "GetMembers: no cache for %s",
        mxid_to_string(multi, truelength, ptr));
  *members = ptr;
  return truelength;
}

/*
 * mxactMemberComparator
 *    qsort comparison function for MultiXactMember
 *
 * We can't use wraparound comparison for XIDs because that does not respect
 * the triangle inequality!  Any old sort order will do.
 */
static int
mxactMemberComparator(const void *arg1, const void *arg2)
{
  MultiXactMember member1 = *(const MultiXactMember *) arg1;
  MultiXactMember member2 = *(const MultiXactMember *) arg2;

  if (member1.xid > member2.xid)
    return 1;
  if (member1.xid < member2.xid)
    return -1;
  if (member1.status > member2.status)
    return 1;
  if (member1.status < member2.status)
    return -1;
  return 0;
}

/*
 * mXactCacheGetBySet
 *    returns a MultiXactId from the cache based on the set of
 *    TransactionIds that compose it, or InvalidMultiXactId if
 *    none matches.
 *
 * This is helpful, for example, if two transactions want to lock a huge
 * table.  By using the cache, the second will use the same MultiXactId
 * for the majority of tuples, thus keeping MultiXactId usage low (saving
 * both I/O and wraparound issues).
 *
 * NB: the passed members array will be sorted in-place.
 */
static MultiXactId
mXactCacheGetBySet(int nmembers, MultiXactMember *members)
{
  dlist_iter  iter;

  debug_elog3(DEBUG2, "CacheGet: looking for %s",
        mxid_to_string(InvalidMultiXactId, nmembers, members));

  /* sort the array so comparison is easy */
  qsort(members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);

  dclist_foreach(iter, &MXactCache)
  {
    mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
                        iter.cur);

    if (entry->nmembers != nmembers)
      continue;

    /*
     * We assume the cache entries are sorted, and that the unused bits in
     * "status" are zeroed.
     */
    if (memcmp(members, entry->members, nmembers * sizeof(MultiXactMember)) == 0)
    {
      debug_elog3(DEBUG2, "CacheGet: found %u", entry->multi);
      dclist_move_head(&MXactCache, iter.cur);
      return entry->multi;
    }
  }

  debug_elog2(DEBUG2, "CacheGet: not found :-(");
  return InvalidMultiXactId;
}

/*
 * mXactCacheGetById
 *    returns the composing MultiXactMember set from the cache for a
 *    given MultiXactId, if present.
 *
 * If successful, *xids is set to the address of a palloc'd copy of the
 * MultiXactMember set.  Return value is number of members, or -1 on failure.
 */
static int
mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
{
  dlist_iter  iter;

  debug_elog3(DEBUG2, "CacheGet: looking for %u", multi);

  dclist_foreach(iter, &MXactCache)
  {
    mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
                        iter.cur);

    if (entry->multi == multi)
    {
      MultiXactMember *ptr;
      Size    size;

      size = sizeof(MultiXactMember) * entry->nmembers;
      ptr = (MultiXactMember *) palloc(size);

      memcpy(ptr, entry->members, size);

      debug_elog3(DEBUG2, "CacheGet: found %s",
            mxid_to_string(multi,
                     entry->nmembers,
                     entry->members));

      /*
       * Note we modify the list while not using a modifiable iterator.
       * This is acceptable only because we exit the iteration
       * immediately afterwards.
       */
      dclist_move_head(&MXactCache, iter.cur);

      *members = ptr;
      return entry->nmembers;
    }
  }

  debug_elog2(DEBUG2, "CacheGet: not found");
  return -1;
}

/*
 * mXactCachePut
 *    Add a new MultiXactId and its composing set into the local cache.
 */
static void
mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
{
  mXactCacheEnt *entry;

  debug_elog3(DEBUG2, "CachePut: storing %s",
        mxid_to_string(multi, nmembers, members));

  if (MXactContext == NULL)
  {
    /* The cache only lives as long as the current transaction */
    debug_elog2(DEBUG2, "CachePut: initializing memory context");
    MXactContext = AllocSetContextCreate(TopTransactionContext,
                       "MultiXact cache context",
                       ALLOCSET_SMALL_SIZES);
  }

  entry = (mXactCacheEnt *)
    MemoryContextAlloc(MXactContext,
               offsetof(mXactCacheEnt, members) +
               nmembers * sizeof(MultiXactMember));

  entry->multi = multi;
  entry->nmembers = nmembers;
  memcpy(entry->members, members, nmembers * sizeof(MultiXactMember));

  /* mXactCacheGetBySet assumes the entries are sorted, so sort them */
  qsort(entry->members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);

  dclist_push_head(&MXactCache, &entry->node);
  if (dclist_count(&MXactCache) > MAX_CACHE_ENTRIES)
  {
    dlist_node *node;

    node = dclist_tail_node(&MXactCache);
    dclist_delete_from(&MXactCache, node);

    entry = dclist_container(mXactCacheEnt, node, node);
    debug_elog3(DEBUG2, "CachePut: pruning cached multi %u",
          entry->multi);

    pfree(entry);
  }
}

static char *
mxstatus_to_string(MultiXactStatus status)
{
  switch (status)
  {
    case MultiXactStatusForKeyShare:
      return "keysh";
    case MultiXactStatusForShare:
      return "sh";
    case MultiXactStatusForNoKeyUpdate:
      return "fornokeyupd";
    case MultiXactStatusForUpdate:
      return "forupd";
    case MultiXactStatusNoKeyUpdate:
      return "nokeyupd";
    case MultiXactStatusUpdate:
      return "upd";
    default:
      elog(ERROR, "unrecognized multixact status %d", status);
      return "";
  }
}

char *
mxid_to_string(MultiXactId multi, int nmembers, MultiXactMember *members)
{
  static char *str = NULL;
  StringInfoData buf;
  int     i;

  if (str != NULL)
    pfree(str);

  initStringInfo(&buf);

  appendStringInfo(&buf, "%u %d[%u (%s)", multi, nmembers, members[0].xid,
           mxstatus_to_string(members[0].status));

  for (i = 1; i < nmembers; i++)
    appendStringInfo(&buf, ", %u (%s)", members[i].xid,
             mxstatus_to_string(members[i].status));

  appendStringInfoChar(&buf, ']');
  str = MemoryContextStrdup(TopMemoryContext, buf.data);
  pfree(buf.data);
  return str;
}

/*
 * AtEOXact_MultiXact
 *    Handle transaction end for MultiXact
 *
 * This is called at top transaction commit or abort (we don't care which).
 */
void
AtEOXact_MultiXact(void)
{
  /*
   * Reset our OldestMemberMXactId and OldestVisibleMXactId values, both of
   * which should only be valid while within a transaction.
   *
   * We assume that storing a MultiXactId is atomic and so we need not take
   * MultiXactGenLock to do this.
   */
  OldestMemberMXactId[MyProcNumber] = InvalidMultiXactId;
  OldestVisibleMXactId[MyProcNumber] = InvalidMultiXactId;

  /*
   * Discard the local MultiXactId cache.  Since MXactContext was created as
   * a child of TopTransactionContext, we needn't delete it explicitly.
   */
  MXactContext = NULL;
  dclist_init(&MXactCache);
}

/*
 * AtPrepare_MultiXact
 *    Save multixact state at 2PC transaction prepare
 *
 * In this phase, we only store our OldestMemberMXactId value in the two-phase
 * state file.
 */
void
AtPrepare_MultiXact(void)
{
  MultiXactId myOldestMember = OldestMemberMXactId[MyProcNumber];

  if (MultiXactIdIsValid(myOldestMember))
    RegisterTwoPhaseRecord(TWOPHASE_RM_MULTIXACT_ID, 0,
                 &myOldestMember, sizeof(MultiXactId));
}

/*
 * PostPrepare_MultiXact
 *    Clean up after successful PREPARE TRANSACTION
 */
void
PostPrepare_MultiXact(TransactionId xid)
{
  MultiXactId myOldestMember;

  /*
   * Transfer our OldestMemberMXactId value to the slot reserved for the
   * prepared transaction.
   */
  myOldestMember = OldestMemberMXactId[MyProcNumber];
  if (MultiXactIdIsValid(myOldestMember))
  {
    ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(xid, false);

