/src/postgres/src/backend/storage/lmgr/proc.c

Source
/*-------------------------------------------------------------------------
 *
 * proc.c
 *    routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/proc.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * Interface (a):
 *    JoinWaitQueue(), ProcSleep(), ProcWakeup()
 *
 * Waiting for a lock causes the backend to be put to sleep.  Whoever releases
 * the lock wakes the process up again (and gives it an error code so it knows
 * whether it was awoken on an error condition).
 *
 * Interface (b):
 *
 * ProcReleaseLocks -- frees the locks associated with current transaction
 *
 * ProcKill -- destroys the shared memory state (and locks)
 * associated with the process.
 */
#include "postgres.h"

#include <signal.h>
#include <unistd.h>
#include <sys/time.h>

#include "access/transam.h"
#include "access/twophase.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "replication/slotsync.h"
#include "replication/syncrep.h"
#include "storage/condition_variable.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/spin.h"
#include "storage/standby.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"

/* GUC variables */
int     DeadlockTimeout = 1000;
int     StatementTimeout = 0;
int     LockTimeout = 0;
int     IdleInTransactionSessionTimeout = 0;
int     TransactionTimeout = 0;
int     IdleSessionTimeout = 0;
bool    log_lock_waits = true;

/* Pointer to this process's PGPROC struct, if any */
PGPROC     *MyProc = NULL;

/*
 * This spinlock protects the freelist of recycled PGPROC structures.
 * We cannot use an LWLock because the LWLock manager depends on already
 * having a PGPROC and a wait semaphore!  But these structures are touched
 * relatively infrequently (only at backend startup or shutdown) and not for
 * very long, so a spinlock is okay.
 */
NON_EXEC_STATIC slock_t *ProcStructLock = NULL;

/* Pointers to shared-memory structures */
PROC_HDR   *ProcGlobal = NULL;
NON_EXEC_STATIC PGPROC *AuxiliaryProcs = NULL;
PGPROC     *PreparedXactProcs = NULL;

static DeadLockState deadlock_state = DS_NOT_YET_CHECKED;

/* Is a deadlock check pending? */
static volatile sig_atomic_t got_deadlock_timeout;

static void RemoveProcFromArray(int code, Datum arg);
static void ProcKill(int code, Datum arg);
static void AuxiliaryProcKill(int code, Datum arg);
static void CheckDeadLock(void);


/*
 * Report shared-memory space needed by PGPROC.
 */
static Size
PGProcShmemSize(void)
{
  Size    size = 0;
  Size    TotalProcs =
    add_size(MaxBackends, add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));

  size = add_size(size, mul_size(TotalProcs, sizeof(PGPROC)));
  size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->xids)));
  size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->subxidStates)));
  size = add_size(size, mul_size(TotalProcs, sizeof(*ProcGlobal->statusFlags)));

  return size;
}

/*
 * Report shared-memory space needed by Fast-Path locks.
 */
static Size
FastPathLockShmemSize(void)
{
  Size    size = 0;
  Size    TotalProcs =
    add_size(MaxBackends, add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));
  Size    fpLockBitsSize,
        fpRelIdSize;

  /*
   * Memory needed for PGPROC fast-path lock arrays. Make sure the sizes are
   * nicely aligned in each backend.
   */
  fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
  fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));

  size = add_size(size, mul_size(TotalProcs, (fpLockBitsSize + fpRelIdSize)));

  return size;
}

/*
 * Report shared-memory space needed by InitProcGlobal.
 */
Size
ProcGlobalShmemSize(void)
{
  Size    size = 0;

  /* ProcGlobal */
  size = add_size(size, sizeof(PROC_HDR));
  size = add_size(size, sizeof(slock_t));

  size = add_size(size, PGProcShmemSize());
  size = add_size(size, FastPathLockShmemSize());

  return size;
}

/*
 * Report number of semaphores needed by InitProcGlobal.
 */
int
ProcGlobalSemas(void)
{
  /*
   * We need a sema per backend (including autovacuum), plus one for each
   * auxiliary process.
   */
  return MaxBackends + NUM_AUXILIARY_PROCS;
}

/*
 * InitProcGlobal -
 *    Initialize the global process table during postmaster or standalone
 *    backend startup.
 *
 *    We also create all the per-process semaphores we will need to support
 *    the requested number of backends.  We used to allocate semaphores
 *    only when backends were actually started up, but that is bad because
 *    it lets Postgres fail under load --- a lot of Unix systems are
 *    (mis)configured with small limits on the number of semaphores, and
 *    running out when trying to start another backend is a common failure.
 *    So, now we grab enough semaphores to support the desired max number
 *    of backends immediately at initialization --- if the sysadmin has set
 *    MaxConnections, max_worker_processes, max_wal_senders, or
 *    autovacuum_worker_slots higher than his kernel will support, he'll
 *    find out sooner rather than later.
 *
 *    Another reason for creating semaphores here is that the semaphore
 *    implementation typically requires us to create semaphores in the
 *    postmaster, not in backends.
 *
 * Note: this is NOT called by individual backends under a postmaster,
 * not even in the EXEC_BACKEND case.  The ProcGlobal and AuxiliaryProcs
 * pointers must be propagated specially for EXEC_BACKEND operation.
 */
void
InitProcGlobal(void)
{
  PGPROC     *procs;
  int     i,
        j;
  bool    found;
  uint32    TotalProcs = MaxBackends + NUM_AUXILIARY_PROCS + max_prepared_xacts;

  /* Used for setup of per-backend fast-path slots. */
  char     *fpPtr,
         *fpEndPtr PG_USED_FOR_ASSERTS_ONLY;
  Size    fpLockBitsSize,
        fpRelIdSize;
  Size    requestSize;
  char     *ptr;

  /* Create the ProcGlobal shared structure */
  ProcGlobal = (PROC_HDR *)
    ShmemInitStruct("Proc Header", sizeof(PROC_HDR), &found);
  Assert(!found);

  /*
   * Initialize the data structures.
   */
  ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;
  dlist_init(&ProcGlobal->freeProcs);
  dlist_init(&ProcGlobal->autovacFreeProcs);
  dlist_init(&ProcGlobal->bgworkerFreeProcs);
  dlist_init(&ProcGlobal->walsenderFreeProcs);
  ProcGlobal->startupBufferPinWaitBufId = -1;
  ProcGlobal->walwriterProc = INVALID_PROC_NUMBER;
  ProcGlobal->checkpointerProc = INVALID_PROC_NUMBER;
  pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PROC_NUMBER);
  pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PROC_NUMBER);

  /*
   * Create and initialize all the PGPROC structures we'll need.  There are
   * six separate consumers: (1) normal backends, (2) autovacuum workers and
   * special workers, (3) background workers, (4) walsenders, (5) auxiliary
   * processes, and (6) prepared transactions.  (For largely-historical
   * reasons, we combine autovacuum and special workers into one category
   * with a single freelist.)  Each PGPROC structure is dedicated to exactly
   * one of these purposes, and they do not move between groups.
   */
  requestSize = PGProcShmemSize();

  ptr = ShmemInitStruct("PGPROC structures",
              requestSize,
              &found);

  MemSet(ptr, 0, requestSize);

  procs = (PGPROC *) ptr;
  ptr = (char *) ptr + TotalProcs * sizeof(PGPROC);

  ProcGlobal->allProcs = procs;
  /* XXX allProcCount isn't really all of them; it excludes prepared xacts */
  ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;

  /*
   * Allocate arrays mirroring PGPROC fields in a dense manner. See
   * PROC_HDR.
   *
   * XXX: It might make sense to increase padding for these arrays, given
   * how hotly they are accessed.
   */
  ProcGlobal->xids = (TransactionId *) ptr;
  ptr = (char *) ptr + (TotalProcs * sizeof(*ProcGlobal->xids));

  ProcGlobal->subxidStates = (XidCacheStatus *) ptr;
  ptr = (char *) ptr + (TotalProcs * sizeof(*ProcGlobal->subxidStates));

  ProcGlobal->statusFlags = (uint8 *) ptr;
  ptr = (char *) ptr + (TotalProcs * sizeof(*ProcGlobal->statusFlags));

  /* make sure wer didn't overflow */
  Assert((ptr > (char *) procs) && (ptr <= (char *) procs + requestSize));

  /*
   * Allocate arrays for fast-path locks. Those are variable-length, so
   * can't be included in PGPROC directly. We allocate a separate piece of
   * shared memory and then divide that between backends.
   */
  fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
  fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));

  requestSize = FastPathLockShmemSize();

  fpPtr = ShmemInitStruct("Fast-Path Lock Array",
              requestSize,
              &found);

  MemSet(fpPtr, 0, requestSize);

