/src/postgres/src/backend/commands/async.c

Source
/*-------------------------------------------------------------------------
 *
 * async.c
 *    Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/commands/async.c
 *
 *-------------------------------------------------------------------------
 */

/*-------------------------------------------------------------------------
 * Async Notification Model as of v19:
 *
 * 1. Multiple backends on same machine.  Multiple backends may be listening
 *    on each of several channels.
 *
 * 2. There is one central queue in disk-based storage (directory pg_notify/),
 *    with actively-used pages mapped into shared memory by the slru.c module.
 *    All notification messages are placed in the queue and later read out
 *    by listening backends.  The single queue allows us to guarantee that
 *    notifications are received in commit order.
 *
 *    Although there is only one queue, notifications are treated as being
 *    database-local; this is done by including the sender's database OID
 *    in each notification message.  Listening backends ignore messages
 *    that don't match their database OID.  This is important because it
 *    ensures senders and receivers have the same database encoding and won't
 *    misinterpret non-ASCII text in the channel name or payload string.
 *
 *    Since notifications are not expected to survive database crashes,
 *    we can simply clean out the pg_notify data at any reboot, and there
 *    is no need for WAL support or fsync'ing.
 *
 * 3. Every backend that is listening on at least one channel registers by
 *    entering its PID into the array in AsyncQueueControl. It then scans all
 *    incoming notifications in the central queue and first compares the
 *    database OID of the notification with its own database OID and then
 *    compares the notified channel with the list of channels that it listens
 *    to. In case there is a match it delivers the notification event to its
 *    frontend.  Non-matching events are simply skipped.
 *
 * 4. The NOTIFY statement (routine Async_Notify) stores the notification in
 *    a backend-local list which will not be processed until transaction end.
 *
 *    Duplicate notifications from the same transaction are sent out as one
 *    notification only. This is done to save work when for example a trigger
 *    on a 2 million row table fires a notification for each row that has been
 *    changed. If the application needs to receive every single notification
 *    that has been sent, it can easily add some unique string into the extra
 *    payload parameter.
 *
 *    When the transaction is ready to commit, PreCommit_Notify() adds the
 *    pending notifications to the head of the queue. The head pointer of the
 *    queue always points to the next free position and a position is just a
 *    page number and the offset in that page. This is done before marking the
 *    transaction as committed in clog. If we run into problems writing the
 *    notifications, we can still call elog(ERROR, ...) and the transaction
 *    will roll back safely.
 *
 *    Once we have put all of the notifications into the queue, we return to
 *    CommitTransaction() which will then do the actual transaction commit.
 *
 *    After commit we are called another time (AtCommit_Notify()). Here we
 *    make any required updates to the effective listen state (see below).
 *    Then we signal any backends that may be interested in our messages
 *    (including our own backend, if listening).  This is done by
 *    SignalBackends(), which sends a PROCSIG_NOTIFY_INTERRUPT signal to
 *    each relevant backend, as described below.
 *
 *    Finally, after we are out of the transaction altogether and about to go
 *    idle, we scan the queue for messages that need to be sent to our
 *    frontend (which might be notifies from other backends, or self-notifies
 *    from our own).  This step is not part of the CommitTransaction sequence
 *    for two important reasons.  First, we could get errors while sending
 *    data to our frontend, and it's really bad for errors to happen in
 *    post-commit cleanup.  Second, in cases where a procedure issues commits
 *    within a single frontend command, we don't want to send notifies to our
 *    frontend until the command is done; but notifies to other backends
 *    should go out immediately after each commit.
 *
 * 5. Upon receipt of a PROCSIG_NOTIFY_INTERRUPT signal, the signal handler
 *    sets the process's latch, which triggers the event to be processed
 *    immediately if this backend is idle (i.e., it is waiting for a frontend
 *    command and is not within a transaction block. C.f.
 *    ProcessClientReadInterrupt()).  Otherwise the handler may only set a
 *    flag, which will cause the processing to occur just before we next go
 *    idle.
 *
 *    Inbound-notify processing consists of reading all of the notifications
 *    that have arrived since scanning last time. We read every notification
 *    until we reach either a notification from an uncommitted transaction or
 *    the head pointer's position.
 *
 * 6. To limit disk space consumption, the tail pointer needs to be advanced
 *    so that old pages can be truncated. This is relatively expensive
 *    (notably, it requires an exclusive lock), so we don't want to do it
 *    often. We make sending backends do this work if they advanced the queue
 *    head into a new page, but only once every QUEUE_CLEANUP_DELAY pages.
 *
 * 7. So far we have not discussed how backends change their listening state,
 *    nor how notification senders know which backends to awaken.  To handle
 *    the latter, we maintain a global channel table (implemented as a dynamic
 *    shared hash table, or dshash) that maps channel names to the set of
 *    backends listening on each channel.  This table is created lazily on the
 *    first LISTEN command and grows dynamically as needed.  There is also a
 *    local channel table (a plain dynahash table) in each listening backend,
 *    tracking which channels that backend is listening to.  The local table
 *    serves to reduce the number of accesses needed to the shared table.
 *
 *    If the current transaction has executed any LISTEN/UNLISTEN actions,
 *    PreCommit_Notify() prepares to commit those.  For LISTEN, it
 *    pre-allocates entries in both the per-backend localChannelTable and the
 *    shared globalChannelTable, marking new shared entries removeOnAbort.
 *    It also records the final per-channel intent in pendingListenActions,
 *    so post-commit/abort processing can apply that in a single step.
 *    Since all these allocations happen before committing to clog, we can
 *    safely abort the transaction on failure.
 *
 *    After commit, AtCommit_Notify() runs through pendingListenActions and
 *    applies the final per-channel listen/unlisten state.  This happens
 *    before sending signals.
 *
 *    SignalBackends() consults the shared global channel table to identify
 *    listeners for the channels that the current transaction sent
 *    notification(s) to.  Each selected backend is marked as having a wakeup
 *    pending to avoid duplicate signals, and a PROCSIG_NOTIFY_INTERRUPT
 *    signal is sent to it.
 *
 * 8. While writing notifications, PreCommit_Notify() records the queue head
 *    position both before and after the write.  Because all writers serialize
 *    on a cluster-wide heavyweight lock, no other backend can insert entries
 *    between these two points.  SignalBackends() uses this fact to directly
 *    advance the queue pointer for any backend that is still positioned at
 *    the old head, or within the range written, but is not interested in any
 *    of our notifications.  This avoids unnecessary wakeups for idle
 *    listeners that have nothing to read.  Backends that are not interested
 *    in our notifications, but cannot be directly advanced, are signaled only
 *    if they are far behind the current queue head; that is to ensure that
 *    we can advance the queue tail without undue delay.
 *
 * An application that listens on the same channel it notifies will get
 * NOTIFY messages for its own NOTIFYs.  These can be ignored, if not useful,
 * by comparing be_pid in the NOTIFY message to the application's own backend's
 * PID.  (As of FE/BE protocol 2.0, the backend's PID is provided to the
 * frontend during startup.)  The above design guarantees that notifies from
 * other backends will never be missed by ignoring self-notifies.
 *
 * The amount of shared memory used for notify management (notify_buffers)
 * can be varied without affecting anything but performance.  The maximum
 * amount of notification data that can be queued at one time is determined
 * by the max_notify_queue_pages GUC.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <limits.h>
#include <unistd.h>
#include <signal.h>

#include "access/parallel.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/pg_database.h"
#include "commands/async.h"
#include "common/hashfn.h"
#include "funcapi.h"
#include "lib/dshash.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/dsm_registry.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lmgr.h"
#include "storage/procsignal.h"
#include "storage/subsystems.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/dsa.h"
#include "utils/guc_hooks.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "utils/tuplestore.h"


/*
 * Maximum size of a NOTIFY payload, including terminating NULL.  This
 * must be kept small enough so that a notification message fits on one
 * SLRU page.  The magic fudge factor here is noncritical as long as it's
 * more than AsyncQueueEntryEmptySize --- we make it significantly bigger
 * than that, so changes in that data structure won't affect user-visible
 * restrictions.
 */
#define NOTIFY_PAYLOAD_MAX_LENGTH (BLCKSZ - NAMEDATALEN - 128)

/*
 * Struct representing an entry in the global notify queue
 *
 * This struct declaration has the maximal length, but in a real queue entry
 * the data area is only big enough for the actual channel and payload strings
 * (each null-terminated).  AsyncQueueEntryEmptySize is the minimum possible
 * entry size, if both channel and payload strings are empty (but note it
 * doesn't include alignment padding).
 *
 * The "length" field should always be rounded up to the next QUEUEALIGN
 * multiple so that all fields are properly aligned.
 */
typedef struct AsyncQueueEntry
{
  int     length;     /* total allocated length of entry */
  Oid     dboid;      /* sender's database OID */
  TransactionId xid;      /* sender's XID */
  int32   srcPid;     /* sender's PID */
  char    data[NAMEDATALEN + NOTIFY_PAYLOAD_MAX_LENGTH];
} AsyncQueueEntry;

/* Currently, no field of AsyncQueueEntry requires more than int alignment */
#define QUEUEALIGN(len)   INTALIGN(len)

#define AsyncQueueEntryEmptySize  (offsetof(AsyncQueueEntry, data) + 2)

/*
 * Struct describing a queue position, and assorted macros for working with it
 */
typedef struct QueuePosition
{
  int64   page;     /* SLRU page number */
  int     offset;     /* byte offset within page */
} QueuePosition;

#define QUEUE_POS_PAGE(x)   ((x).page)
#define QUEUE_POS_OFFSET(x)   ((x).offset)

#define SET_QUEUE_POS(x,y,z) \
  do { \
    (x).page = (y); \
    (x).offset = (z); \
  } while (0)

#define QUEUE_POS_EQUAL(x,y) \
  ((x).page == (y).page && (x).offset == (y).offset)

#define QUEUE_POS_IS_ZERO(x) \
  ((x).page == 0 && (x).offset == 0)

/* choose logically smaller QueuePosition */
#define QUEUE_POS_MIN(x,y) \
  (asyncQueuePagePrecedes((x).page, (y).page) ? (x) : \
   (x).page != (y).page ? (y) : \
   (x).offset < (y).offset ? (x) : (y))

/* choose logically larger QueuePosition */
#define QUEUE_POS_MAX(x,y) \
  (asyncQueuePagePrecedes((x).page, (y).page) ? (y) : \
   (x).page != (y).page ? (x) : \
   (x).offset > (y).offset ? (x) : (y))

/* returns true if x comes before y in queue order */
#define QUEUE_POS_PRECEDES(x,y) \
  (asyncQueuePagePrecedes((x).page, (y).page) || \
   ((x).page == (y).page && (x).offset < (y).offset))

/*
 * Parameter determining how often we try to advance the tail pointer:
 * we do that after every QUEUE_CLEANUP_DELAY pages of NOTIFY data.  This is
 * also the distance by which a backend that's not interested in our
 * notifications needs to be behind before we'll decide we need to wake it
 * up so it can advance its pointer.
 *
 * Resist the temptation to make this really large.  While that would save
 * work in some places, it would add cost in others.  In particular, this
 * should likely be less than notify_buffers, to ensure that backends
 * catch up before the pages they'll need to read fall out of SLRU cache.
 */
#define QUEUE_CLEANUP_DELAY 4

/*
 * Struct describing a listening backend's status
 */
typedef struct QueueBackendStatus
{
  int32   pid;      /* either a PID or InvalidPid */
  Oid     dboid;      /* backend's database OID, or InvalidOid */
  ProcNumber  nextListener; /* id of next listener, or INVALID_PROC_NUMBER */
  QueuePosition pos;      /* backend has read queue up to here */
  bool    wakeupPending;  /* signal sent to backend, not yet processed */
  bool    isAdvancing;  /* backend is advancing its position */
} QueueBackendStatus;

/*
 * Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
 *
 * The AsyncQueueControl structure is protected by the NotifyQueueLock and
 * NotifyQueueTailLock.
 *
 * When holding NotifyQueueLock in SHARED mode, backends may only inspect
 * their own entries as well as the head and tail pointers. Consequently we
 * can allow a backend to update its own record while holding only SHARED lock
 * (since no other backend will inspect it).
 *
 * When holding NotifyQueueLock in EXCLUSIVE mode, backends can inspect the
 * entries of other backends and also change the head pointer. They can
 * also advance other backends' queue positions, unless the other backend
 * has isAdvancing set (i.e., is in process of doing that itself).
 *
 * When holding both NotifyQueueLock and NotifyQueueTailLock in EXCLUSIVE
 * mode, backends can change the tail pointers.
 *
 * SLRU buffer pool is divided in banks and bank wise SLRU lock is used as
 * the control lock for the pg_notify SLRU buffers.
 * In order to avoid deadlocks, whenever we need multiple locks, we first get
 * NotifyQueueTailLock, then NotifyQueueLock, then SLRU bank lock, and lastly
 * globalChannelTable partition locks.
 *
 * Each backend uses the backend[] array entry with index equal to its
 * ProcNumber.  We rely on this to make SendProcSignal fast.
 *
 * The backend[] array entries for actively-listening backends are threaded
 * together using firstListener and the nextListener links, so that we can
 * scan them without having to iterate over inactive entries.  We keep this
 * list in order by ProcNumber so that the scan is cache-friendly when there
 * are many active entries.
 */
typedef struct AsyncQueueControl
{
  QueuePosition head;     /* head points to the next free location */
  QueuePosition tail;     /* tail must be <= the queue position of every
                 * listening backend */
  int64   stopPage;   /* oldest unrecycled page; must be <=
                 * tail.page */
  ProcNumber  firstListener;  /* id of first listener, or
                 * INVALID_PROC_NUMBER */
  TimestampTz lastQueueFillWarn;  /* time of last queue-full msg */
  dsa_handle  globalChannelTableDSA;  /* global channel table's DSA handle */
  dshash_table_handle globalChannelTableDSH;  /* and its dshash handle */
  /* Array with room for MaxBackends entries: */
  QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
} AsyncQueueControl;

static AsyncQueueControl *asyncQueueControl;

static void AsyncShmemRequest(void *arg);
static void AsyncShmemInit(void *arg);

const ShmemCallbacks AsyncShmemCallbacks = {
  .request_fn = AsyncShmemRequest,
  .init_fn = AsyncShmemInit,
};