    /*
     * Even though storing MultiXactId is atomic, acquire lock to make
     * sure others see both changes, not just the reset of the slot of the
     * current backend. Using a volatile pointer might suffice, but this
     * isn't a hot spot.
     */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);

    OldestMemberMXactId[dummyProcNumber] = myOldestMember;
    OldestMemberMXactId[MyProcNumber] = InvalidMultiXactId;

    LWLockRelease(MultiXactGenLock);
  }

  /*
   * We don't need to transfer OldestVisibleMXactId value, because the
   * transaction is not going to be looking at any more multixacts once it's
   * prepared.
   *
   * We assume that storing a MultiXactId is atomic and so we need not take
   * MultiXactGenLock to do this.
   */
  OldestVisibleMXactId[MyProcNumber] = InvalidMultiXactId;

  /*
   * Discard the local MultiXactId cache like in AtEOXact_MultiXact.
   */
  MXactContext = NULL;
  dclist_init(&MXactCache);
}

/*
 * multixact_twophase_recover
 *    Recover the state of a prepared transaction at startup
 */
void
multixact_twophase_recover(TransactionId xid, uint16 info,
               void *recdata, uint32 len)
{
  ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(xid, false);
  MultiXactId oldestMember;

  /*
   * Get the oldest member XID from the state file record, and set it in the
   * OldestMemberMXactId slot reserved for this prepared transaction.
   */
  Assert(len == sizeof(MultiXactId));
  oldestMember = *((MultiXactId *) recdata);

  OldestMemberMXactId[dummyProcNumber] = oldestMember;
}

/*
 * multixact_twophase_postcommit
 *    Similar to AtEOXact_MultiXact but for COMMIT PREPARED
 */
void
multixact_twophase_postcommit(TransactionId xid, uint16 info,
                void *recdata, uint32 len)
{
  ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(xid, true);

  Assert(len == sizeof(MultiXactId));

  OldestMemberMXactId[dummyProcNumber] = InvalidMultiXactId;
}

/*
 * multixact_twophase_postabort
 *    This is actually just the same as the COMMIT case.
 */
void
multixact_twophase_postabort(TransactionId xid, uint16 info,
               void *recdata, uint32 len)
{
  multixact_twophase_postcommit(xid, info, recdata, len);
}

/*
 * Initialization of shared memory for MultiXact.  We use two SLRU areas,
 * thus double memory.  Also, reserve space for the shared MultiXactState
 * struct and the per-backend MultiXactId arrays (two of those, too).
 */
Size
MultiXactShmemSize(void)
{
  Size    size;

  /* We need 2*MaxOldestSlot perBackendXactIds[] entries */
#define SHARED_MULTIXACT_STATE_SIZE \
  add_size(offsetof(MultiXactStateData, perBackendXactIds), \
       mul_size(sizeof(MultiXactId) * 2, MaxOldestSlot))

  size = SHARED_MULTIXACT_STATE_SIZE;
  size = add_size(size, SimpleLruShmemSize(multixact_offset_buffers, 0));
  size = add_size(size, SimpleLruShmemSize(multixact_member_buffers, 0));

  return size;
}

void
MultiXactShmemInit(void)
{
  bool    found;

  debug_elog2(DEBUG2, "Shared Memory Init for MultiXact");

  MultiXactOffsetCtl->PagePrecedes = MultiXactOffsetPagePrecedes;
  MultiXactMemberCtl->PagePrecedes = MultiXactMemberPagePrecedes;

  SimpleLruInit(MultiXactOffsetCtl,
          "multixact_offset", multixact_offset_buffers, 0,
          "pg_multixact/offsets", LWTRANCHE_MULTIXACTOFFSET_BUFFER,
          LWTRANCHE_MULTIXACTOFFSET_SLRU,
          SYNC_HANDLER_MULTIXACT_OFFSET,
          false);
  SlruPagePrecedesUnitTests(MultiXactOffsetCtl, MULTIXACT_OFFSETS_PER_PAGE);
  SimpleLruInit(MultiXactMemberCtl,
          "multixact_member", multixact_member_buffers, 0,
          "pg_multixact/members", LWTRANCHE_MULTIXACTMEMBER_BUFFER,
          LWTRANCHE_MULTIXACTMEMBER_SLRU,
          SYNC_HANDLER_MULTIXACT_MEMBER,
          false);
  /* doesn't call SimpleLruTruncate() or meet criteria for unit tests */

  /* Initialize our shared state struct */
  MultiXactState = ShmemInitStruct("Shared MultiXact State",
                   SHARED_MULTIXACT_STATE_SIZE,
                   &found);
  if (!IsUnderPostmaster)
  {
    Assert(!found);

    /* Make sure we zero out the per-backend state */
    MemSet(MultiXactState, 0, SHARED_MULTIXACT_STATE_SIZE);
    ConditionVariableInit(&MultiXactState->nextoff_cv);
  }
  else
    Assert(found);

  /*
   * Set up array pointers.
   */
  OldestMemberMXactId = MultiXactState->perBackendXactIds;
  OldestVisibleMXactId = OldestMemberMXactId + MaxOldestSlot;
}

/*
 * GUC check_hook for multixact_offset_buffers
 */
bool
check_multixact_offset_buffers(int *newval, void **extra, GucSource source)
{
  return check_slru_buffers("multixact_offset_buffers", newval);
}

/*
 * GUC check_hook for multixact_member_buffers
 */
bool
check_multixact_member_buffers(int *newval, void **extra, GucSource source)
{
  return check_slru_buffers("multixact_member_buffers", newval);
}

/*
 * This func must be called ONCE on system install.  It creates the initial
 * MultiXact segments.  (The MultiXacts directories are assumed to have been
 * created by initdb, and MultiXactShmemInit must have been called already.)
 */
void
BootStrapMultiXact(void)
{
  int     slotno;
  LWLock     *lock;

  lock = SimpleLruGetBankLock(MultiXactOffsetCtl, 0);
  LWLockAcquire(lock, LW_EXCLUSIVE);

  /* Create and zero the first page of the offsets log */
  slotno = ZeroMultiXactOffsetPage(0, false);

  /* Make sure it's written out */
  SimpleLruWritePage(MultiXactOffsetCtl, slotno);
  Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);

  LWLockRelease(lock);

  lock = SimpleLruGetBankLock(MultiXactMemberCtl, 0);
  LWLockAcquire(lock, LW_EXCLUSIVE);

  /* Create and zero the first page of the members log */
  slotno = ZeroMultiXactMemberPage(0, false);

  /* Make sure it's written out */
  SimpleLruWritePage(MultiXactMemberCtl, slotno);
  Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);

  LWLockRelease(lock);
}

/*
 * Initialize (or reinitialize) a page of MultiXactOffset to zeroes.
 * If writeXlog is true, also emit an XLOG record saying we did this.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
static int
ZeroMultiXactOffsetPage(int64 pageno, bool writeXlog)
{
  int     slotno;

  slotno = SimpleLruZeroPage(MultiXactOffsetCtl, pageno);

  if (writeXlog)
    WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_OFF_PAGE);

  return slotno;
}

/*
 * Ditto, for MultiXactMember
 */
static int
ZeroMultiXactMemberPage(int64 pageno, bool writeXlog)
{
  int     slotno;

  slotno = SimpleLruZeroPage(MultiXactMemberCtl, pageno);

  if (writeXlog)
    WriteMZeroPageXlogRec(pageno, XLOG_MULTIXACT_ZERO_MEM_PAGE);

  return slotno;
}

/*
 * MaybeExtendOffsetSlru
 *    Extend the offsets SLRU area, if necessary
 *
 * After a binary upgrade from <= 9.2, the pg_multixact/offsets SLRU area might
 * contain files that are shorter than necessary; this would occur if the old
 * installation had used multixacts beyond the first page (files cannot be
 * copied, because the on-disk representation is different).  pg_upgrade would
 * update pg_control to set the next offset value to be at that position, so
 * that tuples marked as locked by such MultiXacts would be seen as visible
 * without having to consult multixact.  However, trying to create and use a
 * new MultiXactId would result in an error because the page on which the new
 * value would reside does not exist.  This routine is in charge of creating
 * such pages.
 */
static void
MaybeExtendOffsetSlru(void)
{
  int64   pageno;
  LWLock     *lock;

  pageno = MultiXactIdToOffsetPage(MultiXactState->nextMXact);
  lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);

