Update copyright for 2016

[postgresql] / src / backend / commands / async.c

/*-------------------------------------------------------------------------
 *
 * async.c
 *        Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
 *
 * Portions Copyright (c) 1996-2016, 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 the actual updates to the effective listen state (listenChannels).
 *
 *        Finally, after we are out of the transaction altogether, we check if
 *        we need to signal listening backends.  In SignalBackends() we scan the
 *        list of listening backends and send 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.  We don't bother with a self-signal
 *        either, but just process the queue directly.
 *
 * 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. Then we check if we were the laziest
 *        backend: if our pointer is set to the same position as the global tail
 *        pointer is set, then we move the global tail pointer ahead to where the
 *        second-laziest backend is (in general, we take the MIN of the current
 *        head position and all active backends' new tail pointers). Whenever we
 *        move the global tail pointer we also truncate now-unused pages (i.e.,
 *        delete files in pg_notify/ that are no longer used).
 *
 * 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 (NUM_ASYNC_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 slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
 *-------------------------------------------------------------------------
 */

#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 "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/sinval.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/ps_status.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
{
        int                     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)

/* 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))

/*
 * 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 */
        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 AsyncQueueLock.
 *
 * When holding the lock 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 the lock in EXCLUSIVE mode, backends can inspect the entries
 * of other backends and also change the head and tail pointers.
 *
 * AsyncCtlLock is used as the control lock for the pg_notify SLRU buffers.
 * In order to avoid deadlocks, whenever we need both locks, we always first
 * get AsyncQueueLock and then AsyncCtlLock.
 *
 * Each backend uses the backend[] array entry with index equal to its
 * BackendId (which can range from 1 to MaxBackends).  We rely on this to make
 * SendProcSignal fast.
 */
typedef struct AsyncQueueControl
{
        QueuePosition head;                     /* head points to the next free location */
        QueuePosition tail;                     /* the global tail is equivalent to the pos of
                                                                 * the "slowest" backend */
        TimestampTz lastQueueFillWarn;          /* time of last queue-full msg */
        QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
        /* backend[0] is not used; used entries are from [1] to [MaxBackends] */
} AsyncQueueControl;

static AsyncQueueControl *asyncQueueControl;

#define QUEUE_HEAD                                      (asyncQueueControl->head)
#define QUEUE_TAIL                                      (asyncQueueControl->tail)
#define QUEUE_BACKEND_PID(i)            (asyncQueueControl->backend[i].pid)
#define QUEUE_BACKEND_DBOID(i)          (asyncQueueControl->backend[i].dboid)
#define QUEUE_BACKEND_POS(i)            (asyncQueueControl->backend[i].pos)

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

#define AsyncCtl                                        (&AsyncCtlData)
#define QUEUE_PAGESIZE                          BLCKSZ
#define QUEUE_FULL_WARN_INTERVAL        5000            /* warn at most once every 5s */

/*
 * slru.c currently assumes that all filenames are four characters of hex
 * digits. That means that we can use segments 0000 through FFFF.
 * Each segment contains SLRU_PAGES_PER_SEGMENT pages which gives us
 * the pages from 0 to SLRU_PAGES_PER_SEGMENT * 0x10000 - 1.
 *
 * It's of course possible to enhance slru.c, but this gives us so much
 * space already that it doesn't seem worth the trouble.
 *
 * The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
 * pages, because more than that would confuse slru.c into thinking there
 * was a wraparound condition.  With the default BLCKSZ this means there
 * can be up to 8GB of queued-and-not-read data.
 *
 * Note: it's possible to redefine QUEUE_MAX_PAGE with a smaller multiple of
 * SLRU_PAGES_PER_SEGMENT, for easier testing of queue-full behaviour.
 */
#define QUEUE_MAX_PAGE                  (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)

/*
 * 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;

static List *pendingActions = NIL;              /* list of ListenAction */

static List *upperPendingActions = NIL; /* list of upper-xact lists */

/*
 * 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 NIL if no
 * NOTIFYs have been done in the current transaction.
 *
 * 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.
 *
 * 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
{
        char       *channel;            /* channel name */
        char       *payload;            /* payload string (can be empty) */
} Notification;

static List *pendingNotifies = NIL;             /* list of Notifications */

static List *upperPendingNotifies = NIL;                /* list of upper-xact lists */