#define QUEUE_HEAD          (asyncQueueControl->head)
#define QUEUE_TAIL          (asyncQueueControl->tail)
#define QUEUE_STOP_PAGE       (asyncQueueControl->stopPage)
#define QUEUE_FIRST_LISTENER    (asyncQueueControl->firstListener)
#define QUEUE_BACKEND_PID(i)    (asyncQueueControl->backend[i].pid)
#define QUEUE_BACKEND_DBOID(i)    (asyncQueueControl->backend[i].dboid)
#define QUEUE_NEXT_LISTENER(i)    (asyncQueueControl->backend[i].nextListener)
#define QUEUE_BACKEND_POS(i)    (asyncQueueControl->backend[i].pos)
#define QUEUE_BACKEND_WAKEUP_PENDING(i) (asyncQueueControl->backend[i].wakeupPending)
#define QUEUE_BACKEND_IS_ADVANCING(i) (asyncQueueControl->backend[i].isAdvancing)

/*
 * The SLRU buffer area through which we access the notification queue
 */
static inline bool asyncQueuePagePrecedes(int64 p, int64 q);
static int  asyncQueueErrdetailForIoError(const void *opaque_data);

static SlruDesc NotifySlruDesc;


#define NotifyCtl         (&NotifySlruDesc)
#define QUEUE_PAGESIZE        BLCKSZ

#define QUEUE_FULL_WARN_INTERVAL  5000  /* warn at most once every 5s */

/*
 * Global channel table definitions
 *
 * This hash table maps (database OID, channel name) keys to arrays of
 * ProcNumbers representing the backends listening or about to listen
 * on each channel.  The removeOnAbort flags allow us to create hash table
 * entries pre-commit and not have to assume that creating them post-commit
 * will succeed.
 */
#define INITIAL_LISTENERS_ARRAY_SIZE 4

typedef struct GlobalChannelKey
{
  Oid     dboid;
  char    channel[NAMEDATALEN];
} GlobalChannelKey;

typedef struct ListenerEntry
{
  ProcNumber  procNo;     /* listener's ProcNumber */
  bool    removeOnAbort;  /* remove entry if current xact aborts */
} ListenerEntry;

typedef struct GlobalChannelEntry
{
  GlobalChannelKey key;   /* hash key */
  dsa_pointer listenersArray; /* DSA pointer to ListenerEntry array */
  int     numListeners; /* Number of listeners currently stored */
  int     allocatedListeners; /* Allocated size of array */
} GlobalChannelEntry;

static dshash_table *globalChannelTable = NULL;
static dsa_area *globalChannelDSA = NULL;

/*
 * localChannelTable caches the channel names this backend is listening on
 * (including those we have staged to be listened on, but not yet committed).
 * Used by IsListeningOn() for fast lookups when reading notifications.
 */
static HTAB *localChannelTable = NULL;

/* We test this condition to detect that we're not listening at all */
#define LocalChannelTableIsEmpty() \
  (localChannelTable == NULL || hash_get_num_entries(localChannelTable) == 0)

/*
 * State for pending LISTEN/UNLISTEN actions consists of an ordered list of
 * all actions requested in the current transaction.  As explained above,
 * we don't actually change listen state until we reach transaction commit.
 *
 * The list is kept in CurTransactionContext.  In subtransactions, each
 * subtransaction has its own list in its own CurTransactionContext, but
 * successful subtransactions attach their lists to their parent's list.
 * Failed subtransactions simply discard their lists.
 */
typedef enum
{
  LISTEN_LISTEN,
  LISTEN_UNLISTEN,
  LISTEN_UNLISTEN_ALL,
} ListenActionKind;

typedef struct
{
  ListenActionKind action;
  char    channel[FLEXIBLE_ARRAY_MEMBER]; /* nul-terminated string */
} ListenAction;

typedef struct ActionList
{
  int     nestingLevel; /* current transaction nesting depth */
  List     *actions;    /* list of ListenAction structs */
  struct ActionList *upper; /* details for upper transaction levels */
} ActionList;

static ActionList *pendingActions = NULL;

/*
 * Hash table recording the final listen/unlisten intent per channel for
 * the current transaction.  Key is channel name, value is PENDING_LISTEN or
 * PENDING_UNLISTEN.  This keeps critical commit/abort processing to one step
 * per channel instead of replaying every action.  This is built from the
 * pendingActions list by PreCommit_Notify, then used by AtCommit_Notify or
 * AtAbort_Notify.
 */
typedef enum
{
  PENDING_LISTEN,
  PENDING_UNLISTEN,
} PendingListenAction;

typedef struct PendingListenEntry
{
  char    channel[NAMEDATALEN]; /* hash key */
  PendingListenAction action; /* which action should we perform? */
} PendingListenEntry;

static HTAB *pendingListenActions = NULL;

/*
 * State for outbound notifies consists of a list of all channels+payloads
 * NOTIFYed in the current transaction.  We do not actually perform a NOTIFY
 * until and unless the transaction commits.  pendingNotifies is NULL if no
 * NOTIFYs have been done in the current (sub) transaction.
 *
 * We discard duplicate notify events issued in the same transaction.
 * Hence, in addition to the list proper (which we need to track the order
 * of the events, since we guarantee to deliver them in order), we build a
 * hash table which we can probe to detect duplicates.  Since building the
 * hash table is somewhat expensive, we do so only once we have at least
 * MIN_HASHABLE_NOTIFIES events queued in the current (sub) transaction;
 * before that we just scan the events linearly.
 *
 * The list is kept in CurTransactionContext.  In subtransactions, each
 * subtransaction has its own list in its own CurTransactionContext, but
 * successful subtransactions add their entries to their parent's list.
 * Failed subtransactions simply discard their lists.  Since these lists
 * are independent, there may be notify events in a subtransaction's list
 * that duplicate events in some ancestor (sub) transaction; we get rid of
 * the dups when merging the subtransaction's list into its parent's.
 *
 * Note: the action and notify lists do not interact within a transaction.
 * In particular, if a transaction does NOTIFY and then LISTEN on the same
 * condition name, it will get a self-notify at commit.  This is a bit odd
 * but is consistent with our historical behavior.
 */
typedef struct Notification
{
  uint16    channel_len;  /* length of channel-name string */
  uint16    payload_len;  /* length of payload string */
  /* null-terminated channel name, then null-terminated payload follow */
  char    data[FLEXIBLE_ARRAY_MEMBER];
} Notification;

typedef struct NotificationList
{
  int     nestingLevel; /* current transaction nesting depth */
  List     *events;     /* list of Notification structs */
  HTAB     *hashtab;    /* hash of NotificationHash structs, or NULL */
  List     *uniqueChannelNames; /* unique channel names being notified */
  HTAB     *uniqueChannelHash;  /* hash of unique channel names, or NULL */
  struct NotificationList *upper; /* details for upper transaction levels */
} NotificationList;

#define MIN_HASHABLE_NOTIFIES 16  /* threshold to build hashtab */

struct NotificationHash
{
  Notification *event;    /* => the actual Notification struct */
};

static NotificationList *pendingNotifies = NULL;

/*
 * Hash entry in NotificationList.uniqueChannelHash or localChannelTable
 * (both just carry the channel name, with no payload).
 */
typedef struct ChannelName
{
  char    channel[NAMEDATALEN]; /* hash key */
} ChannelName;

/*
 * Inbound notifications are initially processed by HandleNotifyInterrupt(),
 * called from inside a signal handler. That just sets the
 * notifyInterruptPending flag and sets the process
 * latch. ProcessNotifyInterrupt() will then be called whenever it's safe to
 * actually deal with the interrupt.
 */
volatile sig_atomic_t notifyInterruptPending = false;

/* True if we've registered an on_shmem_exit cleanup */
static bool unlistenExitRegistered = false;

/* True if we're currently registered as a listener in asyncQueueControl */
static bool amRegisteredListener = false;

/*
 * Queue head positions for direct advancement.
 * These are captured during PreCommit_Notify while holding the heavyweight
 * lock on database 0, ensuring no other backend can insert notifications
 * between them.  SignalBackends uses these to advance idle backends.
 */
static QueuePosition queueHeadBeforeWrite;
static QueuePosition queueHeadAfterWrite;

/*
 * Workspace arrays for SignalBackends.  These are preallocated in
 * PreCommit_Notify to avoid needing memory allocation after committing to
 * clog.
 */
static int32 *signalPids = NULL;
static ProcNumber *signalProcnos = NULL;

/* have we advanced to a page that's a multiple of QUEUE_CLEANUP_DELAY? */
static bool tryAdvanceTail = false;

/* GUC parameters */
bool    Trace_notify = false;

/* For 8 KB pages this gives 8 GB of disk space */
int     max_notify_queue_pages = 1048576;

/* local function prototypes */
static inline int64 asyncQueuePageDiff(int64 p, int64 q);
static inline void GlobalChannelKeyInit(GlobalChannelKey *key, Oid dboid,
                    const char *channel);
static dshash_hash globalChannelTableHash(const void *key, size_t size,
                      void *arg);
static void initGlobalChannelTable(void);
static void initLocalChannelTable(void);
static void queue_listen(ListenActionKind action, const char *channel);
static void Async_UnlistenOnExit(int code, Datum arg);
static void BecomeRegisteredListener(void);
static void PrepareTableEntriesForListen(const char *channel);
static void PrepareTableEntriesForUnlisten(const char *channel);
static void PrepareTableEntriesForUnlistenAll(void);
static void RemoveListenerFromChannel(GlobalChannelEntry **entry_ptr,
                    ListenerEntry *listeners,
                    int idx);
static void ApplyPendingListenActions(bool isCommit);
static void CleanupListenersOnExit(void);
static bool IsListeningOn(const char *channel);
static void asyncQueueUnregister(void);
static bool asyncQueueIsFull(void);
static bool asyncQueueAdvance(QueuePosition *position, int entryLength);
static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
static double asyncQueueUsage(void);
static void asyncQueueFillWarning(void);
static void SignalBackends(void);
static void asyncQueueReadAllNotifications(void);
static bool asyncQueueProcessPageEntries(QueuePosition *current,
                     QueuePosition stop,
                     Snapshot snapshot);
static void asyncQueueAdvanceTail(void);
static void ProcessIncomingNotify(bool flush);
static bool AsyncExistsPendingNotify(Notification *n);
static void AddEventToPendingNotifies(Notification *n);
static uint32 notification_hash(const void *key, Size keysize);
static int  notification_match(const void *key1, const void *key2, Size keysize);
static void ClearPendingActionsAndNotifies(void);

static int
asyncQueueErrdetailForIoError(const void *opaque_data)
{
  const QueuePosition *position = opaque_data;

  return errdetail("Could not access async queue at page %" PRId64 ", offset %d.",
           position->page, position->offset);
}

/*
 * Compute the difference between two queue page numbers.
 * Previously this function accounted for a wraparound.
 */
static inline int64
asyncQueuePageDiff(int64 p, int64 q)
{
  return p - q;
}

/*
 * Determines whether p precedes q.
 * Previously this function accounted for a wraparound.
 */
static inline bool
asyncQueuePagePrecedes(int64 p, int64 q)
{
  return p < q;
}

/*
 * GlobalChannelKeyInit
 *    Prepare a global channel table key for hashing.
 */
static inline void
GlobalChannelKeyInit(GlobalChannelKey *key, Oid dboid, const char *channel)
{
  memset(key, 0, sizeof(GlobalChannelKey));
  key->dboid = dboid;
  strlcpy(key->channel, channel, NAMEDATALEN);
}

/*
 * globalChannelTableHash
 *    Hash function for global channel table keys.
 */
static dshash_hash
globalChannelTableHash(const void *key, size_t size, void *arg)
{
  const GlobalChannelKey *k = (const GlobalChannelKey *) key;
  dshash_hash h;

  h = DatumGetUInt32(hash_uint32(k->dboid));
  h ^= hash_bytes((const unsigned char *) k->channel,
          strnlen(k->channel, NAMEDATALEN));

  return h;
}

/* parameters for the global channel table */
static const dshash_parameters globalChannelTableDSHParams = {
  sizeof(GlobalChannelKey),
  sizeof(GlobalChannelEntry),
  dshash_memcmp,
  globalChannelTableHash,
  dshash_memcpy,
  LWTRANCHE_NOTIFY_CHANNEL_HASH
};

/*
 * initGlobalChannelTable
 *    Lazy initialization of the global channel table.
 */
static void
initGlobalChannelTable(void)
{
  MemoryContext oldcontext;

  /* Quick exit if we already did this */
  if (asyncQueueControl->globalChannelTableDSH != DSHASH_HANDLE_INVALID &&
    globalChannelTable != NULL)
    return;

  /* Otherwise, use a lock to ensure only one process creates the table */
  LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);