  /* For asserts checking we did not overflow. */
  fpEndPtr = fpPtr + requestSize;

  for (i = 0; i < TotalProcs; i++)
  {
    PGPROC     *proc = &procs[i];

    /* Common initialization for all PGPROCs, regardless of type. */

    /*
     * Set the fast-path lock arrays, and move the pointer. We interleave
     * the two arrays, to (hopefully) get some locality for each backend.
     */
    proc->fpLockBits = (uint64 *) fpPtr;
    fpPtr += fpLockBitsSize;

    proc->fpRelId = (Oid *) fpPtr;
    fpPtr += fpRelIdSize;

    Assert(fpPtr <= fpEndPtr);

    /*
     * Set up per-PGPROC semaphore, latch, and fpInfoLock.  Prepared xact
     * dummy PGPROCs don't need these though - they're never associated
     * with a real process
     */
    if (i < MaxBackends + NUM_AUXILIARY_PROCS)
    {
      proc->sem = PGSemaphoreCreate();
      InitSharedLatch(&(proc->procLatch));
      LWLockInitialize(&(proc->fpInfoLock), LWTRANCHE_LOCK_FASTPATH);
    }

    /*
     * Newly created PGPROCs for normal backends, autovacuum workers,
     * special workers, bgworkers, and walsenders must be queued up on the
     * appropriate free list.  Because there can only ever be a small,
     * fixed number of auxiliary processes, no free list is used in that
     * case; InitAuxiliaryProcess() instead uses a linear search.  PGPROCs
     * for prepared transactions are added to a free list by
     * TwoPhaseShmemInit().
     */
    if (i < MaxConnections)
    {
      /* PGPROC for normal backend, add to freeProcs list */
      dlist_push_tail(&ProcGlobal->freeProcs, &proc->links);
      proc->procgloballist = &ProcGlobal->freeProcs;
    }
    else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS)
    {
      /* PGPROC for AV or special worker, add to autovacFreeProcs list */
      dlist_push_tail(&ProcGlobal->autovacFreeProcs, &proc->links);
      proc->procgloballist = &ProcGlobal->autovacFreeProcs;
    }
    else if (i < MaxConnections + autovacuum_worker_slots + NUM_SPECIAL_WORKER_PROCS + max_worker_processes)
    {
      /* PGPROC for bgworker, add to bgworkerFreeProcs list */
      dlist_push_tail(&ProcGlobal->bgworkerFreeProcs, &proc->links);
      proc->procgloballist = &ProcGlobal->bgworkerFreeProcs;
    }
    else if (i < MaxBackends)
    {
      /* PGPROC for walsender, add to walsenderFreeProcs list */
      dlist_push_tail(&ProcGlobal->walsenderFreeProcs, &proc->links);
      proc->procgloballist = &ProcGlobal->walsenderFreeProcs;
    }

    /* Initialize myProcLocks[] shared memory queues. */
    for (j = 0; j < NUM_LOCK_PARTITIONS; j++)
      dlist_init(&(proc->myProcLocks[j]));

    /* Initialize lockGroupMembers list. */
    dlist_init(&proc->lockGroupMembers);

    /*
     * Initialize the atomic variables, otherwise, it won't be safe to
     * access them for backends that aren't currently in use.
     */
    pg_atomic_init_u32(&(proc->procArrayGroupNext), INVALID_PROC_NUMBER);
    pg_atomic_init_u32(&(proc->clogGroupNext), INVALID_PROC_NUMBER);
    pg_atomic_init_u64(&(proc->waitStart), 0);
  }

  /* Should have consumed exactly the expected amount of fast-path memory. */
  Assert(fpPtr == fpEndPtr);

  /*
   * Save pointers to the blocks of PGPROC structures reserved for auxiliary
   * processes and prepared transactions.
   */
  AuxiliaryProcs = &procs[MaxBackends];
  PreparedXactProcs = &procs[MaxBackends + NUM_AUXILIARY_PROCS];

  /* Create ProcStructLock spinlock, too */
  ProcStructLock = (slock_t *) ShmemInitStruct("ProcStructLock spinlock",
                         sizeof(slock_t),
                         &found);
  SpinLockInit(ProcStructLock);
}

/*
 * InitProcess -- initialize a per-process PGPROC entry for this backend
 */
void
InitProcess(void)
{
  dlist_head *procgloballist;

  /*
   * ProcGlobal should be set up already (if we are a backend, we inherit
   * this by fork() or EXEC_BACKEND mechanism from the postmaster).
   */
  if (ProcGlobal == NULL)
    elog(PANIC, "proc header uninitialized");

  if (MyProc != NULL)
    elog(ERROR, "you already exist");

  /*
   * Before we start accessing the shared memory in a serious way, mark
   * ourselves as an active postmaster child; this is so that the postmaster
   * can detect it if we exit without cleaning up.
   */
  if (IsUnderPostmaster)
    RegisterPostmasterChildActive();

  /*
   * Decide which list should supply our PGPROC.  This logic must match the
   * way the freelists were constructed in InitProcGlobal().
   */
  if (AmAutoVacuumWorkerProcess() || AmSpecialWorkerProcess())
    procgloballist = &ProcGlobal->autovacFreeProcs;
  else if (AmBackgroundWorkerProcess())
    procgloballist = &ProcGlobal->bgworkerFreeProcs;
  else if (AmWalSenderProcess())
    procgloballist = &ProcGlobal->walsenderFreeProcs;
  else
    procgloballist = &ProcGlobal->freeProcs;

  /*
   * Try to get a proc struct from the appropriate free list.  If this
   * fails, we must be out of PGPROC structures (not to mention semaphores).
   *
   * While we are holding the ProcStructLock, also copy the current shared
   * estimate of spins_per_delay to local storage.
   */
  SpinLockAcquire(ProcStructLock);

  set_spins_per_delay(ProcGlobal->spins_per_delay);

  if (!dlist_is_empty(procgloballist))
  {
    MyProc = dlist_container(PGPROC, links, dlist_pop_head_node(procgloballist));
    SpinLockRelease(ProcStructLock);
  }
  else
  {
    /*
     * If we reach here, all the PGPROCs are in use.  This is one of the
     * possible places to detect "too many backends", so give the standard
     * error message.  XXX do we need to give a different failure message
     * in the autovacuum case?
     */
    SpinLockRelease(ProcStructLock);
    if (AmWalSenderProcess())
      ereport(FATAL,
          (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
           errmsg("number of requested standby connections exceeds \"max_wal_senders\" (currently %d)",
              max_wal_senders)));
    ereport(FATAL,
        (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
         errmsg("sorry, too many clients already")));
  }
  MyProcNumber = GetNumberFromPGProc(MyProc);

  /*
   * Cross-check that the PGPROC is of the type we expect; if this were not
   * the case, it would get returned to the wrong list.
   */
  Assert(MyProc->procgloballist == procgloballist);

  /*
   * Initialize all fields of MyProc, except for those previously
   * initialized by InitProcGlobal.
   */
  dlist_node_init(&MyProc->links);
  MyProc->waitStatus = PROC_WAIT_STATUS_OK;
  MyProc->fpVXIDLock = false;
  MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
  MyProc->xid = InvalidTransactionId;
  MyProc->xmin = InvalidTransactionId;
  MyProc->pid = MyProcPid;
  MyProc->vxid.procNumber = MyProcNumber;
  MyProc->vxid.lxid = InvalidLocalTransactionId;
  /* databaseId and roleId will be filled in later */
  MyProc->databaseId = InvalidOid;
  MyProc->roleId = InvalidOid;
  MyProc->tempNamespaceId = InvalidOid;
  MyProc->isRegularBackend = AmRegularBackendProcess();
  MyProc->delayChkptFlags = 0;
  MyProc->statusFlags = 0;
  /* NB -- autovac launcher intentionally does not set IS_AUTOVACUUM */
  if (AmAutoVacuumWorkerProcess())
    MyProc->statusFlags |= PROC_IS_AUTOVACUUM;
  MyProc->lwWaiting = LW_WS_NOT_WAITING;
  MyProc->lwWaitMode = 0;
  MyProc->waitLock = NULL;
  MyProc->waitProcLock = NULL;
  pg_atomic_write_u64(&MyProc->waitStart, 0);
#ifdef USE_ASSERT_CHECKING
  {
    int     i;

    /* Last process should have released all locks. */
    for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
      Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
  }
#endif
  MyProc->recoveryConflictPending = false;

  /* Initialize fields for sync rep */
  MyProc->waitLSN = 0;
  MyProc->syncRepState = SYNC_REP_NOT_WAITING;
  dlist_node_init(&MyProc->syncRepLinks);

