Move routines to manipulate WAL into PostgreSQL::Test::Cluster

[pgsql.git] / src / backend / commands / async.c

/*-------------------------------------------------------------------------
 *
 * async.c
 *        Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
 *
 * Portions Copyright (c) 1996-2025, 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 9.0:
 *
 * 1. Multiple backends on same machine. Multiple backends listening on
 *        several channels. (Channels are also called "conditions" in other
 *        parts of the code.)
 *
 * 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.
 *
 *        There is no central knowledge of which backend listens on which channel;
 *        every backend has its own list of interesting channels.
 *
 *        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.
 *
 *        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 actual updates to the effective listen state (listenChannels).
 *        Then we signal any backends that may be interested in our messages
 *        (including our own backend, if listening).  This is done by
 *        SignalBackends(), which scans the list of listening backends and sends a
 *        PROCSIG_NOTIFY_INTERRUPT signal to every listening backend (we don't
 *        know which backend is listening on which channel so we must signal them
 *        all).  We can exclude backends that are already up to date, though, and
 *        we can also exclude backends that are in other databases (unless they
 *        are way behind and should be kicked to make them advance their
 *        pointers).
 *
 *        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.
 *
 * 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 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 "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/procsignal.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/guc_hooks.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.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))

/*
 * 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 in another database needs to be
 * behind before we'll decide we need to wake it up to 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 */
} 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. 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, and lastly SLRU bank lock.
 *
 * 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 */
        QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
} AsyncQueueControl;

static AsyncQueueControl *asyncQueueControl;

#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)

/*
 * The SLRU buffer area through which we access the notification queue
 */
static SlruCtlData NotifyCtlData;

#define NotifyCtl                                       (&NotifyCtlData)
#define QUEUE_PAGESIZE                          BLCKSZ

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

/*
 * listenChannels identifies the channels we are actually listening to
 * (ie, have committed a LISTEN on).  It is a simple list of channel names,
 * allocated in TopMemoryContext.
 */
static List *listenChannels = NIL;      /* list of C strings */

/*
 * 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 listenChannels 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;

/*
 * 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 */
        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;

/*
 * 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;

/* 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 bool asyncQueuePagePrecedes(int64 p, int64 q);
static void queue_listen(ListenActionKind action, const char *channel);
static void Async_UnlistenOnExit(int code, Datum arg);
static void Exec_ListenPreCommit(void);
static void Exec_ListenCommit(const char *channel);
static void Exec_UnlistenCommit(const char *channel);
static void Exec_UnlistenAllCommit(void);
static bool IsListeningOn(const char *channel);
static void asyncQueueUnregister(void);
static bool asyncQueueIsFull(void);
static bool asyncQueueAdvance(volatile 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(volatile QueuePosition *current,
                                                                                 QueuePosition stop,
                                                                                 char *page_buffer,
                                                                                 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);

/*
 * 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;
}

/*
 * Report space needed for our shared memory area
 */
Size
AsyncShmemSize(void)
{
        Size            size;

        /* This had better match AsyncShmemInit */
        size = mul_size(MaxBackends, sizeof(QueueBackendStatus));
        size = add_size(size, offsetof(AsyncQueueControl, backend));

        size = add_size(size, SimpleLruShmemSize(notify_buffers, 0));

        return size;
}

/*
 * Initialize our shared memory area
 */
void
AsyncShmemInit(void)
{
        bool            found;
        Size            size;

        /*
         * Create or attach to the AsyncQueueControl structure.
         */
        size = mul_size(MaxBackends, sizeof(QueueBackendStatus));
        size = add_size(size, offsetof(AsyncQueueControl, backend));

        asyncQueueControl = (AsyncQueueControl *)
                ShmemInitStruct("Async Queue Control", size, &found);

        if (!found)
        {
                /* First time through, so initialize it */
                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;
                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);
                }
        }