  /* Be sure any local memory allocated by DSA routines is persistent */
  oldcontext = MemoryContextSwitchTo(TopMemoryContext);

  if (asyncQueueControl->globalChannelTableDSH == DSHASH_HANDLE_INVALID)
  {
    /* Initialize dynamic shared hash table for global channels */
    globalChannelDSA = dsa_create(LWTRANCHE_NOTIFY_CHANNEL_HASH);
    dsa_pin(globalChannelDSA);
    dsa_pin_mapping(globalChannelDSA);
    globalChannelTable = dshash_create(globalChannelDSA,
                       &globalChannelTableDSHParams,
                       NULL);

    /* Store handles in shared memory for other backends to use */
    asyncQueueControl->globalChannelTableDSA = dsa_get_handle(globalChannelDSA);
    asyncQueueControl->globalChannelTableDSH =
      dshash_get_hash_table_handle(globalChannelTable);
  }
  else if (!globalChannelTable)
  {
    /* Attach to existing dynamic shared hash table */
    globalChannelDSA = dsa_attach(asyncQueueControl->globalChannelTableDSA);
    dsa_pin_mapping(globalChannelDSA);
    globalChannelTable = dshash_attach(globalChannelDSA,
                       &globalChannelTableDSHParams,
                       asyncQueueControl->globalChannelTableDSH,
                       NULL);
  }

  MemoryContextSwitchTo(oldcontext);
  LWLockRelease(NotifyQueueLock);
}

/*
 * initLocalChannelTable
 *    Lazy initialization of the local channel table.
 *    Once created, this table lasts for the life of the session.
 */
static void
initLocalChannelTable(void)
{
  HASHCTL   hash_ctl;

  /* Quick exit if we already did this */
  if (localChannelTable != NULL)
    return;

  /* Initialize local hash table for this backend's listened channels */
  hash_ctl.keysize = NAMEDATALEN;
  hash_ctl.entrysize = sizeof(ChannelName);

  localChannelTable =
    hash_create("Local Listen Channels",
          64,
          &hash_ctl,
          HASH_ELEM | HASH_STRINGS);
}

/*
 * initPendingListenActions
 *    Lazy initialization of the pending listen actions hash table.
 *    This is allocated in CurTransactionContext during PreCommit_Notify,
 *    and destroyed at transaction end.
 */
static void
initPendingListenActions(void)
{
  HASHCTL   hash_ctl;

  if (pendingListenActions != NULL)
    return;

  hash_ctl.keysize = NAMEDATALEN;
  hash_ctl.entrysize = sizeof(PendingListenEntry);
  hash_ctl.hcxt = CurTransactionContext;

  pendingListenActions =
    hash_create("Pending Listen Actions",
          list_length(pendingActions->actions),
          &hash_ctl,
          HASH_ELEM | HASH_STRINGS | HASH_CONTEXT);
}

/*
 * Register our shared memory needs
 */
static void
AsyncShmemRequest(void *arg)
{
  Size    size;

  size = mul_size(MaxBackends, sizeof(QueueBackendStatus));
  size = add_size(size, offsetof(AsyncQueueControl, backend));

  ShmemRequestStruct(.name = "Async Queue Control",
             .size = size,
             .ptr = (void **) &asyncQueueControl,
    );

  SimpleLruRequest(.desc = &NotifySlruDesc,
           .name = "notify",
           .Dir = "pg_notify",

  /* long segment names are used in order to avoid wraparound */
           .long_segment_names = true,

           .nslots = notify_buffers,

           .sync_handler = SYNC_HANDLER_NONE,
           .PagePrecedes = asyncQueuePagePrecedes,
           .errdetail_for_io_error = asyncQueueErrdetailForIoError,

           .buffer_tranche_id = LWTRANCHE_NOTIFY_BUFFER,
           .bank_tranche_id = LWTRANCHE_NOTIFY_SLRU,
    );
}

static void
AsyncShmemInit(void *arg)
{
  SET_QUEUE_POS(QUEUE_HEAD, 0, 0);
  SET_QUEUE_POS(QUEUE_TAIL, 0, 0);
  QUEUE_STOP_PAGE = 0;
  QUEUE_FIRST_LISTENER = INVALID_PROC_NUMBER;
  asyncQueueControl->lastQueueFillWarn = 0;
  asyncQueueControl->globalChannelTableDSA = DSA_HANDLE_INVALID;
  asyncQueueControl->globalChannelTableDSH = DSHASH_HANDLE_INVALID;
  for (int i = 0; i < MaxBackends; i++)
  {
    QUEUE_BACKEND_PID(i) = InvalidPid;
    QUEUE_BACKEND_DBOID(i) = InvalidOid;
    QUEUE_NEXT_LISTENER(i) = INVALID_PROC_NUMBER;
    SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
    QUEUE_BACKEND_WAKEUP_PENDING(i) = false;
    QUEUE_BACKEND_IS_ADVANCING(i) = false;
  }

  /*
   * During start or reboot, clean out the pg_notify directory.
   */
  (void) SlruScanDirectory(NotifyCtl, SlruScanDirCbDeleteAll, NULL);
}


/*
 * pg_notify -
 *    SQL function to send a notification event
 */
Datum
pg_notify(PG_FUNCTION_ARGS)
{
  const char *channel;
  const char *payload;

  if (PG_ARGISNULL(0))
    channel = "";
  else
    channel = text_to_cstring(PG_GETARG_TEXT_PP(0));

  if (PG_ARGISNULL(1))
    payload = "";
  else
    payload = text_to_cstring(PG_GETARG_TEXT_PP(1));

  /* For NOTIFY as a statement, this is checked in ProcessUtility */
  PreventCommandDuringRecovery("NOTIFY");

  Async_Notify(channel, payload);

  PG_RETURN_VOID();
}


/*
 * Async_Notify
 *
 *    This is executed by the SQL notify command.
 *
 *    Adds the message to the list of pending notifies.
 *    Actual notification happens during transaction commit.
 *    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 */
void
Async_Notify(const char *channel, const char *payload)
{
  int     my_level = GetCurrentTransactionNestLevel();
  size_t    channel_len;
  size_t    payload_len;
  Notification *n;
  MemoryContext oldcontext;

  if (IsParallelWorker())
    elog(ERROR, "cannot send notifications from a parallel worker");

  if (Trace_notify)
    elog(DEBUG1, "Async_Notify(%s)", channel);

  channel_len = channel ? strlen(channel) : 0;
  payload_len = payload ? strlen(payload) : 0;

  /* a channel name must be specified */
  if (channel_len == 0)
    ereport(ERROR,
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
         errmsg("channel name cannot be empty")));

  /* enforce length limits */
  if (channel_len >= NAMEDATALEN)
    ereport(ERROR,
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
         errmsg("channel name too long")));

  if (payload_len >= NOTIFY_PAYLOAD_MAX_LENGTH)
    ereport(ERROR,
        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
         errmsg("payload string too long")));

  /*
   * We must construct the Notification entry, even if we end up not using
   * it, in order to compare it cheaply to existing list entries.
   *
   * The notification list needs to live until end of transaction, so store
   * it in the transaction context.
   */
  oldcontext = MemoryContextSwitchTo(CurTransactionContext);

  n = (Notification *) palloc(offsetof(Notification, data) +
                channel_len + payload_len + 2);
  n->channel_len = channel_len;
  n->payload_len = payload_len;
  strcpy(n->data, channel);
  if (payload)
    strcpy(n->data + channel_len + 1, payload);
  else
    n->data[channel_len + 1] = '\0';

  if (pendingNotifies == NULL || my_level > pendingNotifies->nestingLevel)
  {
    NotificationList *notifies;

    /*
     * First notify event in current (sub)xact. Note that we allocate the
     * NotificationList in TopTransactionContext; the nestingLevel might
     * get changed later by AtSubCommit_Notify.
     */
    notifies = (NotificationList *)
      MemoryContextAlloc(TopTransactionContext,
                 sizeof(NotificationList));
    notifies->nestingLevel = my_level;
    notifies->events = list_make1(n);
    /* We certainly don't need a hashtable yet */
    notifies->hashtab = NULL;
    /* We won't build uniqueChannelNames/Hash till later, either */
    notifies->uniqueChannelNames = NIL;
    notifies->uniqueChannelHash = NULL;
    notifies->upper = pendingNotifies;
    pendingNotifies = notifies;
  }
  else
  {
    /* Now check for duplicates */
    if (AsyncExistsPendingNotify(n))
    {
      /* It's a dup, so forget it */
      pfree(n);
      MemoryContextSwitchTo(oldcontext);
      return;
    }

    /* Append more events to existing list */
    AddEventToPendingNotifies(n);
  }

  MemoryContextSwitchTo(oldcontext);
}

/*
 * queue_listen
 *    Common code for listen, unlisten, unlisten all commands.
 *
 *    Adds the request to the list of pending actions.
 *    Actual update of localChannelTable and globalChannelTable happens during
 *    PreCommit_Notify, with staged changes committed in AtCommit_Notify.
 */
static void
queue_listen(ListenActionKind action, const char *channel)
{
  MemoryContext oldcontext;
  ListenAction *actrec;
  int     my_level = GetCurrentTransactionNestLevel();

  /*
   * Unlike Async_Notify, we don't try to collapse out duplicates here. We
   * keep the ordered list to preserve interactions like UNLISTEN ALL; the
   * final per-channel intent is computed during PreCommit_Notify.
   */
  oldcontext = MemoryContextSwitchTo(CurTransactionContext);

  /* space for terminating null is included in sizeof(ListenAction) */
  actrec = (ListenAction *) palloc(offsetof(ListenAction, channel) +
                   strlen(channel) + 1);
  actrec->action = action;
  strcpy(actrec->channel, channel);

  if (pendingActions == NULL || my_level > pendingActions->nestingLevel)
  {
    ActionList *actions;

    /*
     * First action in current sub(xact). Note that we allocate the
     * ActionList in TopTransactionContext; the nestingLevel might get
     * changed later by AtSubCommit_Notify.
     */
    actions = (ActionList *)
      MemoryContextAlloc(TopTransactionContext, sizeof(ActionList));
    actions->nestingLevel = my_level;
    actions->actions = list_make1(actrec);
    actions->upper = pendingActions;
    pendingActions = actions;
  }
  else
    pendingActions->actions = lappend(pendingActions->actions, actrec);

  MemoryContextSwitchTo(oldcontext);
}

/*
 * Async_Listen
 *
 *    This is executed by the SQL listen command.
 */
void
Async_Listen(const char *channel)
{
  if (Trace_notify)
    elog(DEBUG1, "Async_Listen(%s,%d)", channel, MyProcPid);

  queue_listen(LISTEN_LISTEN, channel);
}

/*
 * Async_Unlisten
 *
 *    This is executed by the SQL unlisten command.
 */
void
Async_Unlisten(const char *channel)
{
  if (Trace_notify)
    elog(DEBUG1, "Async_Unlisten(%s,%d)", channel, MyProcPid);

  /* If we couldn't possibly be listening, no need to queue anything */
  if (pendingActions == NULL && !unlistenExitRegistered)
    return;

  queue_listen(LISTEN_UNLISTEN, channel);
}

/*
 * Async_UnlistenAll
 *
 *    This is invoked by UNLISTEN * command, and also at backend exit.
 */
void
Async_UnlistenAll(void)
{
  if (Trace_notify)
    elog(DEBUG1, "Async_UnlistenAll(%d)", MyProcPid);

  /* If we couldn't possibly be listening, no need to queue anything */
  if (pendingActions == NULL && !unlistenExitRegistered)
    return;

  queue_listen(LISTEN_UNLISTEN_ALL, "");
}

/*
 * SQL function: return a set of the channel names this backend is actively
 * listening to.
 */
Datum
pg_listening_channels(PG_FUNCTION_ARGS)
{
  ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;

  InitMaterializedSRF(fcinfo, MAT_SRF_USE_EXPECTED_DESC);

  /*
   * Complete the hash scan in one call, since the caller might not fetch
   * all the rows and would otherwise leave the scan registered until
   * transaction end.
   */
  if (localChannelTable != NULL)
  {
    HASH_SEQ_STATUS status;
    ChannelName *channelEntry;

    hash_seq_init(&status, localChannelTable);
    while ((channelEntry = (ChannelName *) hash_seq_search(&status)) != NULL)
    {
      Datum   values[1];
      bool    nulls[1];

      values[0] = CStringGetTextDatum(channelEntry->channel);
      nulls[0] = false;
      tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                 values, nulls);
    }
  }

  return (Datum) 0;
}

/*
 * Async_UnlistenOnExit
 *
 * This is executed at backend exit if we have done any LISTENs in this
 * backend.  It might not be necessary anymore, if the user UNLISTENed
 * everything, but we don't try to detect that case.
 */
static void
Async_UnlistenOnExit(int code, Datum arg)
{
  CleanupListenersOnExit();
  asyncQueueUnregister();
}

/*
 * AtPrepare_Notify
 *
 *    This is called at the prepare phase of a two-phase
 *    transaction.  Save the state for possible commit later.
 */
void
AtPrepare_Notify(void)
{
  /* It's not allowed to have any pending LISTEN/UNLISTEN/NOTIFY actions */
  if (pendingActions || pendingNotifies)
    ereport(ERROR,
        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
         errmsg("cannot PREPARE a transaction that has executed LISTEN, UNLISTEN, or NOTIFY")));
}

/*
 * PreCommit_Notify
 *
 *    This is called at transaction commit, before actually committing to
 *    clog.
 *
 *    If there are pending LISTEN actions, make sure we are listed in the
 *    shared-memory listener array.  This must happen before commit to
 *    ensure we don't miss any notifies from transactions that commit
 *    just after ours.
 *
 *    If there are outbound notify requests in the pendingNotifies list,
 *    add them to the global queue.  We do that before commit so that
 *    we can still throw error if we run out of queue space.
 */
void
PreCommit_Notify(void)
{
  ListCell   *p;

  if (!pendingActions && !pendingNotifies)
    return;         /* no relevant statements in this xact */

  if (Trace_notify)
    elog(DEBUG1, "PreCommit_Notify");