  /* Initialize fields for group XID clearing. */
  MyProc->procArrayGroupMember = false;
  MyProc->procArrayGroupMemberXid = InvalidTransactionId;
  Assert(pg_atomic_read_u32(&MyProc->procArrayGroupNext) == INVALID_PROC_NUMBER);

  /* Check that group locking fields are in a proper initial state. */
  Assert(MyProc->lockGroupLeader == NULL);
  Assert(dlist_is_empty(&MyProc->lockGroupMembers));

  /* Initialize wait event information. */
  MyProc->wait_event_info = 0;

  /* Initialize fields for group transaction status update. */
  MyProc->clogGroupMember = false;
  MyProc->clogGroupMemberXid = InvalidTransactionId;
  MyProc->clogGroupMemberXidStatus = TRANSACTION_STATUS_IN_PROGRESS;
  MyProc->clogGroupMemberPage = -1;
  MyProc->clogGroupMemberLsn = InvalidXLogRecPtr;
  Assert(pg_atomic_read_u32(&MyProc->clogGroupNext) == INVALID_PROC_NUMBER);

  /*
   * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
   * on it.  That allows us to repoint the process latch, which so far
   * points to process local one, to the shared one.
   */
  OwnLatch(&MyProc->procLatch);
  SwitchToSharedLatch();

  /* now that we have a proc, report wait events to shared memory */
  pgstat_set_wait_event_storage(&MyProc->wait_event_info);

  /*
   * We might be reusing a semaphore that belonged to a failed process. So
   * be careful and reinitialize its value here.  (This is not strictly
   * necessary anymore, but seems like a good idea for cleanliness.)
   */
  PGSemaphoreReset(MyProc->sem);

  /*
   * Arrange to clean up at backend exit.
   */
  on_shmem_exit(ProcKill, 0);

  /*
   * Now that we have a PGPROC, we could try to acquire locks, so initialize
   * local state needed for LWLocks, and the deadlock checker.
   */
  InitLWLockAccess();
  InitDeadLockChecking();

#ifdef EXEC_BACKEND

  /*
   * Initialize backend-local pointers to all the shared data structures.
   * (We couldn't do this until now because it needs LWLocks.)
   */
  if (IsUnderPostmaster)
    AttachSharedMemoryStructs();
#endif
}

/*
 * InitProcessPhase2 -- make MyProc visible in the shared ProcArray.
 *
 * This is separate from InitProcess because we can't acquire LWLocks until
 * we've created a PGPROC, but in the EXEC_BACKEND case ProcArrayAdd won't
 * work until after we've done AttachSharedMemoryStructs.
 */
void
InitProcessPhase2(void)
{
  Assert(MyProc != NULL);

  /*
   * Add our PGPROC to the PGPROC array in shared memory.
   */
  ProcArrayAdd(MyProc);

  /*
   * Arrange to clean that up at backend exit.
   */
  on_shmem_exit(RemoveProcFromArray, 0);
}

/*
 * InitAuxiliaryProcess -- create a PGPROC entry for an auxiliary process
 *
 * This is called by bgwriter and similar processes so that they will have a
 * MyProc value that's real enough to let them wait for LWLocks.  The PGPROC
 * and sema that are assigned are one of the extra ones created during
 * InitProcGlobal.
 *
 * Auxiliary processes are presently not expected to wait for real (lockmgr)
 * locks, so we need not set up the deadlock checker.  They are never added
 * to the ProcArray or the sinval messaging mechanism, either.  They also
 * don't get a VXID assigned, since this is only useful when we actually
 * hold lockmgr locks.
 *
 * Startup process however uses locks but never waits for them in the
 * normal backend sense. Startup process also takes part in sinval messaging
 * as a sendOnly process, so never reads messages from sinval queue. So
 * Startup process does have a VXID and does show up in pg_locks.
 */
void
InitAuxiliaryProcess(void)
{
  PGPROC     *auxproc;
  int     proctype;

  /*
   * ProcGlobal should be set up already (if we are a backend, we inherit
   * this by fork() or EXEC_BACKEND mechanism from the postmaster).
   */
  if (ProcGlobal == NULL || AuxiliaryProcs == NULL)
    elog(PANIC, "proc header uninitialized");

  if (MyProc != NULL)
    elog(ERROR, "you already exist");

  if (IsUnderPostmaster)
    RegisterPostmasterChildActive();

  /*
   * We use the ProcStructLock to protect assignment and releasing of
   * AuxiliaryProcs entries.
   *
   * While we are holding the ProcStructLock, also copy the current shared
   * estimate of spins_per_delay to local storage.
   */
  SpinLockAcquire(ProcStructLock);

  set_spins_per_delay(ProcGlobal->spins_per_delay);

  /*
   * Find a free auxproc ... *big* trouble if there isn't one ...
   */
  for (proctype = 0; proctype < NUM_AUXILIARY_PROCS; proctype++)
  {
    auxproc = &AuxiliaryProcs[proctype];
    if (auxproc->pid == 0)
      break;
  }
  if (proctype >= NUM_AUXILIARY_PROCS)
  {
    SpinLockRelease(ProcStructLock);
    elog(FATAL, "all AuxiliaryProcs are in use");
  }

  /* Mark auxiliary proc as in use by me */
  /* use volatile pointer to prevent code rearrangement */
  ((volatile PGPROC *) auxproc)->pid = MyProcPid;

  SpinLockRelease(ProcStructLock);

  MyProc = auxproc;
  MyProcNumber = GetNumberFromPGProc(MyProc);

  /*
   * Initialize all fields of MyProc, except for those previously
   * initialized by InitProcGlobal.
   */
  dlist_node_init(&MyProc->links);
  MyProc->waitStatus = PROC_WAIT_STATUS_OK;
  MyProc->fpVXIDLock = false;
  MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
  MyProc->xid = InvalidTransactionId;
  MyProc->xmin = InvalidTransactionId;
  MyProc->vxid.procNumber = INVALID_PROC_NUMBER;
  MyProc->vxid.lxid = InvalidLocalTransactionId;
  MyProc->databaseId = InvalidOid;
  MyProc->roleId = InvalidOid;
  MyProc->tempNamespaceId = InvalidOid;
  MyProc->isRegularBackend = false;
  MyProc->delayChkptFlags = 0;
  MyProc->statusFlags = 0;
  MyProc->lwWaiting = LW_WS_NOT_WAITING;
  MyProc->lwWaitMode = 0;
  MyProc->waitLock = NULL;
  MyProc->waitProcLock = NULL;
  pg_atomic_write_u64(&MyProc->waitStart, 0);
#ifdef USE_ASSERT_CHECKING
  {
    int     i;

    /* Last process should have released all locks. */
    for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
      Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
  }
#endif

  /*
   * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch
   * on it.  That allows us to repoint the process latch, which so far
   * points to process local one, to the shared one.
   */
  OwnLatch(&MyProc->procLatch);
  SwitchToSharedLatch();

  /* now that we have a proc, report wait events to shared memory */
  pgstat_set_wait_event_storage(&MyProc->wait_event_info);

  /* Check that group locking fields are in a proper initial state. */
  Assert(MyProc->lockGroupLeader == NULL);
  Assert(dlist_is_empty(&MyProc->lockGroupMembers));

  /*
   * We might be reusing a semaphore that belonged to a failed process. So
   * be careful and reinitialize its value here.  (This is not strictly
   * necessary anymore, but seems like a good idea for cleanliness.)
   */
  PGSemaphoreReset(MyProc->sem);

  /*
   * Arrange to clean up at process exit.
   */
  on_shmem_exit(AuxiliaryProcKill, Int32GetDatum(proctype));

  /*
   * Now that we have a PGPROC, we could try to acquire lightweight locks.
   * Initialize local state needed for them.  (Heavyweight locks cannot be
   * acquired in aux processes.)
   */
  InitLWLockAccess();

#ifdef EXEC_BACKEND

  /*
   * Initialize backend-local pointers to all the shared data structures.
   * (We couldn't do this until now because it needs LWLocks.)
   */
  if (IsUnderPostmaster)
    AttachSharedMemoryStructs();
#endif
}

/*
 * Used from bufmgr to share the value of the buffer that Startup waits on,
 * or to reset the value to "not waiting" (-1). This allows processing
 * of recovery conflicts for buffer pins. Set is made before backends look
 * at this value, so locking not required, especially since the set is
 * an atomic integer set operation.
 */
void
SetStartupBufferPinWaitBufId(int bufid)
{
  /* use volatile pointer to prevent code rearrangement */
  volatile PROC_HDR *procglobal = ProcGlobal;

  procglobal->startupBufferPinWaitBufId = bufid;
}

/*
 * Used by backends when they receive a request to check for buffer pin waits.
 */
int
GetStartupBufferPinWaitBufId(void)
{
  /* use volatile pointer to prevent code rearrangement */
  volatile PROC_HDR *procglobal = ProcGlobal;

  return procglobal->startupBufferPinWaitBufId;
}

/*
 * Check whether there are at least N free PGPROC objects.  If false is
 * returned, *nfree will be set to the number of free PGPROC objects.
 * Otherwise, *nfree will be set to n.
 *
 * Note: this is designed on the assumption that N will generally be small.
 */
bool
HaveNFreeProcs(int n, int *nfree)
{
  dlist_iter  iter;