/*
 * 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;

/* has this backend sent notifications in the current transaction? */
static bool backendHasSentNotifications = false;

/* GUC parameter */
bool            Trace_notify = false;

/* local function prototypes */
static bool asyncQueuePagePrecedes(int p, int 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 bool SignalBackends(void);
static void asyncQueueReadAllNotifications(void);
static bool asyncQueueProcessPageEntries(volatile QueuePosition *current,
                                                         QueuePosition stop,
                                                         char *page_buffer);
static void asyncQueueAdvanceTail(void);
static void ProcessIncomingNotify(void);
static void NotifyMyFrontEnd(const char *channel,
                                 const char *payload,
                                 int32 srcPid);
static bool AsyncExistsPendingNotify(const char *channel, const char *payload);
static void ClearPendingActionsAndNotifies(void);

/*
 * We will work on the page range of 0..QUEUE_MAX_PAGE.
 */
static bool
asyncQueuePagePrecedes(int p, int q)
{
        int                     diff;

        /*
         * We have to compare modulo (QUEUE_MAX_PAGE+1)/2.  Both inputs should be
         * in the range 0..QUEUE_MAX_PAGE.
         */
        Assert(p >= 0 && p <= QUEUE_MAX_PAGE);
        Assert(q >= 0 && q <= QUEUE_MAX_PAGE);

        diff = p - q;
        if (diff >= ((QUEUE_MAX_PAGE + 1) / 2))
                diff -= QUEUE_MAX_PAGE + 1;
        else if (diff < -((QUEUE_MAX_PAGE + 1) / 2))
                diff += QUEUE_MAX_PAGE + 1;
        return diff < 0;
}

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

        /* This had better match AsyncShmemInit */
        size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
        size = add_size(size, offsetof(AsyncQueueControl, backend));

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

        return size;
}

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

        /*
         * Create or attach to the AsyncQueueControl structure.
         *
         * The used entries in the backend[] array run from 1 to MaxBackends; the
         * zero'th entry is unused but must be allocated.
         */
        size = mul_size(MaxBackends + 1, 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 */
                int                     i;

                SET_QUEUE_POS(QUEUE_HEAD, 0, 0);
                SET_QUEUE_POS(QUEUE_TAIL, 0, 0);
                asyncQueueControl->lastQueueFillWarn = 0;
                /* zero'th entry won't be used, but let's initialize it anyway */
                for (i = 0; i <= MaxBackends; i++)
                {
                        QUEUE_BACKEND_PID(i) = InvalidPid;
                        QUEUE_BACKEND_DBOID(i) = InvalidOid;
                        SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
                }
        }

        /*
         * Set up SLRU management of the pg_notify data.
         */
        AsyncCtl->PagePrecedes = asyncQueuePagePrecedes;
        SimpleLruInit(AsyncCtl, "async", NUM_ASYNC_BUFFERS, 0,
                                  AsyncCtlLock, "pg_notify");
        /* Override default assumption that writes should be fsync'd */
        AsyncCtl->do_fsync = false;

        if (!found)
        {
                /*
                 * During start or reboot, clean out the pg_notify directory.
                 */
                (void) SlruScanDirectory(AsyncCtl, SlruScanDirCbDeleteAll, NULL);

                /* Now initialize page zero to empty */
                LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
                slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(QUEUE_HEAD));
                /* This write is just to verify that pg_notify/ is writable */
                SimpleLruWritePage(AsyncCtl, slotno);
                LWLockRelease(AsyncCtlLock);
        }
}


/*
 * 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)
{
        Notification *n;
        MemoryContext oldcontext;

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

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

        /* a channel name must be specified */
        if (!channel || !strlen(channel))
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("channel name cannot be empty")));

        if (strlen(channel) >= NAMEDATALEN)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("channel name too long")));

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

        /* no point in making duplicate entries in the list ... */
        if (AsyncExistsPendingNotify(channel, payload))
                return;