  /* Preflight for any pending listen/unlisten actions */
  initGlobalChannelTable();

  if (pendingActions != NULL)
  {
    /* Ensure we have a local channel table */
    initLocalChannelTable();
    /* Create pendingListenActions hash table for this transaction */
    initPendingListenActions();

    /* Stage all the actions this transaction wants to perform */
    foreach(p, pendingActions->actions)
    {
      ListenAction *actrec = (ListenAction *) lfirst(p);

      switch (actrec->action)
      {
        case LISTEN_LISTEN:
          BecomeRegisteredListener();
          PrepareTableEntriesForListen(actrec->channel);
          break;
        case LISTEN_UNLISTEN:
          PrepareTableEntriesForUnlisten(actrec->channel);
          break;
        case LISTEN_UNLISTEN_ALL:
          PrepareTableEntriesForUnlistenAll();
          break;
      }
    }
  }

  /* Queue any pending notifies (must happen after the above) */
  if (pendingNotifies)
  {
    ListCell   *nextNotify;
    bool    firstIteration = true;

    /*
     * Build list of unique channel names being notified for use by
     * SignalBackends().
     *
     * If uniqueChannelHash is available, use it to efficiently get the
     * unique channels.  Otherwise, fall back to the O(N^2) approach.
     */
    pendingNotifies->uniqueChannelNames = NIL;
    if (pendingNotifies->uniqueChannelHash != NULL)
    {
      HASH_SEQ_STATUS status;
      ChannelName *channelEntry;

      hash_seq_init(&status, pendingNotifies->uniqueChannelHash);
      while ((channelEntry = (ChannelName *) hash_seq_search(&status)) != NULL)
        pendingNotifies->uniqueChannelNames =
          lappend(pendingNotifies->uniqueChannelNames,
              channelEntry->channel);
    }
    else
    {
      /* O(N^2) approach is better for small number of notifications */
      foreach_ptr(Notification, n, pendingNotifies->events)
      {
        char     *channel = n->data;
        bool    found = false;

        /* Name present in list? */
        foreach_ptr(char, oldchan, pendingNotifies->uniqueChannelNames)
        {
          if (strcmp(oldchan, channel) == 0)
          {
            found = true;
            break;
          }
        }
        /* Add if not already in list */
        if (!found)
          pendingNotifies->uniqueChannelNames =
            lappend(pendingNotifies->uniqueChannelNames,
                channel);
      }
    }

    /* Preallocate workspace that will be needed by SignalBackends() */
    if (signalPids == NULL)
      signalPids = MemoryContextAlloc(TopMemoryContext,
                      MaxBackends * sizeof(int32));

    if (signalProcnos == NULL)
      signalProcnos = MemoryContextAlloc(TopMemoryContext,
                         MaxBackends * sizeof(ProcNumber));

    /*
     * Make sure that we have an XID assigned to the current transaction.
     * GetCurrentTransactionId is cheap if we already have an XID, but not
     * so cheap if we don't, and we'd prefer not to do that work while
     * holding NotifyQueueLock.
     */
    (void) GetCurrentTransactionId();

    /*
     * Serialize writers by acquiring a special lock that we hold till
     * after commit.  This ensures that queue entries appear in commit
     * order, and in particular that there are never uncommitted queue
     * entries ahead of committed ones, so an uncommitted transaction
     * can't block delivery of deliverable notifications.
     *
     * We use a heavyweight lock so that it'll automatically be released
     * after either commit or abort.  This also allows deadlocks to be
     * detected, though really a deadlock shouldn't be possible here.
     *
     * The lock is on "database 0", which is pretty ugly but it doesn't
     * seem worth inventing a special locktag category just for this.
     * (Historical note: before PG 9.0, a similar lock on "database 0" was
     * used by the flatfiles mechanism.)
     */
    LockSharedObject(DatabaseRelationId, InvalidOid, 0,
             AccessExclusiveLock);

    /*
     * For the direct advancement optimization in SignalBackends(), we
     * need to ensure that no other backend can insert queue entries
     * between queueHeadBeforeWrite and queueHeadAfterWrite.  The
     * heavyweight lock above provides this guarantee, since it serializes
     * all writers.
     *
     * Note: if the heavyweight lock were ever removed for scalability
     * reasons, we could achieve the same guarantee by holding
     * NotifyQueueLock in EXCLUSIVE mode across all our insertions, rather
     * than releasing and reacquiring it for each page as we do below.
     */

    /* Initialize values to a safe default in case list is empty */
    SET_QUEUE_POS(queueHeadBeforeWrite, 0, 0);
    SET_QUEUE_POS(queueHeadAfterWrite, 0, 0);

    /* Now push the notifications into the queue */
    nextNotify = list_head(pendingNotifies->events);
    while (nextNotify != NULL)
    {
      /*
       * Add the pending notifications to the queue.  We acquire and
       * release NotifyQueueLock once per page, which might be overkill
       * but it does allow readers to get in while we're doing this.
       *
       * A full queue is very uncommon and should really not happen,
       * given that we have so much space available in the SLRU pages.
       * Nevertheless we need to deal with this possibility. Note that
       * when we get here we are in the process of committing our
       * transaction, but we have not yet committed to clog, so at this
       * point in time we can still roll the transaction back.
       */
      LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
      if (firstIteration)
      {
        queueHeadBeforeWrite = QUEUE_HEAD;
        firstIteration = false;
      }
      asyncQueueFillWarning();
      if (asyncQueueIsFull())
        ereport(ERROR,
            (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
             errmsg("too many notifications in the NOTIFY queue")));
      nextNotify = asyncQueueAddEntries(nextNotify);
      queueHeadAfterWrite = QUEUE_HEAD;
      LWLockRelease(NotifyQueueLock);
    }

    /* Note that we don't clear pendingNotifies; AtCommit_Notify will. */
  }
}

/*
 * AtCommit_Notify
 *
 *    This is called at transaction commit, after committing to clog.
 *
 *    Apply pending listen/unlisten changes and clear transaction-local state.
 *
 *    If we issued any notifications in the transaction, send signals to
 *    listening backends (possibly including ourselves) to process them.
 *    Also, if we filled enough queue pages with new notifies, try to
 *    advance the queue tail pointer.
 */
void
AtCommit_Notify(void)
{
  /*
   * Allow transactions that have not executed LISTEN/UNLISTEN/NOTIFY to
   * return as soon as possible
   */
  if (!pendingActions && !pendingNotifies)
    return;

  if (Trace_notify)
    elog(DEBUG1, "AtCommit_Notify");

  /* Apply staged listen/unlisten changes */
  ApplyPendingListenActions(true);

  /* If no longer listening to anything, get out of listener array */
  if (amRegisteredListener && LocalChannelTableIsEmpty())
    asyncQueueUnregister();

  /*
   * Send signals to listening backends.  We need do this only if there are
   * pending notifies, which were previously added to the shared queue by
   * PreCommit_Notify().
   */
  if (pendingNotifies != NULL)
    SignalBackends();

  /*
   * If it's time to try to advance the global tail pointer, do that.
   *
   * (It might seem odd to do this in the sender, when more than likely the
   * listeners won't yet have read the messages we just sent.  However,
   * there's less contention if only the sender does it, and there is little
   * need for urgency in advancing the global tail.  So this typically will
   * be clearing out messages that were sent some time ago.)
   */
  if (tryAdvanceTail)
  {
    tryAdvanceTail = false;
    asyncQueueAdvanceTail();
  }

  /* And clean up */
  ClearPendingActionsAndNotifies();
}

/*
 * BecomeRegisteredListener --- subroutine for PreCommit_Notify
 *
 * This function must make sure we are ready to catch any incoming messages.
 */
static void
BecomeRegisteredListener(void)
{
  QueuePosition head;
  QueuePosition max;
  ProcNumber  prevListener;

  /*
   * Nothing to do if we are already listening to something, nor if we
   * already ran this routine in this transaction.
   */
  if (amRegisteredListener)
    return;

  if (Trace_notify)
    elog(DEBUG1, "BecomeRegisteredListener(%d)", MyProcPid);

  /*
   * Before registering, make sure we will unlisten before dying. (Note:
   * this action does not get undone if we abort later.)
   */
  if (!unlistenExitRegistered)
  {
    before_shmem_exit(Async_UnlistenOnExit, 0);
    unlistenExitRegistered = true;
  }

  /*
   * This is our first LISTEN, so establish our pointer.
   *
   * We set our pointer to the global tail pointer and then move it forward
   * over already-committed notifications.  This ensures we cannot miss any
   * not-yet-committed notifications.  We might get a few more but that
   * doesn't hurt.
   *
   * In some scenarios there might be a lot of committed notifications that
   * have not yet been pruned away (because some backend is being lazy about
   * reading them).  To reduce our startup time, we can look at other
   * backends and adopt the maximum "pos" pointer of any backend that's in
   * our database; any notifications it's already advanced over are surely
   * committed and need not be re-examined by us.  (We must consider only
   * backends connected to our DB, because others will not have bothered to
   * check committed-ness of notifications in our DB.)
   *
   * We need exclusive lock here so we can look at other backends' entries
   * and manipulate the list links.
   */
  LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
  head = QUEUE_HEAD;
  max = QUEUE_TAIL;
  prevListener = INVALID_PROC_NUMBER;
  for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
  {
    if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
      max = QUEUE_POS_MAX(max, QUEUE_BACKEND_POS(i));
    /* Also find last listening backend before this one */
    if (i < MyProcNumber)
      prevListener = i;
  }
  QUEUE_BACKEND_POS(MyProcNumber) = max;
  QUEUE_BACKEND_PID(MyProcNumber) = MyProcPid;
  QUEUE_BACKEND_DBOID(MyProcNumber) = MyDatabaseId;
  QUEUE_BACKEND_WAKEUP_PENDING(MyProcNumber) = false;
  QUEUE_BACKEND_IS_ADVANCING(MyProcNumber) = false;
  /* Insert backend into list of listeners at correct position */
  if (prevListener != INVALID_PROC_NUMBER)
  {
    QUEUE_NEXT_LISTENER(MyProcNumber) = QUEUE_NEXT_LISTENER(prevListener);
    QUEUE_NEXT_LISTENER(prevListener) = MyProcNumber;
  }
  else
  {
    QUEUE_NEXT_LISTENER(MyProcNumber) = QUEUE_FIRST_LISTENER;
    QUEUE_FIRST_LISTENER = MyProcNumber;
  }
  LWLockRelease(NotifyQueueLock);

  /* Now we are listed in the global array, so remember we're listening */
  amRegisteredListener = true;

  /*
   * Try to move our pointer forward as far as possible.  This will skip
   * over already-committed notifications, which we want to do because they
   * might be quite stale.  Note that we are not yet listening on anything,
   * so we won't deliver such notifications to our frontend.  Also, although
   * our transaction might have executed NOTIFY, those message(s) aren't
   * queued yet so we won't skip them here.
   */
  if (!QUEUE_POS_EQUAL(max, head))
    asyncQueueReadAllNotifications();
}

/*
 * PrepareTableEntriesForListen --- subroutine for PreCommit_Notify
 *
 * Prepare a LISTEN by recording it in pendingListenActions, pre-allocating
 * an entry in localChannelTable, and pre-allocating an entry in the shared
 * globalChannelTable with removeOnAbort set.  AtCommit_Notify will clear
 * removeOnAbort; abort processing will remove entries still marked so.
 */
static void
PrepareTableEntriesForListen(const char *channel)
{
  GlobalChannelKey key;
  GlobalChannelEntry *entry;
  bool    found;
  ListenerEntry *listeners;
  PendingListenEntry *pending;

  /*
   * Record in local pending hash that we want to LISTEN, overwriting any
   * earlier attempt to UNLISTEN.
   */
  pending = (PendingListenEntry *)
    hash_search(pendingListenActions, channel, HASH_ENTER, NULL);
  pending->action = PENDING_LISTEN;

  /*
   * Ensure that there is an entry for the channel in localChannelTable.
   * (Should this fail, we can just roll back.)  If the transaction fails
   * after this point, we will remove the entry if appropriate during
   * ApplyPendingListenActions.  Note that this entry allows IsListeningOn()
   * to return TRUE; we assume nothing is going to consult that before
   * AtCommit_Notify/AtAbort_Notify.  However, if later actions attempt to
   * UNLISTEN this channel or UNLISTEN *, we need to have the local entry
   * present to ensure they do the right things; see
   * PrepareTableEntriesForUnlisten and PrepareTableEntriesForUnlistenAll.
   */
  (void) hash_search(localChannelTable, channel, HASH_ENTER, NULL);

  /* Pre-allocate entry in shared globalChannelTable */
  GlobalChannelKeyInit(&key, MyDatabaseId, channel);
  entry = dshash_find_or_insert(globalChannelTable, &key, &found);

  if (!found)
  {
    /* New channel entry, so initialize it to a safe state */
    entry->listenersArray = InvalidDsaPointer;
    entry->numListeners = 0;
    entry->allocatedListeners = 0;
  }

  /*
   * Create listenersArray if entry doesn't have one.  It's tempting to fold
   * this into the !found case, but this coding allows us to cope in case
   * dsa_allocate() failed in an earlier attempt.
   */
  if (!DsaPointerIsValid(entry->listenersArray))
  {
    entry->listenersArray = dsa_allocate(globalChannelDSA,
                       sizeof(ListenerEntry) * INITIAL_LISTENERS_ARRAY_SIZE);
    entry->allocatedListeners = INITIAL_LISTENERS_ARRAY_SIZE;
  }

  listeners = (ListenerEntry *)
    dsa_get_address(globalChannelDSA, entry->listenersArray);