  LWLockAcquire(lock, LW_EXCLUSIVE);

  if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
  {
    int     slotno;

    /*
     * Fortunately for us, SimpleLruWritePage is already prepared to deal
     * with creating a new segment file even if the page we're writing is
     * not the first in it, so this is enough.
     */
    slotno = ZeroMultiXactOffsetPage(pageno, false);
    SimpleLruWritePage(MultiXactOffsetCtl, slotno);
  }

  LWLockRelease(lock);
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup.
 *
 * StartupXLOG has already established nextMXact/nextOffset by calling
 * MultiXactSetNextMXact and/or MultiXactAdvanceNextMXact, and the oldestMulti
 * info from pg_control and/or MultiXactAdvanceOldest, but we haven't yet
 * replayed WAL.
 */
void
StartupMultiXact(void)
{
  MultiXactId multi = MultiXactState->nextMXact;
  MultiXactOffset offset = MultiXactState->nextOffset;
  int64   pageno;

  /*
   * Initialize offset's idea of the latest page number.
   */
  pageno = MultiXactIdToOffsetPage(multi);
  pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
            pageno);

  /*
   * Initialize member's idea of the latest page number.
   */
  pageno = MXOffsetToMemberPage(offset);
  pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
            pageno);
}

/*
 * This must be called ONCE at the end of startup/recovery.
 */
void
TrimMultiXact(void)
{
  MultiXactId nextMXact;
  MultiXactOffset offset;
  MultiXactId oldestMXact;
  Oid     oldestMXactDB;
  int64   pageno;
  int     entryno;
  int     flagsoff;

  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  nextMXact = MultiXactState->nextMXact;
  offset = MultiXactState->nextOffset;
  oldestMXact = MultiXactState->oldestMultiXactId;
  oldestMXactDB = MultiXactState->oldestMultiXactDB;
  LWLockRelease(MultiXactGenLock);

  /* Clean up offsets state */

  /*
   * (Re-)Initialize our idea of the latest page number for offsets.
   */
  pageno = MultiXactIdToOffsetPage(nextMXact);
  pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
            pageno);

  /*
   * Zero out the remainder of the current offsets page.  See notes in
   * TrimCLOG() for background.  Unlike CLOG, some WAL record covers every
   * pg_multixact SLRU mutation.  Since, also unlike CLOG, we ignore the WAL
   * rule "write xlog before data," nextMXact successors may carry obsolete,
   * nonzero offset values.  Zero those so case 2 of GetMultiXactIdMembers()
   * operates normally.
   */
  entryno = MultiXactIdToOffsetEntry(nextMXact);
  if (entryno != 0)
  {
    int     slotno;
    MultiXactOffset *offptr;
    LWLock     *lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);

    LWLockAcquire(lock, LW_EXCLUSIVE);
    slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact);
    offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
    offptr += entryno;

    MemSet(offptr, 0, BLCKSZ - (entryno * sizeof(MultiXactOffset)));

    MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
    LWLockRelease(lock);
  }

  /*
   * And the same for members.
   *
   * (Re-)Initialize our idea of the latest page number for members.
   */
  pageno = MXOffsetToMemberPage(offset);
  pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
            pageno);

  /*
   * Zero out the remainder of the current members page.  See notes in
   * TrimCLOG() for motivation.
   */
  flagsoff = MXOffsetToFlagsOffset(offset);
  if (flagsoff != 0)
  {
    int     slotno;
    TransactionId *xidptr;
    int     memberoff;
    LWLock     *lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);

    LWLockAcquire(lock, LW_EXCLUSIVE);
    memberoff = MXOffsetToMemberOffset(offset);
    slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, offset);
    xidptr = (TransactionId *)
      (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);

    MemSet(xidptr, 0, BLCKSZ - memberoff);

    /*
     * Note: we don't need to zero out the flag bits in the remaining
     * members of the current group, because they are always reset before
     * writing.
     */

    MultiXactMemberCtl->shared->page_dirty[slotno] = true;
    LWLockRelease(lock);
  }

  /* signal that we're officially up */
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  MultiXactState->finishedStartup = true;
  LWLockRelease(MultiXactGenLock);

  /* Now compute how far away the next members wraparound is. */
  SetMultiXactIdLimit(oldestMXact, oldestMXactDB, true);
}

/*
 * Get the MultiXact data to save in a checkpoint record
 */
void
MultiXactGetCheckptMulti(bool is_shutdown,
             MultiXactId *nextMulti,
             MultiXactOffset *nextMultiOffset,
             MultiXactId *oldestMulti,
             Oid *oldestMultiDB)
{
  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  *nextMulti = MultiXactState->nextMXact;
  *nextMultiOffset = MultiXactState->nextOffset;
  *oldestMulti = MultiXactState->oldestMultiXactId;
  *oldestMultiDB = MultiXactState->oldestMultiXactDB;
  LWLockRelease(MultiXactGenLock);

  debug_elog6(DEBUG2,
        "MultiXact: checkpoint is nextMulti %u, nextOffset %u, oldestMulti %u in DB %u",
        *nextMulti, *nextMultiOffset, *oldestMulti, *oldestMultiDB);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointMultiXact(void)
{
  TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(true);

  /*
   * Write dirty MultiXact pages to disk.  This may result in sync requests
   * queued for later handling by ProcessSyncRequests(), as part of the
   * checkpoint.
   */
  SimpleLruWriteAll(MultiXactOffsetCtl, true);
  SimpleLruWriteAll(MultiXactMemberCtl, true);

  TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(true);
}

/*
 * Set the next-to-be-assigned MultiXactId and offset
 *
 * This is used when we can determine the correct next ID/offset exactly
 * from a checkpoint record.  Although this is only called during bootstrap
 * and XLog replay, we take the lock in case any hot-standby backends are
 * examining the values.
 */
void
MultiXactSetNextMXact(MultiXactId nextMulti,
            MultiXactOffset nextMultiOffset)
{
  debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %u",
        nextMulti, nextMultiOffset);
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  MultiXactState->nextMXact = nextMulti;
  MultiXactState->nextOffset = nextMultiOffset;
  LWLockRelease(MultiXactGenLock);

  /*
   * During a binary upgrade, make sure that the offsets SLRU is large
   * enough to contain the next value that would be created.
   *
   * We need to do this pretty early during the first startup in binary
   * upgrade mode: before StartupMultiXact() in fact, because this routine
   * is called even before that by StartupXLOG().  And we can't do it
   * earlier than at this point, because during that first call of this
   * routine we determine the MultiXactState->nextMXact value that
   * MaybeExtendOffsetSlru needs.
   */
  if (IsBinaryUpgrade)
    MaybeExtendOffsetSlru();
}

/*
 * Determine the last safe MultiXactId to allocate given the currently oldest
 * datminmxid (ie, the oldest MultiXactId that might exist in any database
 * of our cluster), and the OID of the (or a) database with that value.
 *
 * is_startup is true when we are just starting the cluster, false when we
 * are updating state in a running cluster.  This only affects log messages.
 */
void
SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid,
          bool is_startup)
{
  MultiXactId multiVacLimit;
  MultiXactId multiWarnLimit;
  MultiXactId multiStopLimit;
  MultiXactId multiWrapLimit;
  MultiXactId curMulti;
  bool    needs_offset_vacuum;

  Assert(MultiXactIdIsValid(oldest_datminmxid));

  /*
   * We pretend that a wrap will happen halfway through the multixact ID
   * space, but that's not really true, because multixacts wrap differently
   * from transaction IDs.  Note that, separately from any concern about
   * multixact IDs wrapping, we must ensure that multixact members do not
   * wrap.  Limits for that are set in SetOffsetVacuumLimit, not here.
   */
  multiWrapLimit = oldest_datminmxid + (MaxMultiXactId >> 1);
  if (multiWrapLimit < FirstMultiXactId)
    multiWrapLimit += FirstMultiXactId;