  Assert(n > 0);
  Assert(nfree);

  SpinLockAcquire(ProcStructLock);

  *nfree = 0;
  dlist_foreach(iter, &ProcGlobal->freeProcs)
  {
    (*nfree)++;
    if (*nfree == n)
      break;
  }

  SpinLockRelease(ProcStructLock);

  return (*nfree == n);
}

/*
 * Cancel any pending wait for lock, when aborting a transaction, and revert
 * any strong lock count acquisition for a lock being acquired.
 *
 * (Normally, this would only happen if we accept a cancel/die
 * interrupt while waiting; but an ereport(ERROR) before or during the lock
 * wait is within the realm of possibility, too.)
 */
void
LockErrorCleanup(void)
{
  LOCALLOCK  *lockAwaited;
  LWLock     *partitionLock;
  DisableTimeoutParams timeouts[2];

  HOLD_INTERRUPTS();

  AbortStrongLockAcquire();

  /* Nothing to do if we weren't waiting for a lock */
  lockAwaited = GetAwaitedLock();
  if (lockAwaited == NULL)
  {
    RESUME_INTERRUPTS();
    return;
  }

  /*
   * Turn off the deadlock and lock timeout timers, if they are still
   * running (see ProcSleep).  Note we must preserve the LOCK_TIMEOUT
   * indicator flag, since this function is executed before
   * ProcessInterrupts when responding to SIGINT; else we'd lose the
   * knowledge that the SIGINT came from a lock timeout and not an external
   * source.
   */
  timeouts[0].id = DEADLOCK_TIMEOUT;
  timeouts[0].keep_indicator = false;
  timeouts[1].id = LOCK_TIMEOUT;
  timeouts[1].keep_indicator = true;
  disable_timeouts(timeouts, 2);

  /* Unlink myself from the wait queue, if on it (might not be anymore!) */
  partitionLock = LockHashPartitionLock(lockAwaited->hashcode);
  LWLockAcquire(partitionLock, LW_EXCLUSIVE);

  if (!dlist_node_is_detached(&MyProc->links))
  {
    /* We could not have been granted the lock yet */
    RemoveFromWaitQueue(MyProc, lockAwaited->hashcode);
  }
  else
  {
    /*
     * Somebody kicked us off the lock queue already.  Perhaps they
     * granted us the lock, or perhaps they detected a deadlock. If they
     * did grant us the lock, we'd better remember it in our local lock
     * table.
     */
    if (MyProc->waitStatus == PROC_WAIT_STATUS_OK)
      GrantAwaitedLock();
  }

  ResetAwaitedLock();

  LWLockRelease(partitionLock);

  RESUME_INTERRUPTS();
}


/*
 * ProcReleaseLocks() -- release locks associated with current transaction
 *      at main transaction commit or abort
 *
 * At main transaction commit, we release standard locks except session locks.
 * At main transaction abort, we release all locks including session locks.
 *
 * Advisory locks are released only if they are transaction-level;
 * session-level holds remain, whether this is a commit or not.
 *
 * At subtransaction commit, we don't release any locks (so this func is not
 * needed at all); we will defer the releasing to the parent transaction.
 * At subtransaction abort, we release all locks held by the subtransaction;
 * this is implemented by retail releasing of the locks under control of
 * the ResourceOwner mechanism.
 */
void
ProcReleaseLocks(bool isCommit)
{
  if (!MyProc)
    return;
  /* If waiting, get off wait queue (should only be needed after error) */
  LockErrorCleanup();
  /* Release standard locks, including session-level if aborting */
  LockReleaseAll(DEFAULT_LOCKMETHOD, !isCommit);
  /* Release transaction-level advisory locks */
  LockReleaseAll(USER_LOCKMETHOD, false);
}


/*
 * RemoveProcFromArray() -- Remove this process from the shared ProcArray.
 */
static void
RemoveProcFromArray(int code, Datum arg)
{
  Assert(MyProc != NULL);
  ProcArrayRemove(MyProc, InvalidTransactionId);
}

/*
 * ProcKill() -- Destroy the per-proc data structure for
 *    this process. Release any of its held LW locks.
 */
static void
ProcKill(int code, Datum arg)
{
  PGPROC     *proc;
  dlist_head *procgloballist;

  Assert(MyProc != NULL);

  /* not safe if forked by system(), etc. */
  if (MyProc->pid != (int) getpid())
    elog(PANIC, "ProcKill() called in child process");

  /* Make sure we're out of the sync rep lists */
  SyncRepCleanupAtProcExit();

#ifdef USE_ASSERT_CHECKING
  {
    int     i;

    /* Last process should have released all locks. */
    for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
      Assert(dlist_is_empty(&(MyProc->myProcLocks[i])));
  }
#endif

  /*
   * Release any LW locks I am holding.  There really shouldn't be any, but
   * it's cheap to check again before we cut the knees off the LWLock
   * facility by releasing our PGPROC ...
   */
  LWLockReleaseAll();

  /* Cancel any pending condition variable sleep, too */
  ConditionVariableCancelSleep();

  /*
   * Detach from any lock group of which we are a member.  If the leader
   * exits before all other group members, its PGPROC will remain allocated
   * until the last group process exits; that process must return the
   * leader's PGPROC to the appropriate list.
   */
  if (MyProc->lockGroupLeader != NULL)
  {
    PGPROC     *leader = MyProc->lockGroupLeader;
    LWLock     *leader_lwlock = LockHashPartitionLockByProc(leader);

    LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
    Assert(!dlist_is_empty(&leader->lockGroupMembers));
    dlist_delete(&MyProc->lockGroupLink);
    if (dlist_is_empty(&leader->lockGroupMembers))
    {
      leader->lockGroupLeader = NULL;
      if (leader != MyProc)
      {
        procgloballist = leader->procgloballist;

        /* Leader exited first; return its PGPROC. */
        SpinLockAcquire(ProcStructLock);
        dlist_push_head(procgloballist, &leader->links);
        SpinLockRelease(ProcStructLock);
      }
    }
    else if (leader != MyProc)
      MyProc->lockGroupLeader = NULL;
    LWLockRelease(leader_lwlock);
  }

  /*
   * Reset MyLatch to the process local one.  This is so that signal
   * handlers et al can continue using the latch after the shared latch
   * isn't ours anymore.
   *
   * Similarly, stop reporting wait events to MyProc->wait_event_info.
   *
   * After that clear MyProc and disown the shared latch.
   */
  SwitchBackToLocalLatch();
  pgstat_reset_wait_event_storage();

  proc = MyProc;
  MyProc = NULL;
  MyProcNumber = INVALID_PROC_NUMBER;
  DisownLatch(&proc->procLatch);

  /* Mark the proc no longer in use */
  proc->pid = 0;
  proc->vxid.procNumber = INVALID_PROC_NUMBER;
  proc->vxid.lxid = InvalidTransactionId;

  procgloballist = proc->procgloballist;
  SpinLockAcquire(ProcStructLock);

  /*
   * If we're still a member of a locking group, that means we're a leader
   * which has somehow exited before its children.  The last remaining child
   * will release our PGPROC.  Otherwise, release it now.
   */
  if (proc->lockGroupLeader == NULL)
  {
    /* Since lockGroupLeader is NULL, lockGroupMembers should be empty. */
    Assert(dlist_is_empty(&proc->lockGroupMembers));

    /* Return PGPROC structure (and semaphore) to appropriate freelist */
    dlist_push_tail(procgloballist, &proc->links);
  }

  /* Update shared estimate of spins_per_delay */
  ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);

  SpinLockRelease(ProcStructLock);

  /* wake autovac launcher if needed -- see comments in FreeWorkerInfo */
  if (AutovacuumLauncherPid != 0)
    kill(AutovacuumLauncherPid, SIGUSR2);
}

/*
 * AuxiliaryProcKill() -- Cut-down version of ProcKill for auxiliary
 *    processes (bgwriter, etc).  The PGPROC and sema are not released, only
 *    marked as not-in-use.
 */
static void
AuxiliaryProcKill(int code, Datum arg)
{
  int     proctype = DatumGetInt32(arg);
  PGPROC     *auxproc PG_USED_FOR_ASSERTS_ONLY;
  PGPROC     *proc;

  Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);

  /* not safe if forked by system(), etc. */
  if (MyProc->pid != (int) getpid())
    elog(PANIC, "AuxiliaryProcKill() called in child process");

  auxproc = &AuxiliaryProcs[proctype];

  Assert(MyProc == auxproc);

  /* Release any LW locks I am holding (see notes above) */
  LWLockReleaseAll();

  /* Cancel any pending condition variable sleep, too */
  ConditionVariableCancelSleep();

  /* look at the equivalent ProcKill() code for comments */
  SwitchBackToLocalLatch();
  pgstat_reset_wait_event_storage();

  proc = MyProc;
  MyProc = NULL;
  MyProcNumber = INVALID_PROC_NUMBER;
  DisownLatch(&proc->procLatch);

  SpinLockAcquire(ProcStructLock);

  /* Mark auxiliary proc no longer in use */
  proc->pid = 0;
  proc->vxid.procNumber = INVALID_PROC_NUMBER;
  proc->vxid.lxid = InvalidTransactionId;

  /* Update shared estimate of spins_per_delay */
  ProcGlobal->spins_per_delay = update_spins_per_delay(ProcGlobal->spins_per_delay);

  SpinLockRelease(ProcStructLock);
}

/*
 * AuxiliaryPidGetProc -- get PGPROC for an auxiliary process
 * given its PID
 *
 * Returns NULL if not found.
 */
PGPROC *
AuxiliaryPidGetProc(int pid)
{
  PGPROC     *result = NULL;
  int     index;

  if (pid == 0)       /* never match dummy PGPROCs */
    return NULL;

  for (index = 0; index < NUM_AUXILIARY_PROCS; index++)
  {
    PGPROC     *proc = &AuxiliaryProcs[index];

    if (proc->pid == pid)
    {
      result = proc;
      break;
    }
  }
  return result;
}


/*
 * JoinWaitQueue -- join the wait queue on the specified lock
 *
 * It's not actually guaranteed that we need to wait when this function is
 * called, because it could be that when we try to find a position at which
 * to insert ourself into the wait queue, we discover that we must be inserted
 * ahead of everyone who wants a lock that conflict with ours. In that case,
 * we get the lock immediately. Because of this, it's sensible for this function
 * to have a dontWait argument, despite the name.
 *
 * On entry, the caller has already set up LOCK and PROCLOCK entries to
 * reflect that we have "requested" the lock.  The caller is responsible for
 * cleaning that up, if we end up not joining the queue after all.
 *
 * The lock table's partition lock must be held at entry, and is still held
 * at exit.  The caller must release it before calling ProcSleep().
 *
 * Result is one of the following:
 *
 *  PROC_WAIT_STATUS_OK       - lock was immediately granted
 *  PROC_WAIT_STATUS_WAITING  - joined the wait queue; call ProcSleep()
 *  PROC_WAIT_STATUS_ERROR    - immediate deadlock was detected, or would
 *                              need to wait and dontWait == true
 *
 * NOTES: The process queue is now a priority queue for locking.
 */
ProcWaitStatus
JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
{
  LOCKMODE  lockmode = locallock->tag.mode;
  LOCK     *lock = locallock->lock;
  PROCLOCK   *proclock = locallock->proclock;
  uint32    hashcode = locallock->hashcode;
  LWLock     *partitionLock PG_USED_FOR_ASSERTS_ONLY = LockHashPartitionLock(hashcode);
  dclist_head *waitQueue = &lock->waitProcs;
  PGPROC     *insert_before = NULL;
  LOCKMASK  myProcHeldLocks;
  LOCKMASK  myHeldLocks;
  bool    early_deadlock = false;
  PGPROC     *leader = MyProc->lockGroupLeader;

  Assert(LWLockHeldByMeInMode(partitionLock, LW_EXCLUSIVE));

  /*
   * Set bitmask of locks this process already holds on this object.
   */
  myHeldLocks = MyProc->heldLocks = proclock->holdMask;

  /*
   * Determine which locks we're already holding.
   *
   * If group locking is in use, locks held by members of my locking group
   * need to be included in myHeldLocks.  This is not required for relation
   * extension lock which conflict among group members. However, including
   * them in myHeldLocks will give group members the priority to get those
   * locks as compared to other backends which are also trying to acquire
   * those locks.  OTOH, we can avoid giving priority to group members for
   * that kind of locks, but there doesn't appear to be a clear advantage of
   * the same.
   */
  myProcHeldLocks = proclock->holdMask;
  myHeldLocks = myProcHeldLocks;
  if (leader != NULL)
  {
    dlist_iter  iter;

    dlist_foreach(iter, &lock->procLocks)
    {
      PROCLOCK   *otherproclock;

      otherproclock = dlist_container(PROCLOCK, lockLink, iter.cur);

      if (otherproclock->groupLeader == leader)
        myHeldLocks |= otherproclock->holdMask;
    }
  }

  /*
   * Determine where to add myself in the wait queue.
   *
   * Normally I should go at the end of the queue.  However, if I already
   * hold locks that conflict with the request of any previous waiter, put
   * myself in the queue just in front of the first such waiter. This is not
   * a necessary step, since deadlock detection would move me to before that
   * waiter anyway; but it's relatively cheap to detect such a conflict
   * immediately, and avoid delaying till deadlock timeout.
   *
   * Special case: if I find I should go in front of some waiter, check to
   * see if I conflict with already-held locks or the requests before that
   * waiter.  If not, then just grant myself the requested lock immediately.
   * This is the same as the test for immediate grant in LockAcquire, except
   * we are only considering the part of the wait queue before my insertion
   * point.
   */
  if (myHeldLocks != 0 && !dclist_is_empty(waitQueue))
  {
    LOCKMASK  aheadRequests = 0;
    dlist_iter  iter;

    dclist_foreach(iter, waitQueue)
    {
      PGPROC     *proc = dlist_container(PGPROC, links, iter.cur);

      /*
       * If we're part of the same locking group as this waiter, its
       * locks neither conflict with ours nor contribute to
       * aheadRequests.
       */
      if (leader != NULL && leader == proc->lockGroupLeader)
        continue;

      /* Must he wait for me? */
      if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
      {
        /* Must I wait for him ? */
        if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
        {
          /*
           * Yes, so we have a deadlock.  Easiest way to clean up
           * correctly is to call RemoveFromWaitQueue(), but we
           * can't do that until we are *on* the wait queue. So, set
           * a flag to check below, and break out of loop.  Also,
           * record deadlock info for later message.
           */
          RememberSimpleDeadLock(MyProc, lockmode, lock, proc);
          early_deadlock = true;
          break;
        }
        /* I must go before this waiter.  Check special case. */
        if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
          !LockCheckConflicts(lockMethodTable, lockmode, lock,
                    proclock))
        {
          /* Skip the wait and just grant myself the lock. */
          GrantLock(lock, proclock, lockmode);
          return PROC_WAIT_STATUS_OK;
        }

        /* Put myself into wait queue before conflicting process */
        insert_before = proc;
        break;
      }
      /* Nope, so advance to next waiter */
      aheadRequests |= LOCKBIT_ON(proc->waitLockMode);
    }
  }

  /*
   * If we detected deadlock, give up without waiting.  This must agree with
   * CheckDeadLock's recovery code.
   */
  if (early_deadlock)
    return PROC_WAIT_STATUS_ERROR;

  /*
   * At this point we know that we'd really need to sleep. If we've been
   * commanded not to do that, bail out.
   */
  if (dontWait)
    return PROC_WAIT_STATUS_ERROR;

  /*
   * Insert self into queue, at the position determined above.
   */
  if (insert_before)
    dclist_insert_before(waitQueue, &insert_before->links, &MyProc->links);
  else
    dclist_push_tail(waitQueue, &MyProc->links);

  lock->waitMask |= LOCKBIT_ON(lockmode);

  /* Set up wait information in PGPROC object, too */
  MyProc->heldLocks = myProcHeldLocks;
  MyProc->waitLock = lock;
  MyProc->waitProcLock = proclock;
  MyProc->waitLockMode = lockmode;

  MyProc->waitStatus = PROC_WAIT_STATUS_WAITING;

  return PROC_WAIT_STATUS_WAITING;
}

/*
 * ProcSleep -- put process to sleep waiting on lock
 *
 * This must be called when JoinWaitQueue() returns PROC_WAIT_STATUS_WAITING.
 * Returns after the lock has been granted, or if a deadlock is detected.  Can
 * also bail out with ereport(ERROR), if some other error condition, or a
 * timeout or cancellation is triggered.
 *
 * Result is one of the following:
 *
 *  PROC_WAIT_STATUS_OK      - lock was granted
 *  PROC_WAIT_STATUS_ERROR   - a deadlock was detected
 */
ProcWaitStatus
ProcSleep(LOCALLOCK *locallock)
{
  LOCKMODE  lockmode = locallock->tag.mode;
  LOCK     *lock = locallock->lock;
  uint32    hashcode = locallock->hashcode;
  LWLock     *partitionLock = LockHashPartitionLock(hashcode);
  TimestampTz standbyWaitStart = 0;
  bool    allow_autovacuum_cancel = true;
  bool    logged_recovery_conflict = false;
  ProcWaitStatus myWaitStatus;