  /*
   * Check if we already have a ListenerEntry (possibly from earlier in this
   * transaction)
   */
  for (int i = 0; i < entry->numListeners; i++)
  {
    if (listeners[i].procNo == MyProcNumber)
    {
      /* Already have an entry; leave removeOnAbort as-is */
      dshash_release_lock(globalChannelTable, entry);
      return;
    }
  }

  /* Need to add a new entry; grow array if necessary */
  if (entry->numListeners >= entry->allocatedListeners)
  {
    int     new_size = entry->allocatedListeners * 2;
    dsa_pointer old_array = entry->listenersArray;
    dsa_pointer new_array = dsa_allocate(globalChannelDSA,
                       sizeof(ListenerEntry) * new_size);
    ListenerEntry *new_listeners = (ListenerEntry *) dsa_get_address(globalChannelDSA, new_array);

    memcpy(new_listeners, listeners, sizeof(ListenerEntry) * entry->numListeners);
    entry->listenersArray = new_array;
    entry->allocatedListeners = new_size;
    dsa_free(globalChannelDSA, old_array);
    listeners = new_listeners;
  }

  listeners[entry->numListeners].procNo = MyProcNumber;
  listeners[entry->numListeners].removeOnAbort = true;
  entry->numListeners++;

  dshash_release_lock(globalChannelTable, entry);
}

/*
 * PrepareTableEntriesForUnlisten --- subroutine for PreCommit_Notify
 *
 * Prepare an UNLISTEN by recording it in pendingListenActions, but only if
 * we're currently listening (committed or staged).  We don't touch
 * globalChannelTable yet - the listener keeps receiving signals until
 * commit, when the entry is removed.
 */
static void
PrepareTableEntriesForUnlisten(const char *channel)
{
  PendingListenEntry *pending;

  /*
   * If the channel name is not in localChannelTable, then we are neither
   * listening on it nor preparing to listen on it, so we don't need to
   * record an UNLISTEN action.
   */
  Assert(localChannelTable != NULL);
  if (hash_search(localChannelTable, channel, HASH_FIND, NULL) == NULL)
    return;

  /*
   * Record in local pending hash that we want to UNLISTEN, overwriting any
   * earlier attempt to LISTEN.  Don't touch localChannelTable or
   * globalChannelTable yet - we keep receiving signals until commit.
   */
  pending = (PendingListenEntry *)
    hash_search(pendingListenActions, channel, HASH_ENTER, NULL);
  pending->action = PENDING_UNLISTEN;
}

/*
 * PrepareTableEntriesForUnlistenAll --- subroutine for PreCommit_Notify
 *
 * Prepare UNLISTEN * by recording an UNLISTEN for all listened or
 * about-to-be-listened channels in pendingListenActions.
 */
static void
PrepareTableEntriesForUnlistenAll(void)
{
  HASH_SEQ_STATUS seq;
  ChannelName *channelEntry;
  PendingListenEntry *pending;

  /*
   * Scan localChannelTable, which will have the names of all channels that
   * we are listening on or have prepared to listen on.  Record an UNLISTEN
   * action for each one, overwriting any earlier attempt to LISTEN.
   */
  hash_seq_init(&seq, localChannelTable);
  while ((channelEntry = (ChannelName *) hash_seq_search(&seq)) != NULL)
  {
    pending = (PendingListenEntry *)
      hash_search(pendingListenActions, channelEntry->channel, HASH_ENTER, NULL);
    pending->action = PENDING_UNLISTEN;
  }
}

/*
 * RemoveListenerFromChannel --- remove idx'th listener from global channel entry
 *
 * Decrements numListeners, compacts the array, and frees the entry if empty.
 * Sets *entry_ptr to NULL if the entry was deleted.
 *
 * We could get the listeners pointer from the entry, but all callers
 * already have it at hand.
 */
static void
RemoveListenerFromChannel(GlobalChannelEntry **entry_ptr,
              ListenerEntry *listeners,
              int idx)
{
  GlobalChannelEntry *entry = *entry_ptr;

  entry->numListeners--;
  if (idx < entry->numListeners)
    memmove(&listeners[idx], &listeners[idx + 1],
        sizeof(ListenerEntry) * (entry->numListeners - idx));

  if (entry->numListeners == 0)
  {
    dsa_free(globalChannelDSA, entry->listenersArray);
    dshash_delete_entry(globalChannelTable, entry);
    /* tells caller not to release the entry's lock: */
    *entry_ptr = NULL;
  }
}

/*
 * ApplyPendingListenActions
 *
 * Apply, or revert, staged listen/unlisten changes to the local and global
 * hash tables.
 */
static void
ApplyPendingListenActions(bool isCommit)
{
  HASH_SEQ_STATUS seq;
  PendingListenEntry *pending;

  /* Quick exit if nothing to do */
  if (pendingListenActions == NULL)
    return;

  /* We made a globalChannelTable before building pendingListenActions */
  if (globalChannelTable == NULL)
    elog(PANIC, "global channel table missing post-commit/abort");

  /* For each staged action ... */
  hash_seq_init(&seq, pendingListenActions);
  while ((pending = (PendingListenEntry *) hash_seq_search(&seq)) != NULL)
  {
    GlobalChannelKey key;
    GlobalChannelEntry *entry;
    bool    removeLocal = true;
    bool    foundListener = false;

    /*
     * Find the global entry for this channel.  If isCommit, it had better
     * exist (it was created in PreCommit).  In an abort, it might not
     * exist, in which case we are not listening and should discard any
     * local entry that PreCommit may have managed to create.
     */
    GlobalChannelKeyInit(&key, MyDatabaseId, pending->channel);
    entry = dshash_find(globalChannelTable, &key, true);
    if (entry != NULL)
    {
      /* Scan entry to find the ListenerEntry for this backend */
      ListenerEntry *listeners;

      listeners = (ListenerEntry *)
        dsa_get_address(globalChannelDSA, entry->listenersArray);

      for (int i = 0; i < entry->numListeners; i++)
      {
        if (listeners[i].procNo != MyProcNumber)
          continue;
        foundListener = true;
        if (isCommit)
        {
          if (pending->action == PENDING_LISTEN)
          {
            /*
             * LISTEN being committed: entry is now permanent.
             * localChannelTable entry was created during
             * PreCommit and should be kept.
             */
            listeners[i].removeOnAbort = false;
            removeLocal = false;
          }
          else
          {
            /*
             * UNLISTEN being committed: remove pre-allocated
             * entries from both tables.
             */
            RemoveListenerFromChannel(&entry, listeners, i);
          }
        }
        else
        {
          /*
           * Note: this part is reachable only if the transaction
           * aborts after PreCommit_Notify() has made some
           * pendingListenActions entries, so it's pretty hard to
           * test.
           */
          if (listeners[i].removeOnAbort)
          {
            /*
             * Staged LISTEN (or LISTEN+UNLISTEN) being aborted,
             * so remove pre-allocated entries from both tables.
             */
            RemoveListenerFromChannel(&entry, listeners, i);
          }
          else
          {
            /*
             * Entry predates this transaction, so keep the
             * localChannelTable entry.
             */
            removeLocal = false;
          }
        }
        break;      /* there shouldn't be another match */
      }

      /* We might have already released the entry by removing it */
      if (entry != NULL)
        dshash_release_lock(globalChannelTable, entry);
    }

    /*
     * If we're committing a LISTEN action, we should have found a
     * matching ListenerEntry, but otherwise it's okay if we didn't.
     */
    if (isCommit && pending->action == PENDING_LISTEN && !foundListener)
      elog(PANIC, "could not find listener entry for channel \"%s\" backend %d",
         pending->channel, MyProcNumber);

    /*
     * If we did not find a globalChannelTable entry for our backend, or
     * if we are unlistening, remove any localChannelTable entry that may
     * exist.  (Note in particular that this cleans up if we created a
     * localChannelTable entry and then failed while trying to create a
     * globalChannelTable entry.)
     */
    if (removeLocal && localChannelTable != NULL)
      (void) hash_search(localChannelTable, pending->channel,
                 HASH_REMOVE, NULL);
  }
}

/*
 * CleanupListenersOnExit --- called from Async_UnlistenOnExit
 *
 *    Remove this backend from all channels in the shared global table.
 */
static void
CleanupListenersOnExit(void)
{
  dshash_seq_status status;
  GlobalChannelEntry *entry;

  if (Trace_notify)
    elog(DEBUG1, "CleanupListenersOnExit(%d)", MyProcPid);

  /* Clear our local cache (not really necessary, but be consistent) */
  if (localChannelTable != NULL)
  {
    hash_destroy(localChannelTable);
    localChannelTable = NULL;
  }

  /* Now remove our entries from the shared globalChannelTable */
  if (globalChannelTable == NULL)
    return;

  dshash_seq_init(&status, globalChannelTable, true);
  while ((entry = dshash_seq_next(&status)) != NULL)
  {
    ListenerEntry *listeners;

    if (entry->key.dboid != MyDatabaseId)
      continue;     /* not relevant */

    listeners = (ListenerEntry *)
      dsa_get_address(globalChannelDSA, entry->listenersArray);

    for (int i = 0; i < entry->numListeners; i++)
    {
      if (listeners[i].procNo == MyProcNumber)
      {
        entry->numListeners--;
        if (i < entry->numListeners)
          memmove(&listeners[i], &listeners[i + 1],
              sizeof(ListenerEntry) * (entry->numListeners - i));

        if (entry->numListeners == 0)
        {
          dsa_free(globalChannelDSA, entry->listenersArray);
          dshash_delete_current(&status);
        }
        break;
      }
    }
  }
  dshash_seq_term(&status);
}

/*
 * Test whether we are actively listening on the given channel name.
 *
 * Note: this function is executed for every notification found in the queue.
 */
static bool
IsListeningOn(const char *channel)
{
  if (localChannelTable == NULL)
    return false;

  return (hash_search(localChannelTable, channel, HASH_FIND, NULL) != NULL);
}

/*
 * Remove our entry from the listeners array when we are no longer listening
 * on any channel.  NB: must not fail if we're already not listening.
 */
static void
asyncQueueUnregister(void)
{
  Assert(LocalChannelTableIsEmpty()); /* else caller error */

  if (!amRegisteredListener) /* nothing to do */
    return;

  /*
   * Need exclusive lock here to manipulate list links.
   */
  LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
  /* Mark our entry as invalid */
  QUEUE_BACKEND_PID(MyProcNumber) = InvalidPid;
  QUEUE_BACKEND_DBOID(MyProcNumber) = InvalidOid;
  QUEUE_BACKEND_WAKEUP_PENDING(MyProcNumber) = false;
  QUEUE_BACKEND_IS_ADVANCING(MyProcNumber) = false;
  /* and remove it from the list */
  if (QUEUE_FIRST_LISTENER == MyProcNumber)
    QUEUE_FIRST_LISTENER = QUEUE_NEXT_LISTENER(MyProcNumber);
  else
  {
    for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
    {
      if (QUEUE_NEXT_LISTENER(i) == MyProcNumber)
      {
        QUEUE_NEXT_LISTENER(i) = QUEUE_NEXT_LISTENER(MyProcNumber);
        break;
      }
    }
  }
  QUEUE_NEXT_LISTENER(MyProcNumber) = INVALID_PROC_NUMBER;
  LWLockRelease(NotifyQueueLock);

  /* mark ourselves as no longer listed in the global array */
  amRegisteredListener = false;
}

/*
 * Test whether there is room to insert more notification messages.
 *
 * Caller must hold at least shared NotifyQueueLock.
 */
static bool
asyncQueueIsFull(void)
{
  int64   headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
  int64   tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
  int64   occupied = headPage - tailPage;

  return occupied >= max_notify_queue_pages;
}

/*
 * Advance the QueuePosition to the next entry, assuming that the current
 * entry is of length entryLength.  If we jump to a new page the function
 * returns true, else false.
 */
static bool
asyncQueueAdvance(QueuePosition *position, int entryLength)
{
  int64   pageno = QUEUE_POS_PAGE(*position);
  int     offset = QUEUE_POS_OFFSET(*position);
  bool    pageJump = false;

  /*
   * Move to the next writing position: First jump over what we have just
   * written or read.
   */
  offset += entryLength;
  Assert(offset <= QUEUE_PAGESIZE);

  /*
   * In a second step check if another entry can possibly be written to the
   * page. If so, stay here, we have reached the next position. If not, then
   * we need to move on to the next page.
   */
  if (offset + QUEUEALIGN(AsyncQueueEntryEmptySize) > QUEUE_PAGESIZE)
  {
    pageno++;
    offset = 0;
    pageJump = true;
  }

  SET_QUEUE_POS(*position, pageno, offset);
  return pageJump;
}

/*
 * Fill the AsyncQueueEntry at *qe with an outbound notification message.
 */
static void
asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe)
{
  size_t    channellen = n->channel_len;
  size_t    payloadlen = n->payload_len;
  int     entryLength;

  Assert(channellen < NAMEDATALEN);
  Assert(payloadlen < NOTIFY_PAYLOAD_MAX_LENGTH);

  /* The terminators are already included in AsyncQueueEntryEmptySize */
  entryLength = AsyncQueueEntryEmptySize + payloadlen + channellen;
  entryLength = QUEUEALIGN(entryLength);
  qe->length = entryLength;
  qe->dboid = MyDatabaseId;
  qe->xid = GetCurrentTransactionId();
  qe->srcPid = MyProcPid;
  memcpy(qe->data, n->data, channellen + payloadlen + 2);
}