  /*
   * We'll refuse to continue assigning MultiXactIds once we get within 3M
   * multi of data loss.  See SetTransactionIdLimit.
   */
  multiStopLimit = multiWrapLimit - 3000000;
  if (multiStopLimit < FirstMultiXactId)
    multiStopLimit -= FirstMultiXactId;

  /*
   * We'll start complaining loudly when we get within 40M multis of data
   * loss.  This is kind of arbitrary, but if you let your gas gauge get
   * down to 2% of full, would you be looking for the next gas station?  We
   * need to be fairly liberal about this number because there are lots of
   * scenarios where most transactions are done by automatic clients that
   * won't pay attention to warnings.  (No, we're not gonna make this
   * configurable.  If you know enough to configure it, you know enough to
   * not get in this kind of trouble in the first place.)
   */
  multiWarnLimit = multiWrapLimit - 40000000;
  if (multiWarnLimit < FirstMultiXactId)
    multiWarnLimit -= FirstMultiXactId;

  /*
   * We'll start trying to force autovacuums when oldest_datminmxid gets to
   * be more than autovacuum_multixact_freeze_max_age mxids old.
   *
   * Note: autovacuum_multixact_freeze_max_age is a PGC_POSTMASTER parameter
   * so that we don't have to worry about dealing with on-the-fly changes in
   * its value.  See SetTransactionIdLimit.
   */
  multiVacLimit = oldest_datminmxid + autovacuum_multixact_freeze_max_age;
  if (multiVacLimit < FirstMultiXactId)
    multiVacLimit += FirstMultiXactId;

  /* Grab lock for just long enough to set the new limit values */
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  MultiXactState->oldestMultiXactId = oldest_datminmxid;
  MultiXactState->oldestMultiXactDB = oldest_datoid;
  MultiXactState->multiVacLimit = multiVacLimit;
  MultiXactState->multiWarnLimit = multiWarnLimit;
  MultiXactState->multiStopLimit = multiStopLimit;
  MultiXactState->multiWrapLimit = multiWrapLimit;
  curMulti = MultiXactState->nextMXact;
  LWLockRelease(MultiXactGenLock);

  /* Log the info */
  ereport(DEBUG1,
      (errmsg_internal("MultiXactId wrap limit is %u, limited by database with OID %u",
               multiWrapLimit, oldest_datoid)));

  /*
   * Computing the actual limits is only possible once the data directory is
   * in a consistent state. There's no need to compute the limits while
   * still replaying WAL - no decisions about new multis are made even
   * though multixact creations might be replayed. So we'll only do further
   * checks after TrimMultiXact() has been called.
   */
  if (!MultiXactState->finishedStartup)
    return;

  Assert(!InRecovery);

  /* Set limits for offset vacuum. */
  needs_offset_vacuum = SetOffsetVacuumLimit(is_startup);

  /*
   * If past the autovacuum force point, immediately signal an autovac
   * request.  The reason for this is that autovac only processes one
   * database per invocation.  Once it's finished cleaning up the oldest
   * database, it'll call here, and we'll signal the postmaster to start
   * another iteration immediately if there are still any old databases.
   */
  if ((MultiXactIdPrecedes(multiVacLimit, curMulti) ||
     needs_offset_vacuum) && IsUnderPostmaster)
    SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

  /* Give an immediate warning if past the wrap warn point */
  if (MultiXactIdPrecedes(multiWarnLimit, curMulti))
  {
    char     *oldest_datname;

    /*
     * We can be called when not inside a transaction, for example during
     * StartupXLOG().  In such a case we cannot do database access, so we
     * must just report the oldest DB's OID.
     *
     * Note: it's also possible that get_database_name fails and returns
     * NULL, for example because the database just got dropped.  We'll
     * still warn, even though the warning might now be unnecessary.
     */
    if (IsTransactionState())
      oldest_datname = get_database_name(oldest_datoid);
    else
      oldest_datname = NULL;

    if (oldest_datname)
      ereport(WARNING,
          (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
                   "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
                   multiWrapLimit - curMulti,
                   oldest_datname,
                   multiWrapLimit - curMulti),
           errhint("To avoid MultiXactId assignment failures, execute a database-wide VACUUM in that database.\n"
               "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    else
      ereport(WARNING,
          (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
                   "database with OID %u must be vacuumed before %u more MultiXactIds are used",
                   multiWrapLimit - curMulti,
                   oldest_datoid,
                   multiWrapLimit - curMulti),
           errhint("To avoid MultiXactId assignment failures, execute a database-wide VACUUM in that database.\n"
               "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
  }
}

/*
 * Ensure the next-to-be-assigned MultiXactId is at least minMulti,
 * and similarly nextOffset is at least minMultiOffset.
 *
 * This is used when we can determine minimum safe values from an XLog
 * record (either an on-line checkpoint or an mxact creation log entry).
 * Although this is only called during XLog replay, we take the lock in case
 * any hot-standby backends are examining the values.
 */
void
MultiXactAdvanceNextMXact(MultiXactId minMulti,
              MultiXactOffset minMultiOffset)
{
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  if (MultiXactIdPrecedes(MultiXactState->nextMXact, minMulti))
  {
    debug_elog3(DEBUG2, "MultiXact: setting next multi to %u", minMulti);
    MultiXactState->nextMXact = minMulti;
  }
  if (MultiXactOffsetPrecedes(MultiXactState->nextOffset, minMultiOffset))
  {
    debug_elog3(DEBUG2, "MultiXact: setting next offset to %u",
          minMultiOffset);
    MultiXactState->nextOffset = minMultiOffset;
  }
  LWLockRelease(MultiXactGenLock);
}

/*
 * Update our oldestMultiXactId value, but only if it's more recent than what
 * we had.
 *
 * This may only be called during WAL replay.
 */
void
MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB)
{
  Assert(InRecovery);

  if (MultiXactIdPrecedes(MultiXactState->oldestMultiXactId, oldestMulti))
    SetMultiXactIdLimit(oldestMulti, oldestMultiDB, false);
}

/*
 * Make sure that MultiXactOffset has room for a newly-allocated MultiXactId.
 *
 * NB: this is called while holding MultiXactGenLock.  We want it to be very
 * fast most of the time; even when it's not so fast, no actual I/O need
 * happen unless we're forced to write out a dirty log or xlog page to make
 * room in shared memory.
 */
static void
ExtendMultiXactOffset(MultiXactId multi)
{
  int64   pageno;
  LWLock     *lock;

  /*
   * No work except at first MultiXactId of a page.  But beware: just after
   * wraparound, the first MultiXactId of page zero is FirstMultiXactId.
   */
  if (MultiXactIdToOffsetEntry(multi) != 0 &&
    multi != FirstMultiXactId)
    return;

  pageno = MultiXactIdToOffsetPage(multi);
  lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);

  LWLockAcquire(lock, LW_EXCLUSIVE);

  /* Zero the page and make an XLOG entry about it */
  ZeroMultiXactOffsetPage(pageno, true);

  LWLockRelease(lock);
}

/*
 * Make sure that MultiXactMember has room for the members of a newly-
 * allocated MultiXactId.
 *
 * Like the above routine, this is called while holding MultiXactGenLock;
 * same comments apply.
 */
static void
ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
{
  /*
   * It's possible that the members span more than one page of the members
   * file, so we loop to ensure we consider each page.  The coding is not
   * optimal if the members span several pages, but that seems unusual
   * enough to not worry much about.
   */
  while (nmembers > 0)
  {
    int     flagsoff;
    int     flagsbit;
    uint32    difference;

    /*
     * Only zero when at first entry of a page.
     */
    flagsoff = MXOffsetToFlagsOffset(offset);
    flagsbit = MXOffsetToFlagsBitShift(offset);
    if (flagsoff == 0 && flagsbit == 0)
    {
      int64   pageno;
      LWLock     *lock;

      pageno = MXOffsetToMemberPage(offset);
      lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);

      LWLockAcquire(lock, LW_EXCLUSIVE);

      /* Zero the page and make an XLOG entry about it */
      ZeroMultiXactMemberPage(pageno, true);