  /* The caller must've armed the on-error cleanup mechanism */
  Assert(GetAwaitedLock() == locallock);
  Assert(!LWLockHeldByMe(partitionLock));

  /*
   * Now that we will successfully clean up after an ereport, it's safe to
   * check to see if there's a buffer pin deadlock against the Startup
   * process.  Of course, that's only necessary if we're doing Hot Standby
   * and are not the Startup process ourselves.
   */
  if (RecoveryInProgress() && !InRecovery)
    CheckRecoveryConflictDeadlock();

  /* Reset deadlock_state before enabling the timeout handler */
  deadlock_state = DS_NOT_YET_CHECKED;
  got_deadlock_timeout = false;

  /*
   * Set timer so we can wake up after awhile and check for a deadlock. If a
   * deadlock is detected, the handler sets MyProc->waitStatus =
   * PROC_WAIT_STATUS_ERROR, allowing us to know that we must report failure
   * rather than success.
   *
   * By delaying the check until we've waited for a bit, we can avoid
   * running the rather expensive deadlock-check code in most cases.
   *
   * If LockTimeout is set, also enable the timeout for that.  We can save a
   * few cycles by enabling both timeout sources in one call.
   *
   * If InHotStandby we set lock waits slightly later for clarity with other
   * code.
   */
  if (!InHotStandby)
  {
    if (LockTimeout > 0)
    {
      EnableTimeoutParams timeouts[2];

      timeouts[0].id = DEADLOCK_TIMEOUT;
      timeouts[0].type = TMPARAM_AFTER;
      timeouts[0].delay_ms = DeadlockTimeout;
      timeouts[1].id = LOCK_TIMEOUT;
      timeouts[1].type = TMPARAM_AFTER;
      timeouts[1].delay_ms = LockTimeout;
      enable_timeouts(timeouts, 2);
    }
    else
      enable_timeout_after(DEADLOCK_TIMEOUT, DeadlockTimeout);

    /*
     * Use the current time obtained for the deadlock timeout timer as
     * waitStart (i.e., the time when this process started waiting for the
     * lock). Since getting the current time newly can cause overhead, we
     * reuse the already-obtained time to avoid that overhead.
     *
     * Note that waitStart is updated without holding the lock table's
     * partition lock, to avoid the overhead by additional lock
     * acquisition. This can cause "waitstart" in pg_locks to become NULL
     * for a very short period of time after the wait started even though
     * "granted" is false. This is OK in practice because we can assume
     * that users are likely to look at "waitstart" when waiting for the
     * lock for a long time.
     */
    pg_atomic_write_u64(&MyProc->waitStart,
              get_timeout_start_time(DEADLOCK_TIMEOUT));
  }
  else if (log_recovery_conflict_waits)
  {
    /*
     * Set the wait start timestamp if logging is enabled and in hot
     * standby.
     */
    standbyWaitStart = GetCurrentTimestamp();
  }

  /*
   * If somebody wakes us between LWLockRelease and WaitLatch, the latch
   * will not wait. But a set latch does not necessarily mean that the lock
   * is free now, as there are many other sources for latch sets than
   * somebody releasing the lock.
   *
   * We process interrupts whenever the latch has been set, so cancel/die
   * interrupts are processed quickly. This means we must not mind losing
   * control to a cancel/die interrupt here.  We don't, because we have no
   * shared-state-change work to do after being granted the lock (the
   * grantor did it all).  We do have to worry about canceling the deadlock
   * timeout and updating the locallock table, but if we lose control to an
   * error, LockErrorCleanup will fix that up.
   */
  do
  {
    if (InHotStandby)
    {
      bool    maybe_log_conflict =
        (standbyWaitStart != 0 && !logged_recovery_conflict);

      /* Set a timer and wait for that or for the lock to be granted */
      ResolveRecoveryConflictWithLock(locallock->tag.lock,
                      maybe_log_conflict);

      /*
       * Emit the log message if the startup process is waiting longer
       * than deadlock_timeout for recovery conflict on lock.
       */
      if (maybe_log_conflict)
      {
        TimestampTz now = GetCurrentTimestamp();

        if (TimestampDifferenceExceeds(standbyWaitStart, now,
                         DeadlockTimeout))
        {
          VirtualTransactionId *vxids;
          int     cnt;

          vxids = GetLockConflicts(&locallock->tag.lock,
                       AccessExclusiveLock, &cnt);

          /*
           * Log the recovery conflict and the list of PIDs of
           * backends holding the conflicting lock. Note that we do
           * logging even if there are no such backends right now
           * because the startup process here has already waited
           * longer than deadlock_timeout.
           */
          LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_LOCK,
                    standbyWaitStart, now,
                    cnt > 0 ? vxids : NULL, true);
          logged_recovery_conflict = true;
        }
      }
    }
    else
    {
      (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
               PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
      ResetLatch(MyLatch);
      /* check for deadlocks first, as that's probably log-worthy */
      if (got_deadlock_timeout)
      {
        CheckDeadLock();
        got_deadlock_timeout = false;
      }
      CHECK_FOR_INTERRUPTS();
    }

    /*
     * waitStatus could change from PROC_WAIT_STATUS_WAITING to something
     * else asynchronously.  Read it just once per loop to prevent
     * surprising behavior (such as missing log messages).
     */
    myWaitStatus = *((volatile ProcWaitStatus *) &MyProc->waitStatus);

    /*
     * If we are not deadlocked, but are waiting on an autovacuum-induced
     * task, send a signal to interrupt it.
     */
    if (deadlock_state == DS_BLOCKED_BY_AUTOVACUUM && allow_autovacuum_cancel)
    {
      PGPROC     *autovac = GetBlockingAutoVacuumPgproc();
      uint8   statusFlags;
      uint8   lockmethod_copy;
      LOCKTAG   locktag_copy;

      /*
       * Grab info we need, then release lock immediately.  Note this
       * coding means that there is a tiny chance that the process
       * terminates its current transaction and starts a different one
       * before we have a change to send the signal; the worst possible
       * consequence is that a for-wraparound vacuum is canceled.  But
       * that could happen in any case unless we were to do kill() with
       * the lock held, which is much more undesirable.
       */
      LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
      statusFlags = ProcGlobal->statusFlags[autovac->pgxactoff];
      lockmethod_copy = lock->tag.locktag_lockmethodid;
      locktag_copy = lock->tag;
      LWLockRelease(ProcArrayLock);

      /*
       * Only do it if the worker is not working to protect against Xid
       * wraparound.
       */
      if ((statusFlags & PROC_IS_AUTOVACUUM) &&
        !(statusFlags & PROC_VACUUM_FOR_WRAPAROUND))
      {
        int     pid = autovac->pid;

        /* report the case, if configured to do so */
        if (message_level_is_interesting(DEBUG1))
        {
          StringInfoData locktagbuf;
          StringInfoData logbuf;  /* errdetail for server log */

          initStringInfo(&locktagbuf);
          initStringInfo(&logbuf);
          DescribeLockTag(&locktagbuf, &locktag_copy);
          appendStringInfo(&logbuf,
                   "Process %d waits for %s on %s.",
                   MyProcPid,
                   GetLockmodeName(lockmethod_copy, lockmode),
                   locktagbuf.data);

          ereport(DEBUG1,
              (errmsg_internal("sending cancel to blocking autovacuum PID %d",
                       pid),
               errdetail_log("%s", logbuf.data)));

          pfree(locktagbuf.data);
          pfree(logbuf.data);
        }

        /* send the autovacuum worker Back to Old Kent Road */
        if (kill(pid, SIGINT) < 0)
        {
          /*
           * There's a race condition here: once we release the
           * ProcArrayLock, it's possible for the autovac worker to
           * close up shop and exit before we can do the kill().
           * Therefore, we do not whinge about no-such-process.
           * Other errors such as EPERM could conceivably happen if
           * the kernel recycles the PID fast enough, but such cases
           * seem improbable enough that it's probably best to issue
           * a warning if we see some other errno.
           */
          if (errno != ESRCH)
            ereport(WARNING,
                (errmsg("could not send signal to process %d: %m",
                    pid)));
        }
      }