/*
 * Add pending notifications to the queue.
 *
 * We go page by page here, i.e. we stop once we have to go to a new page but
 * we will be called again and then fill that next page. If an entry does not
 * fit into the current page, we write a dummy entry with an InvalidOid as the
 * database OID in order to fill the page. So every page is always used up to
 * the last byte which simplifies reading the page later.
 *
 * We are passed the list cell (in pendingNotifies->events) containing the next
 * notification to write and return the first still-unwritten cell back.
 * Eventually we will return NULL indicating all is done.
 *
 * We are holding NotifyQueueLock already from the caller and grab
 * page specific SLRU bank lock locally in this function.
 */
static ListCell *
asyncQueueAddEntries(ListCell *nextNotify)
{
  AsyncQueueEntry qe;
  QueuePosition queue_head;
  int64   pageno;
  int     offset;
  int     slotno;
  LWLock     *prevlock;

  /*
   * We work with a local copy of QUEUE_HEAD, which we write back to shared
   * memory upon exiting.  The reason for this is that if we have to advance
   * to a new page, SimpleLruZeroPage might fail (out of disk space, for
   * instance), and we must not advance QUEUE_HEAD if it does.  (Otherwise,
   * subsequent insertions would try to put entries into a page that slru.c
   * thinks doesn't exist yet.)  So, use a local position variable.  Note
   * that if we do fail, any already-inserted queue entries are forgotten;
   * this is okay, since they'd be useless anyway after our transaction
   * rolls back.
   */
  queue_head = QUEUE_HEAD;

  /*
   * If this is the first write since the postmaster started, we need to
   * initialize the first page of the async SLRU.  Otherwise, the current
   * page should be initialized already, so just fetch it.
   */
  pageno = QUEUE_POS_PAGE(queue_head);
  prevlock = SimpleLruGetBankLock(NotifyCtl, pageno);

  /* We hold both NotifyQueueLock and SLRU bank lock during this operation */
  LWLockAcquire(prevlock, LW_EXCLUSIVE);

  if (QUEUE_POS_IS_ZERO(queue_head))
    slotno = SimpleLruZeroPage(NotifyCtl, pageno);
  else
    slotno = SimpleLruReadPage(NotifyCtl, pageno, true, &queue_head);

  /* Note we mark the page dirty before writing in it */
  NotifyCtl->shared->page_dirty[slotno] = true;

  while (nextNotify != NULL)
  {
    Notification *n = (Notification *) lfirst(nextNotify);

    /* Construct a valid queue entry in local variable qe */
    asyncQueueNotificationToEntry(n, &qe);

    offset = QUEUE_POS_OFFSET(queue_head);

    /* Check whether the entry really fits on the current page */
    if (offset + qe.length <= QUEUE_PAGESIZE)
    {
      /* OK, so advance nextNotify past this item */
      nextNotify = lnext(pendingNotifies->events, nextNotify);
    }
    else
    {
      /*
       * Write a dummy entry to fill up the page. Actually readers will
       * only check dboid and since it won't match any reader's database
       * OID, they will ignore this entry and move on.
       */
      qe.length = QUEUE_PAGESIZE - offset;
      qe.dboid = InvalidOid;
      qe.xid = InvalidTransactionId;
      qe.data[0] = '\0';  /* empty channel */
      qe.data[1] = '\0';  /* empty payload */
    }

    /* Now copy qe into the shared buffer page */
    memcpy(NotifyCtl->shared->page_buffer[slotno] + offset,
         &qe,
         qe.length);

    /* Advance queue_head appropriately, and detect if page is full */
    if (asyncQueueAdvance(&(queue_head), qe.length))
    {
      LWLock     *lock;

      pageno = QUEUE_POS_PAGE(queue_head);
      lock = SimpleLruGetBankLock(NotifyCtl, pageno);
      if (lock != prevlock)
      {
        LWLockRelease(prevlock);
        LWLockAcquire(lock, LW_EXCLUSIVE);
        prevlock = lock;
      }

      /*
       * Page is full, so we're done here, but first fill the next page
       * with zeroes.  The reason to do this is to ensure that slru.c's
       * idea of the head page is always the same as ours, which avoids
       * boundary problems in SimpleLruTruncate.  The test in
       * asyncQueueIsFull() ensured that there is room to create this
       * page without overrunning the queue.
       */
      slotno = SimpleLruZeroPage(NotifyCtl, QUEUE_POS_PAGE(queue_head));

      /*
       * If the new page address is a multiple of QUEUE_CLEANUP_DELAY,
       * set flag to remember that we should try to advance the tail
       * pointer (we don't want to actually do that right here).
       */
      if (QUEUE_POS_PAGE(queue_head) % QUEUE_CLEANUP_DELAY == 0)
        tryAdvanceTail = true;

      /* And exit the loop */
      break;
    }
  }

  /* Success, so update the global QUEUE_HEAD */
  QUEUE_HEAD = queue_head;

  LWLockRelease(prevlock);

  return nextNotify;
}

/*
 * SQL function to return the fraction of the notification queue currently
 * occupied.
 */
Datum
pg_notification_queue_usage(PG_FUNCTION_ARGS)
{
  double    usage;

  /* Advance the queue tail so we don't report a too-large result */
  asyncQueueAdvanceTail();

  LWLockAcquire(NotifyQueueLock, LW_SHARED);
  usage = asyncQueueUsage();
  LWLockRelease(NotifyQueueLock);

  PG_RETURN_FLOAT8(usage);
}

/*
 * Return the fraction of the queue that is currently occupied.
 *
 * The caller must hold NotifyQueueLock in (at least) shared mode.
 *
 * Note: we measure the distance to the logical tail page, not the physical
 * tail page.  In some sense that's wrong, but the relative position of the
 * physical tail is affected by details such as SLRU segment boundaries,
 * so that a result based on that is unpleasantly unstable.
 */
static double
asyncQueueUsage(void)
{
  int64   headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
  int64   tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
  int64   occupied = headPage - tailPage;

  if (occupied == 0)
    return (double) 0;   /* fast exit for common case */

  return (double) occupied / (double) max_notify_queue_pages;
}

/*
 * Check whether the queue is at least half full, and emit a warning if so.
 *
 * This is unlikely given the size of the queue, but possible.
 * The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
 *
 * Caller must hold exclusive NotifyQueueLock.
 */
static void
asyncQueueFillWarning(void)
{
  double    fillDegree;
  TimestampTz t;

  fillDegree = asyncQueueUsage();
  if (fillDegree < 0.5)
    return;

  t = GetCurrentTimestamp();

  if (TimestampDifferenceExceeds(asyncQueueControl->lastQueueFillWarn,
                   t, QUEUE_FULL_WARN_INTERVAL))
  {
    QueuePosition min = QUEUE_HEAD;
    int32   minPid = InvalidPid;

    for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
    {
      Assert(QUEUE_BACKEND_PID(i) != InvalidPid);
      min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
      if (QUEUE_POS_EQUAL(min, QUEUE_BACKEND_POS(i)))
        minPid = QUEUE_BACKEND_PID(i);
    }

    ereport(WARNING,
        (errmsg("NOTIFY queue is %.0f%% full", fillDegree * 100),
         (minPid != InvalidPid ?
          errdetail("The server process with PID %d is among those with the oldest transactions.", minPid)
          : 0),
         (minPid != InvalidPid ?
          errhint("The NOTIFY queue cannot be emptied until that process ends its current transaction.")
          : 0)));

    asyncQueueControl->lastQueueFillWarn = t;
  }
}

/*
 * Send signals to listening backends.
 *
 * Normally we signal only backends that are interested in the notifies that
 * we just sent.  However, that will leave idle listeners falling further and
 * further behind.  Waken them anyway if they're far enough behind, so they'll
 * advance their queue position pointers, allowing the global tail to advance.
 *
 * Since we know the ProcNumber and the Pid the signaling is quite cheap.
 *
 * This is called during CommitTransaction(), so it's important for it
 * to have very low probability of failure.
 */
static void
SignalBackends(void)
{
  int     count;

  /* Can't get here without PreCommit_Notify having made the global table */
  Assert(globalChannelTable != NULL);

  /* It should have set up these arrays, too */
  Assert(signalPids != NULL && signalProcnos != NULL);

  /*
   * Identify backends that we need to signal.  We don't want to send
   * signals while holding the NotifyQueueLock, so this part just builds a
   * list of target PIDs in signalPids[] and signalProcnos[].
   */
  count = 0;

  LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);

  /* Scan each channel name that we notified in this transaction */
  foreach_ptr(char, channel, pendingNotifies->uniqueChannelNames)
  {
    GlobalChannelKey key;
    GlobalChannelEntry *entry;
    ListenerEntry *listeners;

    GlobalChannelKeyInit(&key, MyDatabaseId, channel);
    entry = dshash_find(globalChannelTable, &key, false);
    if (entry == NULL)
      continue;     /* nobody is listening */

    listeners = (ListenerEntry *) dsa_get_address(globalChannelDSA,
                            entry->listenersArray);

    /*
     * Identify listeners that now need waking, add them to arrays.
     *
     * Note that we signal listeners regardless of the state of their
     * removeOnAbort flags.  Hence a new listener that reached PreCommit,
     * but then failed before AtCommit_Notify, can receive a signal even
     * though it was never really listening.  This is okay because it will
     * not do anything in response to that signal.  If we did not do it
     * like this then a new listener might miss some messages due to the
     * direct-advance logic below.
     */
    for (int j = 0; j < entry->numListeners; j++)
    {
      ProcNumber  i = listeners[j].procNo;
      int32   pid;
      QueuePosition pos;

      if (QUEUE_BACKEND_WAKEUP_PENDING(i))
        continue;   /* already signaled, no need to repeat */

      pid = QUEUE_BACKEND_PID(i);
      pos = QUEUE_BACKEND_POS(i);

      if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
        continue;   /* it's fully caught up already */

      Assert(pid != InvalidPid);

      QUEUE_BACKEND_WAKEUP_PENDING(i) = true;
      signalPids[count] = pid;
      signalProcnos[count] = i;
      count++;
    }

    dshash_release_lock(globalChannelTable, entry);
  }

  /*
   * Scan all listeners.  Any that are not already pending wakeup must not
   * be interested in our notifications (else we'd have set their wakeup
   * flags above).  Check to see if we can directly advance their queue
   * pointers to save a wakeup.  Otherwise, if they are far behind, wake
   * them anyway so they will catch up.
   */
  for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
  {
    int32   pid;
    QueuePosition pos;

    if (QUEUE_BACKEND_WAKEUP_PENDING(i))
      continue;

    /* If it's currently advancing, we should not touch it */
    if (QUEUE_BACKEND_IS_ADVANCING(i))
      continue;

    pid = QUEUE_BACKEND_PID(i);
    pos = QUEUE_BACKEND_POS(i);

    /*
     * We can directly advance the other backend's queue pointer if it's
     * not currently advancing (else there are race conditions), and its
     * current pointer is not behind queueHeadBeforeWrite (else we'd make
     * it miss some older messages), and we'd not be moving the pointer
     * backward.
     */
    if (!QUEUE_POS_PRECEDES(pos, queueHeadBeforeWrite) &&
      QUEUE_POS_PRECEDES(pos, queueHeadAfterWrite))
    {
      /* We can directly advance its pointer past what we wrote */
      QUEUE_BACKEND_POS(i) = queueHeadAfterWrite;
    }
    else if (asyncQueuePageDiff(QUEUE_POS_PAGE(QUEUE_HEAD),
                  QUEUE_POS_PAGE(pos)) >= QUEUE_CLEANUP_DELAY)
    {
      /* It's idle and far behind, so wake it up */
      Assert(pid != InvalidPid);

      QUEUE_BACKEND_WAKEUP_PENDING(i) = true;
      signalPids[count] = pid;
      signalProcnos[count] = i;
      count++;
    }
  }

  LWLockRelease(NotifyQueueLock);

  /* Now send signals */
  for (int i = 0; i < count; i++)
  {
    int32   pid = signalPids[i];

    /*
     * If we are signaling our own process, no need to involve the kernel;
     * just set the flag directly.
     */
    if (pid == MyProcPid)
    {
      notifyInterruptPending = true;
      continue;
    }

    /*
     * Note: assuming things aren't broken, a signal failure here could
     * only occur if the target backend exited since we released
     * NotifyQueueLock; which is unlikely but certainly possible. So we
     * just log a low-level debug message if it happens.
     */
    if (SendProcSignal(pid, PROCSIG_NOTIFY_INTERRUPT, signalProcnos[i]) < 0)
      elog(DEBUG3, "could not signal backend with PID %d: %m", pid);
  }
}

/*
 * AtAbort_Notify
 *
 *  This is called at transaction abort.
 *
 *  Revert any staged listen/unlisten changes and clean up transaction state.
 */
void
AtAbort_Notify(void)
{
  /*
   * If we failed during asyncQueueReadAllNotifications(), clear our
   * isAdvancing flag, else SignalBackends() would never wake us again.
   * Since only our own process changes this flag, it is safe to check it
   * without the lock.
   */
  if (amRegisteredListener && QUEUE_BACKEND_IS_ADVANCING(MyProcNumber))
  {
    LWLockAcquire(NotifyQueueLock, LW_SHARED);
    QUEUE_BACKEND_IS_ADVANCING(MyProcNumber) = false;
    LWLockRelease(NotifyQueueLock);
  }

  /* Revert any staged listen/unlisten changes */
  ApplyPendingListenActions(false);