      LWLockRelease(lock);
    }

    /*
     * Compute the number of items till end of current page.  Careful: if
     * addition of unsigned ints wraps around, we're at the last page of
     * the last segment; since that page holds a different number of items
     * than other pages, we need to do it differently.
     */
    if (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE < offset)
    {
      /*
       * This is the last page of the last segment; we can compute the
       * number of items left to allocate in it without modulo
       * arithmetic.
       */
      difference = MaxMultiXactOffset - offset + 1;
    }
    else
      difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;

    /*
     * Advance to next page, taking care to properly handle the wraparound
     * case.  OK if nmembers goes negative.
     */
    nmembers -= difference;
    offset += difference;
  }
}

/*
 * GetOldestMultiXactId
 *
 * Return the oldest MultiXactId that's still possibly still seen as live by
 * any running transaction.  Older ones might still exist on disk, but they no
 * longer have any running member transaction.
 *
 * It's not safe to truncate MultiXact SLRU segments on the value returned by
 * this function; however, it can be set as the new relminmxid for any table
 * that VACUUM knows has no remaining MXIDs < the same value.  It is only safe
 * to truncate SLRUs when no table can possibly still have a referencing MXID.
 */
MultiXactId
GetOldestMultiXactId(void)
{
  MultiXactId oldestMXact;
  MultiXactId nextMXact;
  int     i;

  /*
   * This is the oldest valid value among all the OldestMemberMXactId[] and
   * OldestVisibleMXactId[] entries, or nextMXact if none are valid.
   */
  LWLockAcquire(MultiXactGenLock, LW_SHARED);

  /*
   * We have to beware of the possibility that nextMXact is in the
   * wrapped-around state.  We don't fix the counter itself here, but we
   * must be sure to use a valid value in our calculation.
   */
  nextMXact = MultiXactState->nextMXact;
  if (nextMXact < FirstMultiXactId)
    nextMXact = FirstMultiXactId;

  oldestMXact = nextMXact;
  for (i = 0; i < MaxOldestSlot; i++)
  {
    MultiXactId thisoldest;

    thisoldest = OldestMemberMXactId[i];
    if (MultiXactIdIsValid(thisoldest) &&
      MultiXactIdPrecedes(thisoldest, oldestMXact))
      oldestMXact = thisoldest;
    thisoldest = OldestVisibleMXactId[i];
    if (MultiXactIdIsValid(thisoldest) &&
      MultiXactIdPrecedes(thisoldest, oldestMXact))
      oldestMXact = thisoldest;
  }

  LWLockRelease(MultiXactGenLock);

  return oldestMXact;
}

/*
 * Determine how aggressively we need to vacuum in order to prevent member
 * wraparound.
 *
 * To do so determine what's the oldest member offset and install the limit
 * info in MultiXactState, where it can be used to prevent overrun of old data
 * in the members SLRU area.
 *
 * The return value is true if emergency autovacuum is required and false
 * otherwise.
 */
static bool
SetOffsetVacuumLimit(bool is_startup)
{
  MultiXactId oldestMultiXactId;
  MultiXactId nextMXact;
  MultiXactOffset oldestOffset = 0; /* placate compiler */
  MultiXactOffset prevOldestOffset;
  MultiXactOffset nextOffset;
  bool    oldestOffsetKnown = false;
  bool    prevOldestOffsetKnown;
  MultiXactOffset offsetStopLimit = 0;
  MultiXactOffset prevOffsetStopLimit;

  /*
   * NB: Have to prevent concurrent truncation, we might otherwise try to
   * lookup an oldestMulti that's concurrently getting truncated away.
   */
  LWLockAcquire(MultiXactTruncationLock, LW_SHARED);

  /* Read relevant fields from shared memory. */
  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  oldestMultiXactId = MultiXactState->oldestMultiXactId;
  nextMXact = MultiXactState->nextMXact;
  nextOffset = MultiXactState->nextOffset;
  prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
  prevOldestOffset = MultiXactState->oldestOffset;
  prevOffsetStopLimit = MultiXactState->offsetStopLimit;
  Assert(MultiXactState->finishedStartup);
  LWLockRelease(MultiXactGenLock);

  /*
   * Determine the offset of the oldest multixact.  Normally, we can read
   * the offset from the multixact itself, but there's an important special
   * case: if there are no multixacts in existence at all, oldestMXact
   * obviously can't point to one.  It will instead point to the multixact
   * ID that will be assigned the next time one is needed.
   */
  if (oldestMultiXactId == nextMXact)
  {
    /*
     * When the next multixact gets created, it will be stored at the next
     * offset.
     */
    oldestOffset = nextOffset;
    oldestOffsetKnown = true;
  }
  else
  {
    /*
     * Figure out where the oldest existing multixact's offsets are
     * stored. Due to bugs in early release of PostgreSQL 9.3.X and 9.4.X,
     * the supposedly-earliest multixact might not really exist.  We are
     * careful not to fail in that case.
     */
    oldestOffsetKnown =
      find_multixact_start(oldestMultiXactId, &oldestOffset);

    if (oldestOffsetKnown)
      ereport(DEBUG1,
          (errmsg_internal("oldest MultiXactId member is at offset %u",
                   oldestOffset)));
    else
      ereport(LOG,
          (errmsg("MultiXact member wraparound protections are disabled because oldest checkpointed MultiXact %u does not exist on disk",
              oldestMultiXactId)));
  }

  LWLockRelease(MultiXactTruncationLock);

  /*
   * If we can, compute limits (and install them MultiXactState) to prevent
   * overrun of old data in the members SLRU area. We can only do so if the
   * oldest offset is known though.
   */
  if (oldestOffsetKnown)
  {
    /* move back to start of the corresponding segment */
    offsetStopLimit = oldestOffset - (oldestOffset %
                      (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT));

    /* always leave one segment before the wraparound point */
    offsetStopLimit -= (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);

    if (!prevOldestOffsetKnown && !is_startup)
      ereport(LOG,
          (errmsg("MultiXact member wraparound protections are now enabled")));

    ereport(DEBUG1,
        (errmsg_internal("MultiXact member stop limit is now %u based on MultiXact %u",
                 offsetStopLimit, oldestMultiXactId)));
  }
  else if (prevOldestOffsetKnown)
  {
    /*
     * If we failed to get the oldest offset this time, but we have a
     * value from a previous pass through this function, use the old
     * values rather than automatically forcing an emergency autovacuum
     * cycle again.
     */
    oldestOffset = prevOldestOffset;
    oldestOffsetKnown = true;
    offsetStopLimit = prevOffsetStopLimit;
  }

  /* Install the computed values */
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  MultiXactState->oldestOffset = oldestOffset;
  MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
  MultiXactState->offsetStopLimit = offsetStopLimit;
  LWLockRelease(MultiXactGenLock);

  /*
   * Do we need an emergency autovacuum?  If we're not sure, assume yes.
   */
  return !oldestOffsetKnown ||
    (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD);
}

/*
 * Return whether adding "distance" to "start" would move past "boundary".
 *
 * We use this to determine whether the addition is "wrapping around" the
 * boundary point, hence the name.  The reason we don't want to use the regular
 * 2^31-modulo arithmetic here is that we want to be able to use the whole of
 * the 2^32-1 space here, allowing for more multixacts than would fit
 * otherwise.
 */
static bool
MultiXactOffsetWouldWrap(MultiXactOffset boundary, MultiXactOffset start,
             uint32 distance)
{
  MultiXactOffset finish;

  /*
   * Note that offset number 0 is not used (see GetMultiXactIdMembers), so
   * if the addition wraps around the UINT_MAX boundary, skip that value.
   */
  finish = start + distance;
  if (finish < start)
    finish++;

  /*-----------------------------------------------------------------------
   * When the boundary is numerically greater than the starting point, any
   * value numerically between the two is not wrapped:
   *
   *  <----S----B---->
   *  [---)      = F wrapped past B (and UINT_MAX)
   *     [---)     = F not wrapped
   *        [----] = F wrapped past B
   *
   * When the boundary is numerically less than the starting point (i.e. the
   * UINT_MAX wraparound occurs somewhere in between) then all values in
   * between are wrapped:
   *
   *  <----B----S---->
   *  [---)      = F not wrapped past B (but wrapped past UINT_MAX)
   *     [---)     = F wrapped past B (and UINT_MAX)
   *        [----] = F not wrapped
   *-----------------------------------------------------------------------
   */
  if (start < boundary)
    return finish >= boundary || finish < start;
  else
    return finish >= boundary && finish < start;
}