        /*
         * Set up SLRU management of the pg_notify data. Note that long segment
         * names are used in order to avoid wraparound.
         */
        NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
        SimpleLruInit(NotifyCtl, "notify", notify_buffers, 0,
                                  "pg_notify", LWTRANCHE_NOTIFY_BUFFER, LWTRANCHE_NOTIFY_SLRU,
                                  SYNC_HANDLER_NONE, true);

        if (!found)
        {
                /*
                 * 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;
                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 the listenChannels list happens during transaction
 *              commit.
 */
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. It would
         * be too complicated to ensure we get the right interactions of
         * conflicting LISTEN/UNLISTEN/UNLISTEN_ALL, and it's unlikely that there
         * would be any performance benefit anyway in sane applications.
         */
        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.
 *
 * Note: this coding relies on the fact that the listenChannels list cannot
 * change within a transaction.
 */
Datum
pg_listening_channels(PG_FUNCTION_ARGS)
{
        FuncCallContext *funcctx;

        /* stuff done only on the first call of the function */
        if (SRF_IS_FIRSTCALL())
        {
                /* create a function context for cross-call persistence */
                funcctx = SRF_FIRSTCALL_INIT();
        }

        /* stuff done on every call of the function */
        funcctx = SRF_PERCALL_SETUP();

        if (funcctx->call_cntr < list_length(listenChannels))
        {
                char       *channel = (char *) list_nth(listenChannels,
                                                                                                funcctx->call_cntr);

                SRF_RETURN_NEXT(funcctx, CStringGetTextDatum(channel));
        }

        SRF_RETURN_DONE(funcctx);
}

/*
 * 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)
{
        Exec_UnlistenAllCommit();
        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 */
        if (pendingActions != NULL)
        {
                foreach(p, pendingActions->actions)
                {
                        ListenAction *actrec = (ListenAction *) lfirst(p);

                        switch (actrec->action)
                        {
                                case LISTEN_LISTEN:
                                        Exec_ListenPreCommit();
                                        break;
                                case LISTEN_UNLISTEN:
                                        /* there is no Exec_UnlistenPreCommit() */
                                        break;
                                case LISTEN_UNLISTEN_ALL:
                                        /* there is no Exec_UnlistenAllPreCommit() */
                                        break;
                        }
                }
        }

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

                /*
                 * 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);

                /* 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);
                        asyncQueueFillWarning();
                        if (asyncQueueIsFull())
                                ereport(ERROR,
                                                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                                                 errmsg("too many notifications in the NOTIFY queue")));
                        nextNotify = asyncQueueAddEntries(nextNotify);
                        LWLockRelease(NotifyQueueLock);
                }

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

/*
 * AtCommit_Notify
 *
 *              This is called at transaction commit, after committing to clog.
 *
 *              Update listenChannels 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)
{
        ListCell   *p;

        /*
         * 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");

        /* Perform any pending listen/unlisten actions */
        if (pendingActions != NULL)
        {
                foreach(p, pendingActions->actions)
                {
                        ListenAction *actrec = (ListenAction *) lfirst(p);

                        switch (actrec->action)
                        {
                                case LISTEN_LISTEN:
                                        Exec_ListenCommit(actrec->channel);
                                        break;
                                case LISTEN_UNLISTEN:
                                        Exec_UnlistenCommit(actrec->channel);
                                        break;
                                case LISTEN_UNLISTEN_ALL:
                                        Exec_UnlistenAllCommit();
                                        break;
                        }
                }
        }

        /* If no longer listening to anything, get out of listener array */
        if (amRegisteredListener && listenChannels == NIL)
                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();
}

/*
 * Exec_ListenPreCommit --- subroutine for PreCommit_Notify
 *
 * This function must make sure we are ready to catch any incoming messages.
 */
static void
Exec_ListenPreCommit(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, "Exec_ListenPreCommit(%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;
        /* 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();
}

/*
 * Exec_ListenCommit --- subroutine for AtCommit_Notify
 *
 * Add the channel to the list of channels we are listening on.
 */
static void
Exec_ListenCommit(const char *channel)
{
        MemoryContext oldcontext;

        /* Do nothing if we are already listening on this channel */
        if (IsListeningOn(channel))
                return;

        /*
         * Add the new channel name to listenChannels.
         *
         * XXX It is theoretically possible to get an out-of-memory failure here,
         * which would be bad because we already committed.  For the moment it
         * doesn't seem worth trying to guard against that, but maybe improve this
         * later.
         */
        oldcontext = MemoryContextSwitchTo(TopMemoryContext);
        listenChannels = lappend(listenChannels, pstrdup(channel));
        MemoryContextSwitchTo(oldcontext);
}