  /* If we're no longer listening on anything, unregister */
  if (amRegisteredListener && LocalChannelTableIsEmpty())
    asyncQueueUnregister();

  /* And clean up */
  ClearPendingActionsAndNotifies();
}

/*
 * AtSubCommit_Notify() --- Take care of subtransaction commit.
 *
 * Reassign all items in the pending lists to the parent transaction.
 */
void
AtSubCommit_Notify(void)
{
  int     my_level = GetCurrentTransactionNestLevel();

  /* If there are actions at our nesting level, we must reparent them. */
  if (pendingActions != NULL &&
    pendingActions->nestingLevel >= my_level)
  {
    if (pendingActions->upper == NULL ||
      pendingActions->upper->nestingLevel < my_level - 1)
    {
      /* nothing to merge; give the whole thing to the parent */
      --pendingActions->nestingLevel;
    }
    else
    {
      ActionList *childPendingActions = pendingActions;

      pendingActions = pendingActions->upper;

      /*
       * Mustn't try to eliminate duplicates here --- see queue_listen()
       */
      pendingActions->actions =
        list_concat(pendingActions->actions,
              childPendingActions->actions);
      pfree(childPendingActions);
    }
  }

  /* If there are notifies at our nesting level, we must reparent them. */
  if (pendingNotifies != NULL &&
    pendingNotifies->nestingLevel >= my_level)
  {
    Assert(pendingNotifies->nestingLevel == my_level);

    if (pendingNotifies->upper == NULL ||
      pendingNotifies->upper->nestingLevel < my_level - 1)
    {
      /* nothing to merge; give the whole thing to the parent */
      --pendingNotifies->nestingLevel;
    }
    else
    {
      /*
       * Formerly, we didn't bother to eliminate duplicates here, but
       * now we must, else we fall foul of "Assert(!found)", either here
       * or during a later attempt to build the parent-level hashtable.
       */
      NotificationList *childPendingNotifies = pendingNotifies;
      ListCell   *l;

      pendingNotifies = pendingNotifies->upper;
      /* Insert all the subxact's events into parent, except for dups */
      foreach(l, childPendingNotifies->events)
      {
        Notification *childn = (Notification *) lfirst(l);

        if (!AsyncExistsPendingNotify(childn))
          AddEventToPendingNotifies(childn);
      }
      pfree(childPendingNotifies);
    }
  }
}

/*
 * AtSubAbort_Notify() --- Take care of subtransaction abort.
 */
void
AtSubAbort_Notify(void)
{
  int     my_level = GetCurrentTransactionNestLevel();

  /*
   * All we have to do is pop the stack --- the actions/notifies made in
   * this subxact are no longer interesting, and the space will be freed
   * when CurTransactionContext is recycled. We still have to free the
   * ActionList and NotificationList objects themselves, though, because
   * those are allocated in TopTransactionContext.
   *
   * Note that there might be no entries at all, or no entries for the
   * current subtransaction level, either because none were ever created, or
   * because we reentered this routine due to trouble during subxact abort.
   */
  while (pendingActions != NULL &&
       pendingActions->nestingLevel >= my_level)
  {
    ActionList *childPendingActions = pendingActions;

    pendingActions = pendingActions->upper;
    pfree(childPendingActions);
  }

  while (pendingNotifies != NULL &&
       pendingNotifies->nestingLevel >= my_level)
  {
    NotificationList *childPendingNotifies = pendingNotifies;

    pendingNotifies = pendingNotifies->upper;
    pfree(childPendingNotifies);
  }
}

/*
 * HandleNotifyInterrupt
 *
 *    Signal handler portion of interrupt handling. Let the backend know
 *    that there's a pending notify interrupt. If we're currently reading
 *    from the client, this will interrupt the read and
 *    ProcessClientReadInterrupt() will call ProcessNotifyInterrupt().
 */
void
HandleNotifyInterrupt(void)
{
  /*
   * Note: this is called by a SIGNAL HANDLER. You must be very wary what
   * you do here.
   */

  /* signal that work needs to be done */
  notifyInterruptPending = true;

  /* latch will be set by procsignal_sigusr1_handler */
}

/*
 * ProcessNotifyInterrupt
 *
 *    This is called if we see notifyInterruptPending set, just before
 *    transmitting ReadyForQuery at the end of a frontend command, and
 *    also if a notify signal occurs while reading from the frontend.
 *    HandleNotifyInterrupt() will cause the read to be interrupted
 *    via the process's latch, and this routine will get called.
 *    If we are truly idle (ie, *not* inside a transaction block),
 *    process the incoming notifies.
 *
 *    If "flush" is true, force any frontend messages out immediately.
 *    This can be false when being called at the end of a frontend command,
 *    since we'll flush after sending ReadyForQuery.
 */
void
ProcessNotifyInterrupt(bool flush)
{
  if (IsTransactionOrTransactionBlock())
    return;         /* not really idle */

  /* Loop in case another signal arrives while sending messages */
  while (notifyInterruptPending)
    ProcessIncomingNotify(flush);
}


/*
 * Read all pending notifications from the queue, and deliver appropriate
 * ones to my frontend.  Stop when we reach queue head or an uncommitted
 * notification.
 */
static void
asyncQueueReadAllNotifications(void)
{
  QueuePosition pos;
  QueuePosition head;
  Snapshot  snapshot;

  /*
   * Fetch current state, indicate to others that we have woken up, and that
   * we are in process of advancing our position.
   */
  LWLockAcquire(NotifyQueueLock, LW_SHARED);
  /* Assert checks that we have a valid state entry */
  Assert(MyProcPid == QUEUE_BACKEND_PID(MyProcNumber));
  QUEUE_BACKEND_WAKEUP_PENDING(MyProcNumber) = false;
  pos = QUEUE_BACKEND_POS(MyProcNumber);
  head = QUEUE_HEAD;

  if (QUEUE_POS_EQUAL(pos, head))
  {
    /* Nothing to do, we have read all notifications already. */
    LWLockRelease(NotifyQueueLock);
    return;
  }

  QUEUE_BACKEND_IS_ADVANCING(MyProcNumber) = true;
  LWLockRelease(NotifyQueueLock);

  /*----------
   * Get snapshot we'll use to decide which xacts are still in progress.
   * This is trickier than it might seem, because of race conditions.
   * Consider the following example:
   *
   * Backend 1:          Backend 2:
   *
   * transaction starts
   * UPDATE foo SET ...;
   * NOTIFY foo;
   * commit starts
   * queue the notify message
   *                 transaction starts
   *                 LISTEN foo;  -- first LISTEN in session
   *                 SELECT * FROM foo WHERE ...;
   * commit to clog
   *                 commit starts
   *                 add backend 2 to array of listeners
   *                 advance to queue head (this code)
   *                 commit to clog
   *
   * Transaction 2's SELECT has not seen the UPDATE's effects, since that
   * wasn't committed yet.  Ideally we'd ensure that client 2 would
   * eventually get transaction 1's notify message, but there's no way
   * to do that; until we're in the listener array, there's no guarantee
   * that the notify message doesn't get removed from the queue.
   *
   * Therefore the coding technique transaction 2 is using is unsafe:
   * applications must commit a LISTEN before inspecting database state,
   * if they want to ensure they will see notifications about subsequent
   * changes to that state.
   *
   * What we do guarantee is that we'll see all notifications from
   * transactions committing after the snapshot we take here.
   * BecomeRegisteredListener has already added us to the listener array,
   * so no not-yet-committed messages can be removed from the queue
   * before we see them.
   *
   * Since we'll only use this snapshot to check whether XIDs are still
   * running, and we will release it before returning, it's okay to use
   * GetSecondarySnapshot(); that avoids possibly blocking on other
   * transactions in SERIALIZABLE READ ONLY DEFERRABLE mode.
   *----------
   */
  snapshot = RegisterSnapshot(GetSecondarySnapshot());

  /*
   * It is possible that we fail while trying to send a message to our
   * frontend (for example, because of encoding conversion failure).  If
   * that happens it is critical that we not try to send the same message
   * over and over again.  Therefore, we set ExitOnAnyError to upgrade any
   * ERRORs to FATAL, causing the client connection to be closed on error.
   *
   * We used to only skip over the offending message and try to soldier on,
   * but it was somewhat questionable to lose a notification and give the
   * client an ERROR instead.  A client application is not be prepared for
   * that and can't tell that a notification was missed.  It was also not
   * very useful in practice because notifications are often processed while
   * a connection is idle and reading a message from the client, and in that
   * state, any error is upgraded to FATAL anyway.  Closing the connection
   * is a clear signal to the application that it might have missed
   * notifications.
   */
  {
    bool    save_ExitOnAnyError = ExitOnAnyError;
    bool    reachedStop;

    ExitOnAnyError = true;

    do
    {
      /*
       * Process messages up to the stop position, end of page, or an
       * uncommitted message.
       *
       * Our stop position is what we found to be the head's position
       * when we entered this function. It might have changed already.
       * But if it has, we will receive (or have already received and
       * queued) another signal and come here again.
       *
       * We are not holding NotifyQueueLock here! The queue can only
       * extend beyond the head pointer (see above) and we leave our
       * backend's pointer where it is so nobody will truncate or
       * rewrite pages under us. Especially we don't want to hold a lock
       * while sending the notifications to the frontend.
       */
      reachedStop = asyncQueueProcessPageEntries(&pos, head, snapshot);
    } while (!reachedStop);

    /* Update shared state */
    LWLockAcquire(NotifyQueueLock, LW_SHARED);
    QUEUE_BACKEND_POS(MyProcNumber) = pos;
    QUEUE_BACKEND_IS_ADVANCING(MyProcNumber) = false;
    LWLockRelease(NotifyQueueLock);

    ExitOnAnyError = save_ExitOnAnyError;
  }

  /* Done with snapshot */
  UnregisterSnapshot(snapshot);
}

/*
 * Fetch notifications from the shared queue, beginning at position current,
 * and deliver relevant ones to my frontend.
 *
 * The function returns true once we have reached the stop position or an
 * uncommitted notification, and false if we have finished with the page.
 * In other words: once it returns true there is no need to look further.
 * The QueuePosition *current is advanced past all processed messages.
 */
static bool
asyncQueueProcessPageEntries(QueuePosition *current,
               QueuePosition stop,
               Snapshot snapshot)
{
  int64   curpage = QUEUE_POS_PAGE(*current);
  int     slotno;
  char     *page_buffer;
  bool    reachedStop = false;
  bool    reachedEndOfPage;

  /*
   * We copy the entries into a local buffer to avoid holding the SLRU lock
   * while we transmit them to our frontend.  The local buffer must be
   * adequately aligned.
   */
  alignas(AsyncQueueEntry) char local_buf[QUEUE_PAGESIZE];
  char     *local_buf_end = local_buf;

  slotno = SimpleLruReadPage_ReadOnly(NotifyCtl, curpage, current);
  page_buffer = NotifyCtl->shared->page_buffer[slotno];

  do
  {
    QueuePosition thisentry = *current;
    AsyncQueueEntry *qe;

    if (QUEUE_POS_EQUAL(thisentry, stop))
      break;

    qe = (AsyncQueueEntry *) (page_buffer + QUEUE_POS_OFFSET(thisentry));

    /*
     * Advance *current over this message, possibly to the next page. As
     * noted in the comments for asyncQueueReadAllNotifications, we must
     * do this before possibly failing while processing the message.
     */
    reachedEndOfPage = asyncQueueAdvance(current, qe->length);

    /* Ignore messages destined for other databases */
    if (qe->dboid == MyDatabaseId)
    {
      if (XidInMVCCSnapshot(qe->xid, snapshot))
      {
        /*
         * The source transaction is still in progress, so we can't
         * process this message yet.  Break out of the loop, but first
         * back up *current so we will reprocess the message next
         * time.  (Note: it is unlikely but not impossible for
         * TransactionIdDidCommit to fail, so we can't really avoid
         * this advance-then-back-up behavior when dealing with an
         * uncommitted message.)
         *
         * Note that we must test XidInMVCCSnapshot before we test
         * TransactionIdDidCommit, else we might return a message from
         * a transaction that is not yet visible to snapshots; compare
         * the comments at the head of heapam_visibility.c.
         *
         * Also, while our own xact won't be listed in the snapshot,
         * we need not check for TransactionIdIsCurrentTransactionId
         * because our transaction cannot (yet) have queued any
         * messages.
         */
        *current = thisentry;
        reachedStop = true;
        break;
      }

      /*
       * Quick check for the case that we're not listening on any
       * channels, before calling TransactionIdDidCommit().  This makes
       * that case a little faster, but more importantly, it ensures
       * that if there's a bad entry in the queue for which
       * TransactionIdDidCommit() fails for some reason, we can skip
       * over it on the first LISTEN in a session, and not get stuck on
       * it indefinitely.  (This is a little trickier than it looks: it
       * works because BecomeRegisteredListener runs this code before we
       * have made the first entry in localChannelTable.)
       */
      if (LocalChannelTableIsEmpty())
        continue;

      if (TransactionIdDidCommit(qe->xid))
      {
        memcpy(local_buf_end, qe, qe->length);
        local_buf_end += qe->length;
      }
      else
      {
        /*
         * The source transaction aborted or crashed, so we just
         * ignore its notifications.
         */
      }
    }

    /* Loop back if we're not at end of page */
  } while (!reachedEndOfPage);

  /* Release lock that we got from SimpleLruReadPage_ReadOnly() */
  LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));

  /*
   * Now that we have let go of the SLRU bank lock, send the notifications
   * to our backend
   */
  Assert(local_buf_end - local_buf <= BLCKSZ);
  for (char *p = local_buf; p < local_buf_end;)
  {
    AsyncQueueEntry *qe = (AsyncQueueEntry *) p;