/*
 * Find the starting offset of the given MultiXactId.
 *
 * Returns false if the file containing the multi does not exist on disk.
 * Otherwise, returns true and sets *result to the starting member offset.
 *
 * This function does not prevent concurrent truncation, so if that's
 * required, the caller has to protect against that.
 */
static bool
find_multixact_start(MultiXactId multi, MultiXactOffset *result)
{
  MultiXactOffset offset;
  int64   pageno;
  int     entryno;
  int     slotno;
  MultiXactOffset *offptr;

  Assert(MultiXactState->finishedStartup);

  pageno = MultiXactIdToOffsetPage(multi);
  entryno = MultiXactIdToOffsetEntry(multi);

  /*
   * Write out dirty data, so PhysicalPageExists can work correctly.
   */
  SimpleLruWriteAll(MultiXactOffsetCtl, true);
  SimpleLruWriteAll(MultiXactMemberCtl, true);

  if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
    return false;

  /* lock is acquired by SimpleLruReadPage_ReadOnly */
  slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, multi);
  offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
  offptr += entryno;
  offset = *offptr;
  LWLockRelease(SimpleLruGetBankLock(MultiXactOffsetCtl, pageno));

  *result = offset;
  return true;
}

/*
 * Determine how many multixacts, and how many multixact members, currently
 * exist.  Return false if unable to determine.
 */
static bool
ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
{
  MultiXactOffset nextOffset;
  MultiXactOffset oldestOffset;
  MultiXactId oldestMultiXactId;
  MultiXactId nextMultiXactId;
  bool    oldestOffsetKnown;

  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  nextOffset = MultiXactState->nextOffset;
  oldestMultiXactId = MultiXactState->oldestMultiXactId;
  nextMultiXactId = MultiXactState->nextMXact;
  oldestOffset = MultiXactState->oldestOffset;
  oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
  LWLockRelease(MultiXactGenLock);

  if (!oldestOffsetKnown)
    return false;

  *members = nextOffset - oldestOffset;
  *multixacts = nextMultiXactId - oldestMultiXactId;
  return true;
}

/*
 * Multixact members can be removed once the multixacts that refer to them
 * are older than every datminmxid.  autovacuum_multixact_freeze_max_age and
 * vacuum_multixact_freeze_table_age work together to make sure we never have
 * too many multixacts; we hope that, at least under normal circumstances,
 * this will also be sufficient to keep us from using too many offsets.
 * However, if the average multixact has many members, we might exhaust the
 * members space while still using few enough members that these limits fail
 * to trigger relminmxid advancement by VACUUM.  At that point, we'd have no
 * choice but to start failing multixact-creating operations with an error.
 *
 * To prevent that, if more than a threshold portion of the members space is
 * used, we effectively reduce autovacuum_multixact_freeze_max_age and
 * to a value just less than the number of multixacts in use.  We hope that
 * this will quickly trigger autovacuuming on the table or tables with the
 * oldest relminmxid, thus allowing datminmxid values to advance and removing
 * some members.
 *
 * As the fraction of the member space currently in use grows, we become
 * more aggressive in clamping this value.  That not only causes autovacuum
 * to ramp up, but also makes any manual vacuums the user issues more
 * aggressive.  This happens because vacuum_get_cutoffs() will clamp the
 * freeze table and the minimum freeze age cutoffs based on the effective
 * autovacuum_multixact_freeze_max_age this function returns.  In the worst
 * case, we'll claim the freeze_max_age to zero, and every vacuum of any
 * table will freeze every multixact.
 */
int
MultiXactMemberFreezeThreshold(void)
{
  MultiXactOffset members;
  uint32    multixacts;
  uint32    victim_multixacts;
  double    fraction;
  int     result;

  /* If we can't determine member space utilization, assume the worst. */
  if (!ReadMultiXactCounts(&multixacts, &members))
    return 0;

  /* If member space utilization is low, no special action is required. */
  if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
    return autovacuum_multixact_freeze_max_age;

  /*
   * Compute a target for relminmxid advancement.  The number of multixacts
   * we try to eliminate from the system is based on how far we are past
   * MULTIXACT_MEMBER_SAFE_THRESHOLD.
   */
  fraction = (double) (members - MULTIXACT_MEMBER_SAFE_THRESHOLD) /
    (MULTIXACT_MEMBER_DANGER_THRESHOLD - MULTIXACT_MEMBER_SAFE_THRESHOLD);
  victim_multixacts = multixacts * fraction;

  /* fraction could be > 1.0, but lowest possible freeze age is zero */
  if (victim_multixacts > multixacts)
    return 0;
  result = multixacts - victim_multixacts;

  /*
   * Clamp to autovacuum_multixact_freeze_max_age, so that we never make
   * autovacuum less aggressive than it would otherwise be.
   */
  return Min(result, autovacuum_multixact_freeze_max_age);
}

typedef struct mxtruncinfo
{
  int64   earliestExistingPage;
} mxtruncinfo;

/*
 * SlruScanDirectory callback
 *    This callback determines the earliest existing page number.
 */
static bool
SlruScanDirCbFindEarliest(SlruCtl ctl, char *filename, int64 segpage, void *data)
{
  mxtruncinfo *trunc = (mxtruncinfo *) data;

  if (trunc->earliestExistingPage == -1 ||
    ctl->PagePrecedes(segpage, trunc->earliestExistingPage))
  {
    trunc->earliestExistingPage = segpage;
  }

  return false;       /* keep going */
}


/*
 * Delete members segments [oldest, newOldest)
 *
 * The members SLRU can, in contrast to the offsets one, be filled to almost
 * the full range at once. This means SimpleLruTruncate() can't trivially be
 * used - instead the to-be-deleted range is computed using the offsets
 * SLRU. C.f. TruncateMultiXact().
 */
static void
PerformMembersTruncation(MultiXactOffset oldestOffset, MultiXactOffset newOldestOffset)
{
  const int64 maxsegment = MXOffsetToMemberSegment(MaxMultiXactOffset);
  int64   startsegment = MXOffsetToMemberSegment(oldestOffset);
  int64   endsegment = MXOffsetToMemberSegment(newOldestOffset);
  int64   segment = startsegment;

  /*
   * Delete all the segments but the last one. The last segment can still
   * contain, possibly partially, valid data.
   */
  while (segment != endsegment)
  {
    elog(DEBUG2, "truncating multixact members segment %" PRIx64,
       segment);
    SlruDeleteSegment(MultiXactMemberCtl, segment);

    /* move to next segment, handling wraparound correctly */
    if (segment == maxsegment)
      segment = 0;
    else
      segment += 1;
  }
}

/*
 * Delete offsets segments [oldest, newOldest)
 */
static void
PerformOffsetsTruncation(MultiXactId oldestMulti, MultiXactId newOldestMulti)
{
  /*
   * We step back one multixact to avoid passing a cutoff page that hasn't
   * been created yet in the rare case that oldestMulti would be the first
   * item on a page and oldestMulti == nextMulti.  In that case, if we
   * didn't subtract one, we'd trigger SimpleLruTruncate's wraparound
   * detection.
   */
  SimpleLruTruncate(MultiXactOffsetCtl,
            MultiXactIdToOffsetPage(PreviousMultiXactId(newOldestMulti)));
}

/*
 * Remove all MultiXactOffset and MultiXactMember segments before the oldest
 * ones still of interest.
 *
 * This is only called on a primary as part of vacuum (via
 * vac_truncate_clog()). During recovery truncation is done by replaying
 * truncation WAL records logged here.
 *
 * newOldestMulti is the oldest currently required multixact, newOldestMultiDB
 * is one of the databases preventing newOldestMulti from increasing.
 */
void
TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
{
  MultiXactId oldestMulti;
  MultiXactId nextMulti;
  MultiXactOffset newOldestOffset;
  MultiXactOffset oldestOffset;
  MultiXactOffset nextOffset;
  mxtruncinfo trunc;
  MultiXactId earliest;