/*
 * Exec_UnlistenCommit --- subroutine for AtCommit_Notify
 *
 * Remove the specified channel name from listenChannels.
 */
static void
Exec_UnlistenCommit(const char *channel)
{
        ListCell   *q;

        if (Trace_notify)
                elog(DEBUG1, "Exec_UnlistenCommit(%s,%d)", channel, MyProcPid);

        foreach(q, listenChannels)
        {
                char       *lchan = (char *) lfirst(q);

                if (strcmp(lchan, channel) == 0)
                {
                        listenChannels = foreach_delete_current(listenChannels, q);
                        pfree(lchan);
                        break;
                }
        }

        /*
         * We do not complain about unlistening something not being listened;
         * should we?
         */
}

/*
 * Exec_UnlistenAllCommit --- subroutine for AtCommit_Notify
 *
 *              Unlisten on all channels for this backend.
 */
static void
Exec_UnlistenAllCommit(void)
{
        if (Trace_notify)
                elog(DEBUG1, "Exec_UnlistenAllCommit(%d)", MyProcPid);

        list_free_deep(listenChannels);
        listenChannels = NIL;
}

/*
 * Test whether we are actively listening on the given channel name.
 *
 * Note: this function is executed for every notification found in the queue.
 * Perhaps it is worth further optimization, eg convert the list to a sorted
 * array so we can binary-search it.  In practice the list is likely to be
 * fairly short, though.
 */
static bool
IsListeningOn(const char *channel)
{
        ListCell   *p;

        foreach(p, listenChannels)
        {
                char       *lchan = (char *) lfirst(p);

                if (strcmp(lchan, channel) == 0)
                        return true;
        }
        return false;
}

/*
 * 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(listenChannels == NIL);  /* 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;
        /* 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(volatile 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,
                                                                   InvalidTransactionId);

        /* 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.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 in our own database, since only those
 * backends could be interested in notifies we send.  However, if there's
 * notify traffic in our database but no traffic in another database that
 * does have listener(s), those listeners will fall further and further
 * behind.  Waken them anyway if they're far enough behind, so that 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)
{
        int32      *pids;
        ProcNumber *procnos;
        int                     count;

        /*
         * Identify backends that we need to signal.  We don't want to send
         * signals while holding the NotifyQueueLock, so this loop just builds a
         * list of target PIDs.
         *
         * XXX in principle these pallocs could fail, which would be bad. Maybe
         * preallocate the arrays?  They're not that large, though.
         */
        pids = (int32 *) palloc(MaxBackends * sizeof(int32));
        procnos = (ProcNumber *) palloc(MaxBackends * sizeof(ProcNumber));
        count = 0;

        LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
        for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
        {
                int32           pid = QUEUE_BACKEND_PID(i);
                QueuePosition pos;

                Assert(pid != InvalidPid);
                pos = QUEUE_BACKEND_POS(i);
                if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
                {
                        /*
                         * Always signal listeners in our own database, unless they're
                         * already caught up (unlikely, but possible).
                         */
                        if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
                                continue;
                }
                else
                {
                        /*
                         * Listeners in other databases should be signaled only if they
                         * are far behind.
                         */
                        if (asyncQueuePageDiff(QUEUE_POS_PAGE(QUEUE_HEAD),
                                                                   QUEUE_POS_PAGE(pos)) < QUEUE_CLEANUP_DELAY)
                                continue;
                }
                /* OK, need to signal this one */
                pids[count] = pid;
                procnos[count] = i;
                count++;
        }
        LWLockRelease(NotifyQueueLock);

        /* Now send signals */
        for (int i = 0; i < count; i++)
        {
                int32           pid = pids[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, procnos[i]) < 0)
                        elog(DEBUG3, "could not signal backend with PID %d: %m", pid);
        }

        pfree(pids);
        pfree(procnos);
}

/*
 * AtAbort_Notify
 *
 *      This is called at transaction abort.
 *
 *      Gets rid of pending actions and outbound notifies that we would have
 *      executed if the transaction got committed.
 */
void
AtAbort_Notify(void)
{
        /*
         * If we LISTEN but then roll back the transaction after PreCommit_Notify,
         * we have registered as a listener but have not made any entry in
         * listenChannels.  In that case, deregister again.
         */
        if (amRegisteredListener && listenChannels == NIL)
                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;