      /* prevent signal from being sent again more than once */
      allow_autovacuum_cancel = false;
    }

    /*
     * If awoken after the deadlock check interrupt has run, and
     * log_lock_waits is on, then report about the wait.
     */
    if (log_lock_waits && deadlock_state != DS_NOT_YET_CHECKED)
    {
      StringInfoData buf,
            lock_waiters_sbuf,
            lock_holders_sbuf;
      const char *modename;
      long    secs;
      int     usecs;
      long    msecs;
      int     lockHoldersNum = 0;

      initStringInfo(&buf);
      initStringInfo(&lock_waiters_sbuf);
      initStringInfo(&lock_holders_sbuf);

      DescribeLockTag(&buf, &locallock->tag.lock);
      modename = GetLockmodeName(locallock->tag.lock.locktag_lockmethodid,
                     lockmode);
      TimestampDifference(get_timeout_start_time(DEADLOCK_TIMEOUT),
                GetCurrentTimestamp(),
                &secs, &usecs);
      msecs = secs * 1000 + usecs / 1000;
      usecs = usecs % 1000;

      /* Gather a list of all lock holders and waiters */
      LWLockAcquire(partitionLock, LW_SHARED);
      GetLockHoldersAndWaiters(locallock, &lock_holders_sbuf,
                   &lock_waiters_sbuf, &lockHoldersNum);
      LWLockRelease(partitionLock);

      if (deadlock_state == DS_SOFT_DEADLOCK)
        ereport(LOG,
            (errmsg("process %d avoided deadlock for %s on %s by rearranging queue order after %ld.%03d ms",
                MyProcPid, modename, buf.data, msecs, usecs),
             (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                         "Processes holding the lock: %s. Wait queue: %s.",
                         lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
      else if (deadlock_state == DS_HARD_DEADLOCK)
      {
        /*
         * This message is a bit redundant with the error that will be
         * reported subsequently, but in some cases the error report
         * might not make it to the log (eg, if it's caught by an
         * exception handler), and we want to ensure all long-wait
         * events get logged.
         */
        ereport(LOG,
            (errmsg("process %d detected deadlock while waiting for %s on %s after %ld.%03d ms",
                MyProcPid, modename, buf.data, msecs, usecs),
             (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                         "Processes holding the lock: %s. Wait queue: %s.",
                         lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
      }

      if (myWaitStatus == PROC_WAIT_STATUS_WAITING)
        ereport(LOG,
            (errmsg("process %d still waiting for %s on %s after %ld.%03d ms",
                MyProcPid, modename, buf.data, msecs, usecs),
             (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                         "Processes holding the lock: %s. Wait queue: %s.",
                         lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
      else if (myWaitStatus == PROC_WAIT_STATUS_OK)
        ereport(LOG,
            (errmsg("process %d acquired %s on %s after %ld.%03d ms",
                MyProcPid, modename, buf.data, msecs, usecs)));
      else
      {
        Assert(myWaitStatus == PROC_WAIT_STATUS_ERROR);

        /*
         * Currently, the deadlock checker always kicks its own
         * process, which means that we'll only see
         * PROC_WAIT_STATUS_ERROR when deadlock_state ==
         * DS_HARD_DEADLOCK, and there's no need to print redundant
         * messages.  But for completeness and future-proofing, print
         * a message if it looks like someone else kicked us off the
         * lock.
         */
        if (deadlock_state != DS_HARD_DEADLOCK)
          ereport(LOG,
              (errmsg("process %d failed to acquire %s on %s after %ld.%03d ms",
                  MyProcPid, modename, buf.data, msecs, usecs),
               (errdetail_log_plural("Process holding the lock: %s. Wait queue: %s.",
                           "Processes holding the lock: %s. Wait queue: %s.",
                           lockHoldersNum, lock_holders_sbuf.data, lock_waiters_sbuf.data))));
      }

      /*
       * At this point we might still need to wait for the lock. Reset
       * state so we don't print the above messages again.
       */
      deadlock_state = DS_NO_DEADLOCK;

      pfree(buf.data);
      pfree(lock_holders_sbuf.data);
      pfree(lock_waiters_sbuf.data);
    }
  } while (myWaitStatus == PROC_WAIT_STATUS_WAITING);

  /*
   * Disable the timers, if they are still running.  As in LockErrorCleanup,
   * we must preserve the LOCK_TIMEOUT indicator flag: if a lock timeout has
   * already caused QueryCancelPending to become set, we want the cancel to
   * be reported as a lock timeout, not a user cancel.
   */
  if (!InHotStandby)
  {
    if (LockTimeout > 0)
    {
      DisableTimeoutParams timeouts[2];

      timeouts[0].id = DEADLOCK_TIMEOUT;
      timeouts[0].keep_indicator = false;
      timeouts[1].id = LOCK_TIMEOUT;
      timeouts[1].keep_indicator = true;
      disable_timeouts(timeouts, 2);
    }
    else
      disable_timeout(DEADLOCK_TIMEOUT, false);
  }

  /*
   * Emit the log message if recovery conflict on lock was resolved but the
   * startup process waited longer than deadlock_timeout for it.
   */
  if (InHotStandby && logged_recovery_conflict)
    LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_LOCK,
              standbyWaitStart, GetCurrentTimestamp(),
              NULL, false);

  /*
   * We don't have to do anything else, because the awaker did all the
   * necessary updates of the lock table and MyProc. (The caller is
   * responsible for updating the local lock table.)
   */
  return myWaitStatus;
}


/*
 * ProcWakeup -- wake up a process by setting its latch.
 *
 *   Also remove the process from the wait queue and set its links invalid.
 *
 * The appropriate lock partition lock must be held by caller.
 *
 * XXX: presently, this code is only used for the "success" case, and only
 * works correctly for that case.  To clean up in failure case, would need
 * to twiddle the lock's request counts too --- see RemoveFromWaitQueue.
 * Hence, in practice the waitStatus parameter must be PROC_WAIT_STATUS_OK.
 */
void
ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus)
{
  if (dlist_node_is_detached(&proc->links))
    return;

  Assert(proc->waitStatus == PROC_WAIT_STATUS_WAITING);

  /* Remove process from wait queue */
  dclist_delete_from_thoroughly(&proc->waitLock->waitProcs, &proc->links);

  /* Clean up process' state and pass it the ok/fail signal */
  proc->waitLock = NULL;
  proc->waitProcLock = NULL;
  proc->waitStatus = waitStatus;
  pg_atomic_write_u64(&MyProc->waitStart, 0);

  /* And awaken it */
  SetLatch(&proc->procLatch);
}

/*
 * ProcLockWakeup -- routine for waking up processes when a lock is
 *    released (or a prior waiter is aborted).  Scan all waiters
 *    for lock, waken any that are no longer blocked.
 *
 * The appropriate lock partition lock must be held by caller.
 */
void
ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
{
  dclist_head *waitQueue = &lock->waitProcs;
  LOCKMASK  aheadRequests = 0;
  dlist_mutable_iter miter;

  if (dclist_is_empty(waitQueue))
    return;

  dclist_foreach_modify(miter, waitQueue)
  {
    PGPROC     *proc = dlist_container(PGPROC, links, miter.cur);
    LOCKMODE  lockmode = proc->waitLockMode;

    /*
     * Waken if (a) doesn't conflict with requests of earlier waiters, and
     * (b) doesn't conflict with already-held locks.
     */
    if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
      !LockCheckConflicts(lockMethodTable, lockmode, lock,
                proc->waitProcLock))
    {
      /* OK to waken */
      GrantLock(lock, proc->waitProcLock, lockmode);
      /* removes proc from the lock's waiting process queue */
      ProcWakeup(proc, PROC_WAIT_STATUS_OK);
    }
    else
    {
      /*
       * Lock conflicts: Don't wake, but remember requested mode for
       * later checks.
       */
      aheadRequests |= LOCKBIT_ON(lockmode);
    }
  }
}

/*
 * CheckDeadLock
 *
 * We only get to this routine, if DEADLOCK_TIMEOUT fired while waiting for a
 * lock to be released by some other process.  Check if there's a deadlock; if
 * not, just return.  (But signal ProcSleep to log a message, if
 * log_lock_waits is true.)  If we have a real deadlock, remove ourselves from
 * the lock's wait queue and signal an error to ProcSleep.
 */
static void
CheckDeadLock(void)
{
  int     i;

  /*
   * Acquire exclusive lock on the entire shared lock data structures. Must
   * grab LWLocks in partition-number order to avoid LWLock deadlock.
   *
   * Note that the deadlock check interrupt had better not be enabled
   * anywhere that this process itself holds lock partition locks, else this
   * will wait forever.  Also note that LWLockAcquire creates a critical
   * section, so that this routine cannot be interrupted by cancel/die
   * interrupts.
   */
  for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
    LWLockAcquire(LockHashPartitionLockByIndex(i), LW_EXCLUSIVE);