  Assert(!RecoveryInProgress());
  Assert(MultiXactState->finishedStartup);

  /*
   * We can only allow one truncation to happen at once. Otherwise parts of
   * members might vanish while we're doing lookups or similar. There's no
   * need to have an interlock with creating new multis or such, since those
   * are constrained by the limits (which only grow, never shrink).
   */
  LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);

  LWLockAcquire(MultiXactGenLock, LW_SHARED);
  nextMulti = MultiXactState->nextMXact;
  nextOffset = MultiXactState->nextOffset;
  oldestMulti = MultiXactState->oldestMultiXactId;
  LWLockRelease(MultiXactGenLock);
  Assert(MultiXactIdIsValid(oldestMulti));

  /*
   * Make sure to only attempt truncation if there's values to truncate
   * away. In normal processing values shouldn't go backwards, but there's
   * some corner cases (due to bugs) where that's possible.
   */
  if (MultiXactIdPrecedesOrEquals(newOldestMulti, oldestMulti))
  {
    LWLockRelease(MultiXactTruncationLock);
    return;
  }

  /*
   * Note we can't just plow ahead with the truncation; it's possible that
   * there are no segments to truncate, which is a problem because we are
   * going to attempt to read the offsets page to determine where to
   * truncate the members SLRU.  So we first scan the directory to determine
   * the earliest offsets page number that we can read without error.
   *
   * When nextMXact is less than one segment away from multiWrapLimit,
   * SlruScanDirCbFindEarliest can find some early segment other than the
   * actual earliest.  (MultiXactOffsetPagePrecedes(EARLIEST, LATEST)
   * returns false, because not all pairs of entries have the same answer.)
   * That can also arise when an earlier truncation attempt failed unlink()
   * or returned early from this function.  The only consequence is
   * returning early, which wastes space that we could have liberated.
   *
   * NB: It's also possible that the page that oldestMulti is on has already
   * been truncated away, and we crashed before updating oldestMulti.
   */
  trunc.earliestExistingPage = -1;
  SlruScanDirectory(MultiXactOffsetCtl, SlruScanDirCbFindEarliest, &trunc);
  earliest = trunc.earliestExistingPage * MULTIXACT_OFFSETS_PER_PAGE;
  if (earliest < FirstMultiXactId)
    earliest = FirstMultiXactId;

  /* If there's nothing to remove, we can bail out early. */
  if (MultiXactIdPrecedes(oldestMulti, earliest))
  {
    LWLockRelease(MultiXactTruncationLock);
    return;
  }

  /*
   * First, compute the safe truncation point for MultiXactMember. This is
   * the starting offset of the oldest multixact.
   *
   * Hopefully, find_multixact_start will always work here, because we've
   * already checked that it doesn't precede the earliest MultiXact on disk.
   * But if it fails, don't truncate anything, and log a message.
   */
  if (oldestMulti == nextMulti)
  {
    /* there are NO MultiXacts */
    oldestOffset = nextOffset;
  }
  else if (!find_multixact_start(oldestMulti, &oldestOffset))
  {
    ereport(LOG,
        (errmsg("oldest MultiXact %u not found, earliest MultiXact %u, skipping truncation",
            oldestMulti, earliest)));
    LWLockRelease(MultiXactTruncationLock);
    return;
  }

  /*
   * Secondly compute up to where to truncate. Lookup the corresponding
   * member offset for newOldestMulti for that.
   */
  if (newOldestMulti == nextMulti)
  {
    /* there are NO MultiXacts */
    newOldestOffset = nextOffset;
  }
  else if (!find_multixact_start(newOldestMulti, &newOldestOffset))
  {
    ereport(LOG,
        (errmsg("cannot truncate up to MultiXact %u because it does not exist on disk, skipping truncation",
            newOldestMulti)));
    LWLockRelease(MultiXactTruncationLock);
    return;
  }

  elog(DEBUG1, "performing multixact truncation: "
     "offsets [%u, %u), offsets segments [%" PRIx64 ", %" PRIx64 "), "
     "members [%u, %u), members segments [%" PRIx64 ", %" PRIx64 ")",
     oldestMulti, newOldestMulti,
     MultiXactIdToOffsetSegment(oldestMulti),
     MultiXactIdToOffsetSegment(newOldestMulti),
     oldestOffset, newOldestOffset,
     MXOffsetToMemberSegment(oldestOffset),
     MXOffsetToMemberSegment(newOldestOffset));

  /*
   * Do truncation, and the WAL logging of the truncation, in a critical
   * section. That way offsets/members cannot get out of sync anymore, i.e.
   * once consistent the newOldestMulti will always exist in members, even
   * if we crashed in the wrong moment.
   */
  START_CRIT_SECTION();

  /*
   * Prevent checkpoints from being scheduled concurrently. This is critical
   * because otherwise a truncation record might not be replayed after a
   * crash/basebackup, even though the state of the data directory would
   * require it.
   */
  Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
  MyProc->delayChkptFlags |= DELAY_CHKPT_START;

  /* WAL log truncation */
  WriteMTruncateXlogRec(newOldestMultiDB,
              oldestMulti, newOldestMulti,
              oldestOffset, newOldestOffset);

  /*
   * Update in-memory limits before performing the truncation, while inside
   * the critical section: Have to do it before truncation, to prevent
   * concurrent lookups of those values. Has to be inside the critical
   * section as otherwise a future call to this function would error out,
   * while looking up the oldest member in offsets, if our caller crashes
   * before updating the limits.
   */
  LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
  MultiXactState->oldestMultiXactId = newOldestMulti;
  MultiXactState->oldestMultiXactDB = newOldestMultiDB;
  LWLockRelease(MultiXactGenLock);

  /* First truncate members */
  PerformMembersTruncation(oldestOffset, newOldestOffset);

  /* Then offsets */
  PerformOffsetsTruncation(oldestMulti, newOldestMulti);

  MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;

  END_CRIT_SECTION();
  LWLockRelease(MultiXactTruncationLock);
}

/*
 * Decide whether a MultiXactOffset page number is "older" for truncation
 * purposes.  Analogous to CLOGPagePrecedes().
 *
 * Offsetting the values is optional, because MultiXactIdPrecedes() has
 * translational symmetry.
 */
static bool
MultiXactOffsetPagePrecedes(int64 page1, int64 page2)
{
  MultiXactId multi1;
  MultiXactId multi2;

  multi1 = ((MultiXactId) page1) * MULTIXACT_OFFSETS_PER_PAGE;
  multi1 += FirstMultiXactId + 1;
  multi2 = ((MultiXactId) page2) * MULTIXACT_OFFSETS_PER_PAGE;
  multi2 += FirstMultiXactId + 1;

  return (MultiXactIdPrecedes(multi1, multi2) &&
      MultiXactIdPrecedes(multi1,
                multi2 + MULTIXACT_OFFSETS_PER_PAGE - 1));
}

/*
 * Decide whether a MultiXactMember page number is "older" for truncation
 * purposes.  There is no "invalid offset number" so use the numbers verbatim.
 */
static bool
MultiXactMemberPagePrecedes(int64 page1, int64 page2)
{
  MultiXactOffset offset1;
  MultiXactOffset offset2;

  offset1 = ((MultiXactOffset) page1) * MULTIXACT_MEMBERS_PER_PAGE;
  offset2 = ((MultiXactOffset) page2) * MULTIXACT_MEMBERS_PER_PAGE;

  return (MultiXactOffsetPrecedes(offset1, offset2) &&
      MultiXactOffsetPrecedes(offset1,
                  offset2 + MULTIXACT_MEMBERS_PER_PAGE - 1));
}