        /* make sure the event is processed in due course */
        SetLatch(MyLatch);
}

/*
 * 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)
{
        volatile QueuePosition pos;
        QueuePosition head;
        Snapshot        snapshot;

        /* page_buffer must be adequately aligned, so use a union */
        union
        {
                char            buf[QUEUE_PAGESIZE];
                AsyncQueueEntry align;
        }                       page_buffer;

        /* Fetch current state */
        LWLockAcquire(NotifyQueueLock, LW_SHARED);
        /* Assert checks that we have a valid state entry */
        Assert(MyProcPid == QUEUE_BACKEND_PID(MyProcNumber));
        pos = QUEUE_BACKEND_POS(MyProcNumber);
        head = QUEUE_HEAD;
        LWLockRelease(NotifyQueueLock);

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

        /*----------
         * 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.
         * Exec_ListenPreCommit has already added us to the listener array,
         * so no not-yet-committed messages can be removed from the queue
         * before we see them.
         *----------
         */
        snapshot = RegisterSnapshot(GetLatestSnapshot());

        /*
         * 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 place a PG_TRY block here that will
         * forcibly advance our queue position before we lose control to an error.
         * (We could alternatively retake NotifyQueueLock and move the position
         * before handling each individual message, but that seems like too much
         * lock traffic.)
         */
        PG_TRY();
        {
                bool            reachedStop;

                do
                {
                        int64           curpage = QUEUE_POS_PAGE(pos);
                        int                     curoffset = QUEUE_POS_OFFSET(pos);
                        int                     slotno;
                        int                     copysize;

                        /*
                         * We copy the data from SLRU into a local buffer, so as to avoid
                         * holding the SLRU lock while we are examining the entries and
                         * possibly transmitting them to our frontend.  Copy only the part
                         * of the page we will actually inspect.
                         */
                        slotno = SimpleLruReadPage_ReadOnly(NotifyCtl, curpage,
                                                                                                InvalidTransactionId);
                        if (curpage == QUEUE_POS_PAGE(head))
                        {
                                /* we only want to read as far as head */
                                copysize = QUEUE_POS_OFFSET(head) - curoffset;
                                if (copysize < 0)
                                        copysize = 0;   /* just for safety */
                        }
                        else
                        {
                                /* fetch all the rest of the page */
                                copysize = QUEUE_PAGESIZE - curoffset;
                        }
                        memcpy(page_buffer.buf + curoffset,
                                   NotifyCtl->shared->page_buffer[slotno] + curoffset,
                                   copysize);
                        /* Release lock that we got from SimpleLruReadPage_ReadOnly() */
                        LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));

                        /*
                         * 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,
                                                                                                           page_buffer.buf,
                                                                                                           snapshot);
                } while (!reachedStop);
        }
        PG_FINALLY();
        {
                /* Update shared state */
                LWLockAcquire(NotifyQueueLock, LW_SHARED);
                QUEUE_BACKEND_POS(MyProcNumber) = pos;
                LWLockRelease(NotifyQueueLock);
        }
        PG_END_TRY();

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

/*
 * Fetch notifications from the shared queue, beginning at position current,
 * and deliver relevant ones to my frontend.
 *
 * The current page must have been fetched into page_buffer from shared
 * memory.  (We could access the page right in shared memory, but that
 * would imply holding the SLRU bank lock throughout this routine.)
 *
 * We stop if we reach the "stop" position, or reach a notification from an
 * uncommitted transaction, or reach the end of the page.
 *
 * 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(volatile QueuePosition *current,
                                                         QueuePosition stop,
                                                         char *page_buffer,
                                                         Snapshot snapshot)
{
        bool            reachedStop = false;
        bool            reachedEndOfPage;
        AsyncQueueEntry *qe;

        do
        {
                QueuePosition thisentry = *current;

                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;
                        }
                        else if (TransactionIdDidCommit(qe->xid))
                        {
                                /* 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);
                                }
                        }
                        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);

        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);
}

/*
 * 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 (listenChannels == NIL)
                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 table 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);

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

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

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

        /* Add event to the hash table if needed */
        if (pendingNotifies->hashtab != NULL)
        {
                bool            found;

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

/*
 * 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 DatumGetUInt32(hash_any((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;
}

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