    /* qe->data is the null-terminated channel name */
    char     *channel = qe->data;

    if (IsListeningOn(channel))
    {
      /* payload follows channel name */
      char     *payload = qe->data + strlen(channel) + 1;

      NotifyMyFrontEnd(channel, payload, qe->srcPid);
    }

    p += qe->length;
  }

  if (QUEUE_POS_EQUAL(*current, stop))
    reachedStop = true;

  return reachedStop;
}

/*
 * Advance the shared queue tail variable to the minimum of all the
 * per-backend tail pointers.  Truncate pg_notify space if possible.
 *
 * This is (usually) called during CommitTransaction(), so it's important for
 * it to have very low probability of failure.
 */
static void
asyncQueueAdvanceTail(void)
{
  QueuePosition min;
  int64   oldtailpage;
  int64   newtailpage;
  int64   boundary;

  /* Restrict task to one backend per cluster; see SimpleLruTruncate(). */
  LWLockAcquire(NotifyQueueTailLock, LW_EXCLUSIVE);

  /*
   * Compute the new tail.  Pre-v13, it's essential that QUEUE_TAIL be exact
   * (ie, exactly match at least one backend's queue position), so it must
   * be updated atomically with the actual computation.  Since v13, we could
   * get away with not doing it like that, but it seems prudent to keep it
   * so.
   *
   * Also, because incoming backends will scan forward from QUEUE_TAIL, that
   * must be advanced before we can truncate any data.  Thus, QUEUE_TAIL is
   * the logical tail, while QUEUE_STOP_PAGE is the physical tail, or oldest
   * un-truncated page.  When QUEUE_STOP_PAGE != QUEUE_POS_PAGE(QUEUE_TAIL),
   * there are pages we can truncate but haven't yet finished doing so.
   *
   * For concurrency's sake, we don't want to hold NotifyQueueLock while
   * performing SimpleLruTruncate.  This is OK because no backend will try
   * to access the pages we are in the midst of truncating.
   */
  LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
  min = QUEUE_HEAD;
  for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
  {
    Assert(QUEUE_BACKEND_PID(i) != InvalidPid);
    min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
  }
  QUEUE_TAIL = min;
  oldtailpage = QUEUE_STOP_PAGE;
  LWLockRelease(NotifyQueueLock);

  /*
   * We can truncate something if the global tail advanced across an SLRU
   * segment boundary.
   *
   * XXX it might be better to truncate only once every several segments, to
   * reduce the number of directory scans.
   */
  newtailpage = QUEUE_POS_PAGE(min);
  boundary = newtailpage - (newtailpage % SLRU_PAGES_PER_SEGMENT);
  if (asyncQueuePagePrecedes(oldtailpage, boundary))
  {
    /*
     * SimpleLruTruncate() will ask for SLRU bank locks but will also
     * release the lock again.
     */
    SimpleLruTruncate(NotifyCtl, newtailpage);

    LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
    QUEUE_STOP_PAGE = newtailpage;
    LWLockRelease(NotifyQueueLock);
  }

  LWLockRelease(NotifyQueueTailLock);
}

/*
 * AsyncNotifyFreezeXids
 *
 * Prepare the async notification queue for CLOG truncation by freezing
 * transaction IDs that are about to become inaccessible.
 *
 * This function is called by VACUUM before advancing datfrozenxid. It scans
 * the notification queue and replaces XIDs that would become inaccessible
 * after CLOG truncation with special markers:
 * - Committed transactions are set to FrozenTransactionId
 * - Aborted/crashed transactions are set to InvalidTransactionId
 *
 * Only XIDs < newFrozenXid are processed, as those are the ones whose CLOG
 * pages will be truncated. If XID < newFrozenXid, it cannot still be running
 * (or it would have held back newFrozenXid through ProcArray).
 * Therefore, if TransactionIdDidCommit returns false, we know the transaction
 * either aborted explicitly or crashed, and we can safely mark it invalid.
 */
void
AsyncNotifyFreezeXids(TransactionId newFrozenXid)
{
  QueuePosition pos;
  QueuePosition head;
  int64   curpage = -1;
  int     slotno = -1;
  char     *page_buffer = NULL;
  bool    page_dirty = false;

  /*
   * Acquire locks in the correct order to avoid deadlocks. As per the
   * locking protocol: NotifyQueueTailLock, then NotifyQueueLock, then SLRU
   * bank locks.
   *
   * We only need SHARED mode since we're just reading the head/tail
   * positions, not modifying them.
   */
  LWLockAcquire(NotifyQueueTailLock, LW_SHARED);
  LWLockAcquire(NotifyQueueLock, LW_SHARED);

  pos = QUEUE_TAIL;
  head = QUEUE_HEAD;

  /* Release NotifyQueueLock early, we only needed to read the positions */
  LWLockRelease(NotifyQueueLock);

  /*
   * Scan the queue from tail to head, freezing XIDs as needed. We hold
   * NotifyQueueTailLock throughout to ensure the tail doesn't move while
   * we're working.
   */
  while (!QUEUE_POS_EQUAL(pos, head))
  {
    AsyncQueueEntry *qe;
    TransactionId xid;
    int64   pageno = QUEUE_POS_PAGE(pos);
    int     offset = QUEUE_POS_OFFSET(pos);

    /* If we need a different page, release old lock and get new one */
    if (pageno != curpage)
    {
      LWLock     *lock;

      /* Release previous page if any */
      if (slotno >= 0)
      {
        if (page_dirty)
        {
          NotifyCtl->shared->page_dirty[slotno] = true;
          page_dirty = false;
        }
        LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));
      }

      lock = SimpleLruGetBankLock(NotifyCtl, pageno);
      LWLockAcquire(lock, LW_EXCLUSIVE);
      slotno = SimpleLruReadPage(NotifyCtl, pageno, true, &pos);
      page_buffer = NotifyCtl->shared->page_buffer[slotno];
      curpage = pageno;
    }

    qe = (AsyncQueueEntry *) (page_buffer + offset);
    xid = qe->xid;

    if (TransactionIdIsNormal(xid) &&
      TransactionIdPrecedes(xid, newFrozenXid))
    {
      if (TransactionIdDidCommit(xid))
      {
        qe->xid = FrozenTransactionId;
        page_dirty = true;
      }
      else
      {
        qe->xid = InvalidTransactionId;
        page_dirty = true;
      }
    }

    /* Advance to next entry */
    asyncQueueAdvance(&pos, qe->length);
  }

  /* Release final page lock if we acquired one */
  if (slotno >= 0)
  {
    if (page_dirty)
      NotifyCtl->shared->page_dirty[slotno] = true;
    LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));
  }

  LWLockRelease(NotifyQueueTailLock);
}

/*
 * ProcessIncomingNotify
 *
 *    Scan the queue for arriving notifications and report them to the front
 *    end.  The notifications might be from other sessions, or our own;
 *    there's no need to distinguish here.
 *
 *    If "flush" is true, force any frontend messages out immediately.
 *
 *    NOTE: since we are outside any transaction, we must create our own.
 */
static void
ProcessIncomingNotify(bool flush)
{
  /* We *must* reset the flag */
  notifyInterruptPending = false;

  /* Do nothing else if we aren't actively listening */
  if (LocalChannelTableIsEmpty())
    return;

  if (Trace_notify)
    elog(DEBUG1, "ProcessIncomingNotify");

  set_ps_display("notify interrupt");

  /*
   * We must run asyncQueueReadAllNotifications inside a transaction, else
   * bad things happen if it gets an error.
   */
  StartTransactionCommand();

  asyncQueueReadAllNotifications();

  CommitTransactionCommand();

  /*
   * If this isn't an end-of-command case, we must flush the notify messages
   * to ensure frontend gets them promptly.
   */
  if (flush)
    pq_flush();

  set_ps_display("idle");

  if (Trace_notify)
    elog(DEBUG1, "ProcessIncomingNotify: done");
}

/*
 * Send NOTIFY message to my front end.
 */
void
NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
{
  if (whereToSendOutput == DestRemote)
  {
    StringInfoData buf;

    pq_beginmessage(&buf, PqMsg_NotificationResponse);
    pq_sendint32(&buf, srcPid);
    pq_sendstring(&buf, channel);
    pq_sendstring(&buf, payload);
    pq_endmessage(&buf);

    /*
     * NOTE: we do not do pq_flush() here.  Some level of caller will
     * handle it later, allowing this message to be combined into a packet
     * with other ones.
     */
  }
  else
    elog(INFO, "NOTIFY for \"%s\" payload \"%s\"", channel, payload);
}

/* Does pendingNotifies include a match for the given event? */
static bool
AsyncExistsPendingNotify(Notification *n)
{
  if (pendingNotifies == NULL)
    return false;

  if (pendingNotifies->hashtab != NULL)
  {
    /* Use the hash table to probe for a match */
    if (hash_search(pendingNotifies->hashtab,
            &n,
            HASH_FIND,
            NULL))
      return true;
  }
  else
  {
    /* Must scan the event list */
    ListCell   *l;

    foreach(l, pendingNotifies->events)
    {
      Notification *oldn = (Notification *) lfirst(l);

      if (n->channel_len == oldn->channel_len &&
        n->payload_len == oldn->payload_len &&
        memcmp(n->data, oldn->data,
             n->channel_len + n->payload_len + 2) == 0)
        return true;
    }
  }

  return false;
}

/*
 * Add a notification event to a pre-existing pendingNotifies list.
 *
 * Because pendingNotifies->events is already nonempty, this works
 * correctly no matter what CurrentMemoryContext is.
 */
static void
AddEventToPendingNotifies(Notification *n)
{
  Assert(pendingNotifies->events != NIL);

  /* Create the hash tables if it's time to */
  if (list_length(pendingNotifies->events) >= MIN_HASHABLE_NOTIFIES &&
    pendingNotifies->hashtab == NULL)
  {
    HASHCTL   hash_ctl;
    ListCell   *l;

    /* Create the hash table */
    hash_ctl.keysize = sizeof(Notification *);
    hash_ctl.entrysize = sizeof(struct NotificationHash);
    hash_ctl.hash = notification_hash;
    hash_ctl.match = notification_match;
    hash_ctl.hcxt = CurTransactionContext;
    pendingNotifies->hashtab =
      hash_create("Pending Notifies",
            256L,
            &hash_ctl,
            HASH_ELEM | HASH_FUNCTION | HASH_COMPARE | HASH_CONTEXT);

    /* Create the unique channel name table */
    Assert(pendingNotifies->uniqueChannelHash == NULL);
    hash_ctl.keysize = NAMEDATALEN;
    hash_ctl.entrysize = sizeof(ChannelName);
    hash_ctl.hcxt = CurTransactionContext;
    pendingNotifies->uniqueChannelHash =
      hash_create("Pending Notify Channel Names",
            64L,
            &hash_ctl,
            HASH_ELEM | HASH_STRINGS | HASH_CONTEXT);

    /* Insert all the already-existing events */
    foreach(l, pendingNotifies->events)
    {
      Notification *oldn = (Notification *) lfirst(l);
      char     *channel = oldn->data;
      bool    found;

      (void) hash_search(pendingNotifies->hashtab,
                 &oldn,
                 HASH_ENTER,
                 &found);
      Assert(!found);

      /* Add channel name to uniqueChannelHash; might be there already */
      (void) hash_search(pendingNotifies->uniqueChannelHash,
                 channel,
                 HASH_ENTER,
                 NULL);
    }
  }

  /* Add new event to the list, in order */
  pendingNotifies->events = lappend(pendingNotifies->events, n);

  /* Add event to the hash tables if needed */
  if (pendingNotifies->hashtab != NULL)
  {
    char     *channel = n->data;
    bool    found;

    (void) hash_search(pendingNotifies->hashtab,
               &n,
               HASH_ENTER,
               &found);
    Assert(!found);

    /* Add channel name to uniqueChannelHash; might be there already */
    (void) hash_search(pendingNotifies->uniqueChannelHash,
               channel,
               HASH_ENTER,
               NULL);
  }
}

/*
 * notification_hash: hash function for notification hash table
 *
 * The hash "keys" are pointers to Notification structs.
 */
static uint32
notification_hash(const void *key, Size keysize)
{
  const Notification *k = *(const Notification *const *) key;

  Assert(keysize == sizeof(Notification *));
  /* We don't bother to include the payload's trailing null in the hash */
  return hash_bytes((const unsigned char *) k->data,
            k->channel_len + k->payload_len + 1);
}

/*
 * notification_match: match function to use with notification_hash
 */
static int
notification_match(const void *key1, const void *key2, Size keysize)
{
  const Notification *k1 = *(const Notification *const *) key1;
  const Notification *k2 = *(const Notification *const *) key2;

  Assert(keysize == sizeof(Notification *));
  if (k1->channel_len == k2->channel_len &&
    k1->payload_len == k2->payload_len &&
    memcmp(k1->data, k2->data,
         k1->channel_len + k1->payload_len + 2) == 0)
    return 0;       /* equal */
  return 1;         /* not equal */
}

/* Clear the pendingActions and pendingNotifies lists. */
static void
ClearPendingActionsAndNotifies(void)
{
  /*
   * Everything's allocated in either TopTransactionContext or the context
   * for the subtransaction to which it corresponds.  So, there's nothing to
   * do here except reset the pointers; the space will be reclaimed when the
   * contexts are deleted.
   */
  pendingActions = NULL;
  pendingNotifies = NULL;
  /* Also clear pendingListenActions, which is derived from pendingActions */
  pendingListenActions = NULL;
}

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

Coverage Report

Created: 2026-11-12 07:20