  /*
   * Check to see if we've been awoken by anyone in the interim.
   *
   * If we have, we can return and resume our transaction -- happy day.
   * Before we are awoken the process releasing the lock grants it to us so
   * we know that we don't have to wait anymore.
   *
   * We check by looking to see if we've been unlinked from the wait queue.
   * This is safe because we hold the lock partition lock.
   */
  if (MyProc->links.prev == NULL ||
    MyProc->links.next == NULL)
    goto check_done;

#ifdef LOCK_DEBUG
  if (Debug_deadlocks)
    DumpAllLocks();
#endif

  /* Run the deadlock check, and set deadlock_state for use by ProcSleep */
  deadlock_state = DeadLockCheck(MyProc);

  if (deadlock_state == DS_HARD_DEADLOCK)
  {
    /*
     * Oops.  We have a deadlock.
     *
     * Get this process out of wait state. (Note: we could do this more
     * efficiently by relying on lockAwaited, but use this coding to
     * preserve the flexibility to kill some other transaction than the
     * one detecting the deadlock.)
     *
     * RemoveFromWaitQueue sets MyProc->waitStatus to
     * PROC_WAIT_STATUS_ERROR, so ProcSleep will report an error after we
     * return from the signal handler.
     */
    Assert(MyProc->waitLock != NULL);
    RemoveFromWaitQueue(MyProc, LockTagHashCode(&(MyProc->waitLock->tag)));

    /*
     * We're done here.  Transaction abort caused by the error that
     * ProcSleep will raise will cause any other locks we hold to be
     * released, thus allowing other processes to wake up; we don't need
     * to do that here.  NOTE: an exception is that releasing locks we
     * hold doesn't consider the possibility of waiters that were blocked
     * behind us on the lock we just failed to get, and might now be
     * wakable because we're not in front of them anymore.  However,
     * RemoveFromWaitQueue took care of waking up any such processes.
     */
  }

  /*
   * And release locks.  We do this in reverse order for two reasons: (1)
   * Anyone else who needs more than one of the locks will be trying to lock
   * them in increasing order; we don't want to release the other process
   * until it can get all the locks it needs. (2) This avoids O(N^2)
   * behavior inside LWLockRelease.
   */
check_done:
  for (i = NUM_LOCK_PARTITIONS; --i >= 0;)
    LWLockRelease(LockHashPartitionLockByIndex(i));
}

/*
 * CheckDeadLockAlert - Handle the expiry of deadlock_timeout.
 *
 * NB: Runs inside a signal handler, be careful.
 */
void
CheckDeadLockAlert(void)
{
  int     save_errno = errno;

  got_deadlock_timeout = true;

  /*
   * Have to set the latch again, even if handle_sig_alarm already did. Back
   * then got_deadlock_timeout wasn't yet set... It's unlikely that this
   * ever would be a problem, but setting a set latch again is cheap.
   *
   * Note that, when this function runs inside procsignal_sigusr1_handler(),
   * the handler function sets the latch again after the latch is set here.
   */
  SetLatch(MyLatch);
  errno = save_errno;
}

/*
 * GetLockHoldersAndWaiters - get lock holders and waiters for a lock
 *
 * Fill lock_holders_sbuf and lock_waiters_sbuf with the PIDs of processes holding
 * and waiting for the lock, and set lockHoldersNum to the number of lock holders.
 *
 * The lock table's partition lock must be held on entry and remains held on exit.
 */
void
GetLockHoldersAndWaiters(LOCALLOCK *locallock, StringInfo lock_holders_sbuf,
             StringInfo lock_waiters_sbuf, int *lockHoldersNum)
{
  dlist_iter  proc_iter;
  PROCLOCK   *curproclock;
  LOCK     *lock = locallock->lock;
  bool    first_holder = true,
        first_waiter = true;

#ifdef USE_ASSERT_CHECKING
  {
    uint32    hashcode = locallock->hashcode;
    LWLock     *partitionLock = LockHashPartitionLock(hashcode);

    Assert(LWLockHeldByMe(partitionLock));
  }
#endif

  *lockHoldersNum = 0;

  /*
   * Loop over the lock's procLocks to gather a list of all holders and
   * waiters. Thus we will be able to provide more detailed information for
   * lock debugging purposes.
   *
   * lock->procLocks contains all processes which hold or wait for this
   * lock.
   */
  dlist_foreach(proc_iter, &lock->procLocks)
  {
    curproclock =
      dlist_container(PROCLOCK, lockLink, proc_iter.cur);

    /*
     * We are a waiter if myProc->waitProcLock == curproclock; we are a
     * holder if it is NULL or something different.
     */
    if (curproclock->tag.myProc->waitProcLock == curproclock)
    {
      if (first_waiter)
      {
        appendStringInfo(lock_waiters_sbuf, "%d",
                 curproclock->tag.myProc->pid);
        first_waiter = false;
      }
      else
        appendStringInfo(lock_waiters_sbuf, ", %d",
                 curproclock->tag.myProc->pid);
    }
    else
    {
      if (first_holder)
      {
        appendStringInfo(lock_holders_sbuf, "%d",
                 curproclock->tag.myProc->pid);
        first_holder = false;
      }
      else
        appendStringInfo(lock_holders_sbuf, ", %d",
                 curproclock->tag.myProc->pid);

      (*lockHoldersNum)++;
    }
  }
}

/*
 * ProcWaitForSignal - wait for a signal from another backend.
 *
 * As this uses the generic process latch the caller has to be robust against
 * unrelated wakeups: Always check that the desired state has occurred, and
 * wait again if not.
 */
void
ProcWaitForSignal(uint32 wait_event_info)
{
  (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
           wait_event_info);
  ResetLatch(MyLatch);
  CHECK_FOR_INTERRUPTS();
}

/*
 * ProcSendSignal - set the latch of a backend identified by ProcNumber
 */
void
ProcSendSignal(ProcNumber procNumber)
{
  if (procNumber < 0 || procNumber >= ProcGlobal->allProcCount)
    elog(ERROR, "procNumber out of range");

  SetLatch(&ProcGlobal->allProcs[procNumber].procLatch);
}

/*
 * BecomeLockGroupLeader - designate process as lock group leader
 *
 * Once this function has returned, other processes can join the lock group
 * by calling BecomeLockGroupMember.
 */
void
BecomeLockGroupLeader(void)
{
  LWLock     *leader_lwlock;

  /* If we already did it, we don't need to do it again. */
  if (MyProc->lockGroupLeader == MyProc)
    return;

  /* We had better not be a follower. */
  Assert(MyProc->lockGroupLeader == NULL);

  /* Create single-member group, containing only ourselves. */
  leader_lwlock = LockHashPartitionLockByProc(MyProc);
  LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);
  MyProc->lockGroupLeader = MyProc;
  dlist_push_head(&MyProc->lockGroupMembers, &MyProc->lockGroupLink);
  LWLockRelease(leader_lwlock);
}

/*
 * BecomeLockGroupMember - designate process as lock group member
 *
 * This is pretty straightforward except for the possibility that the leader
 * whose group we're trying to join might exit before we manage to do so;
 * and the PGPROC might get recycled for an unrelated process.  To avoid
 * that, we require the caller to pass the PID of the intended PGPROC as
 * an interlock.  Returns true if we successfully join the intended lock
 * group, and false if not.
 */
bool
BecomeLockGroupMember(PGPROC *leader, int pid)
{
  LWLock     *leader_lwlock;
  bool    ok = false;

  /* Group leader can't become member of group */
  Assert(MyProc != leader);

  /* Can't already be a member of a group */
  Assert(MyProc->lockGroupLeader == NULL);

  /* PID must be valid. */
  Assert(pid != 0);

  /*
   * Get lock protecting the group fields.  Note LockHashPartitionLockByProc
   * calculates the proc number based on the PGPROC slot without looking at
   * its contents, so we will acquire the correct lock even if the leader
   * PGPROC is in process of being recycled.
   */
  leader_lwlock = LockHashPartitionLockByProc(leader);
  LWLockAcquire(leader_lwlock, LW_EXCLUSIVE);

  /* Is this the leader we're looking for? */
  if (leader->pid == pid && leader->lockGroupLeader == leader)
  {
    /* OK, join the group */
    ok = true;
    MyProc->lockGroupLeader = leader;
    dlist_push_tail(&leader->lockGroupMembers, &MyProc->lockGroupLink);
  }
  LWLockRelease(leader_lwlock);

  return ok;
}

Coverage Report

Created: 2025-09-27 06:52