/*
 * Decide which of two MultiXactIds is earlier.
 *
 * XXX do we need to do something special for InvalidMultiXactId?
 * (Doesn't look like it.)
 */
bool
MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2)
{
  int32   diff = (int32) (multi1 - multi2);

  return (diff < 0);
}

/*
 * MultiXactIdPrecedesOrEquals -- is multi1 logically <= multi2?
 *
 * XXX do we need to do something special for InvalidMultiXactId?
 * (Doesn't look like it.)
 */
bool
MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
{
  int32   diff = (int32) (multi1 - multi2);

  return (diff <= 0);
}


/*
 * Decide which of two offsets is earlier.
 */
static bool
MultiXactOffsetPrecedes(MultiXactOffset offset1, MultiXactOffset offset2)
{
  int32   diff = (int32) (offset1 - offset2);

  return (diff < 0);
}

/*
 * Write an xlog record reflecting the zeroing of either a MEMBERs or
 * OFFSETs page (info shows which)
 */
static void
WriteMZeroPageXlogRec(int64 pageno, uint8 info)
{
  XLogBeginInsert();
  XLogRegisterData(&pageno, sizeof(pageno));
  (void) XLogInsert(RM_MULTIXACT_ID, info);
}

/*
 * Write a TRUNCATE xlog record
 *
 * We must flush the xlog record to disk before returning --- see notes in
 * TruncateCLOG().
 */
static void
WriteMTruncateXlogRec(Oid oldestMultiDB,
            MultiXactId startTruncOff, MultiXactId endTruncOff,
            MultiXactOffset startTruncMemb, MultiXactOffset endTruncMemb)
{
  XLogRecPtr  recptr;
  xl_multixact_truncate xlrec;

  xlrec.oldestMultiDB = oldestMultiDB;

  xlrec.startTruncOff = startTruncOff;
  xlrec.endTruncOff = endTruncOff;

  xlrec.startTruncMemb = startTruncMemb;
  xlrec.endTruncMemb = endTruncMemb;

  XLogBeginInsert();
  XLogRegisterData(&xlrec, SizeOfMultiXactTruncate);
  recptr = XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_TRUNCATE_ID);
  XLogFlush(recptr);
}

/*
 * MULTIXACT resource manager's routines
 */
void
multixact_redo(XLogReaderState *record)
{
  uint8   info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;

  /* Backup blocks are not used in multixact records */
  Assert(!XLogRecHasAnyBlockRefs(record));

  if (info == XLOG_MULTIXACT_ZERO_OFF_PAGE)
  {
    int64   pageno;
    int     slotno;
    LWLock     *lock;

    memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));

    lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);

    slotno = ZeroMultiXactOffsetPage(pageno, false);
    SimpleLruWritePage(MultiXactOffsetCtl, slotno);
    Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);

    LWLockRelease(lock);
  }
  else if (info == XLOG_MULTIXACT_ZERO_MEM_PAGE)
  {
    int64   pageno;
    int     slotno;
    LWLock     *lock;

    memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));

    lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);

    slotno = ZeroMultiXactMemberPage(pageno, false);
    SimpleLruWritePage(MultiXactMemberCtl, slotno);
    Assert(!MultiXactMemberCtl->shared->page_dirty[slotno]);

    LWLockRelease(lock);
  }
  else if (info == XLOG_MULTIXACT_CREATE_ID)
  {
    xl_multixact_create *xlrec =
      (xl_multixact_create *) XLogRecGetData(record);
    TransactionId max_xid;
    int     i;

    /* Store the data back into the SLRU files */
    RecordNewMultiXact(xlrec->mid, xlrec->moff, xlrec->nmembers,
               xlrec->members);

    /* Make sure nextMXact/nextOffset are beyond what this record has */
    MultiXactAdvanceNextMXact(xlrec->mid + 1,
                  xlrec->moff + xlrec->nmembers);

    /*
     * Make sure nextXid is beyond any XID mentioned in the record. This
     * should be unnecessary, since any XID found here ought to have other
     * evidence in the XLOG, but let's be safe.
     */
    max_xid = XLogRecGetXid(record);
    for (i = 0; i < xlrec->nmembers; i++)
    {
      if (TransactionIdPrecedes(max_xid, xlrec->members[i].xid))
        max_xid = xlrec->members[i].xid;
    }

    AdvanceNextFullTransactionIdPastXid(max_xid);
  }
  else if (info == XLOG_MULTIXACT_TRUNCATE_ID)
  {
    xl_multixact_truncate xlrec;
    int64   pageno;

    memcpy(&xlrec, XLogRecGetData(record),
         SizeOfMultiXactTruncate);

    elog(DEBUG1, "replaying multixact truncation: "
       "offsets [%u, %u), offsets segments [%" PRIx64 ", %" PRIx64 "), "
       "members [%u, %u), members segments [%" PRIx64 ", %" PRIx64 ")",
       xlrec.startTruncOff, xlrec.endTruncOff,
       MultiXactIdToOffsetSegment(xlrec.startTruncOff),
       MultiXactIdToOffsetSegment(xlrec.endTruncOff),
       xlrec.startTruncMemb, xlrec.endTruncMemb,
       MXOffsetToMemberSegment(xlrec.startTruncMemb),
       MXOffsetToMemberSegment(xlrec.endTruncMemb));

    /* should not be required, but more than cheap enough */
    LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);

    /*
     * Advance the horizon values, so they're current at the end of
     * recovery.
     */
    SetMultiXactIdLimit(xlrec.endTruncOff, xlrec.oldestMultiDB, false);

    PerformMembersTruncation(xlrec.startTruncMemb, xlrec.endTruncMemb);

    /*
     * During XLOG replay, latest_page_number isn't necessarily set up
     * yet; insert a suitable value to bypass the sanity test in
     * SimpleLruTruncate.
     */
    pageno = MultiXactIdToOffsetPage(xlrec.endTruncOff);
    pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
              pageno);
    PerformOffsetsTruncation(xlrec.startTruncOff, xlrec.endTruncOff);

    LWLockRelease(MultiXactTruncationLock);
  }
  else
    elog(PANIC, "multixact_redo: unknown op code %u", info);
}

Datum
pg_get_multixact_members(PG_FUNCTION_ARGS)
{
  typedef struct
  {
    MultiXactMember *members;
    int     nmembers;
    int     iter;
  } mxact;
  MultiXactId mxid = PG_GETARG_TRANSACTIONID(0);
  mxact    *multi;
  FuncCallContext *funccxt;

  if (mxid < FirstMultiXactId)
    ereport(ERROR,
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
         errmsg("invalid MultiXactId: %u", mxid)));

  if (SRF_IS_FIRSTCALL())
  {
    MemoryContext oldcxt;
    TupleDesc tupdesc;

    funccxt = SRF_FIRSTCALL_INIT();
    oldcxt = MemoryContextSwitchTo(funccxt->multi_call_memory_ctx);

    multi = palloc(sizeof(mxact));
    /* no need to allow for old values here */
    multi->nmembers = GetMultiXactIdMembers(mxid, &multi->members, false,
                        false);
    multi->iter = 0;

    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
      elog(ERROR, "return type must be a row type");
    funccxt->tuple_desc = tupdesc;
    funccxt->attinmeta = TupleDescGetAttInMetadata(tupdesc);
    funccxt->user_fctx = multi;

    MemoryContextSwitchTo(oldcxt);
  }

  funccxt = SRF_PERCALL_SETUP();
  multi = (mxact *) funccxt->user_fctx;

  while (multi->iter < multi->nmembers)
  {
    HeapTuple tuple;
    char     *values[2];

    values[0] = psprintf("%u", multi->members[multi->iter].xid);
    values[1] = mxstatus_to_string(multi->members[multi->iter].status);

    tuple = BuildTupleFromCStrings(funccxt->attinmeta, values);

    multi->iter++;
    pfree(values[0]);
    SRF_RETURN_NEXT(funccxt, HeapTupleGetDatum(tuple));
  }

  SRF_RETURN_DONE(funccxt);
}

/*
 * Entrypoint for sync.c to sync offsets files.
 */
int
multixactoffsetssyncfiletag(const FileTag *ftag, char *path)
{
  return SlruSyncFileTag(MultiXactOffsetCtl, ftag, path);
}

/*
 * Entrypoint for sync.c to sync members files.
 */
int
multixactmemberssyncfiletag(const FileTag *ftag, char *path)
{
  return SlruSyncFileTag(MultiXactMemberCtl, ftag, path);
}

Coverage Report

Created: 2025-06-15 06:31