        /*
         * The notification list needs to live until end of transaction, so store
         * it in the transaction context.
         */
        oldcontext = MemoryContextSwitchTo(CurTransactionContext);

        n = (Notification *) palloc(sizeof(Notification));
        n->channel = pstrdup(channel);
        if (payload)
                n->payload = pstrdup(payload);
        else
                n->payload = "";

        /*
         * We want to preserve the order so we need to append every notification.
         * See comments at AsyncExistsPendingNotify().
         */
        pendingNotifies = lappend(pendingNotifies, 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;

        /*
         * 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);

        pendingActions = lappend(pendingActions, 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 == NIL && !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 == NIL && !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;
        ListCell  **lcp;

        /* stuff done only on the first call of the function */
        if (SRF_IS_FIRSTCALL())
        {
                MemoryContext oldcontext;

                /* create a function context for cross-call persistence */
                funcctx = SRF_FIRSTCALL_INIT();

                /* switch to memory context appropriate for multiple function calls */
                oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

                /* allocate memory for user context */
                lcp = (ListCell **) palloc(sizeof(ListCell *));
                *lcp = list_head(listenChannels);
                funcctx->user_fctx = (void *) lcp;

                MemoryContextSwitchTo(oldcontext);
        }

        /* stuff done on every call of the function */
        funcctx = SRF_PERCALL_SETUP();
        lcp = (ListCell **) funcctx->user_fctx;

        while (*lcp != NULL)
        {
                char       *channel = (char *) lfirst(*lcp);

                *lcp = lnext(*lcp);
                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 == NIL && pendingNotifies == NIL)
                return;                                 /* no relevant statements in this xact */

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

        /* Preflight for any pending listen/unlisten actions */
        foreach(p, pendingActions)
        {
                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 */
        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 AsyncQueueLock.
                 */
                (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 */
                backendHasSentNotifications = true;

                nextNotify = list_head(pendingNotifies);
                while (nextNotify != NULL)
                {
                        /*
                         * Add the pending notifications to the queue.  We acquire and
                         * release AsyncQueueLock 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(AsyncQueueLock, LW_EXCLUSIVE);
                        asyncQueueFillWarning();
                        if (asyncQueueIsFull())
                                ereport(ERROR,
                                                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                                          errmsg("too many notifications in the NOTIFY queue")));
                        nextNotify = asyncQueueAddEntries(nextNotify);
                        LWLockRelease(AsyncQueueLock);
                }
        }
}

/*
 * AtCommit_Notify
 *
 *              This is called at transaction commit, after committing to clog.
 *
 *              Update listenChannels and clear transaction-local state.
 */
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 */
        foreach(p, pendingActions)
        {
                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();

        /* 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;
        int                     i;

        /*
         * 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.)  But we only bother
         * with that if there's more than a page worth of notifications
         * outstanding, otherwise scanning all the other backends isn't worth it.
         *
         * We need exclusive lock here so we can look at other backends' entries.
         */
        LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
        head = QUEUE_HEAD;
        max = QUEUE_TAIL;
        if (QUEUE_POS_PAGE(max) != QUEUE_POS_PAGE(head))
        {
                for (i = 1; i <= MaxBackends; i++)
                {
                        if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
                                max = QUEUE_POS_MAX(max, QUEUE_BACKEND_POS(i));
                }
        }
        QUEUE_BACKEND_POS(MyBackendId) = max;
        QUEUE_BACKEND_PID(MyBackendId) = MyProcPid;
        QUEUE_BACKEND_DBOID(MyBackendId) = MyDatabaseId;
        LWLockRelease(AsyncQueueLock);

        /* 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. Still, we could get notifications that
         * have already committed before we started to LISTEN.
         *
         * Note that we are not yet listening on anything, so we won't deliver any
         * notification to the frontend.
         *
         * This will also advance the global tail pointer if possible.
         */
        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;
        ListCell   *prev;

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

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

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

        /*
         * 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;
}

/*
 * ProcessCompletedNotifies --- send out signals and self-notifies
 *
 * This is called from postgres.c just before going idle at the completion
 * of a transaction.  If we issued any notifications in the just-completed
 * transaction, send signals to other backends to process them, and also
 * process the queue ourselves to send messages to our own frontend.
 *
 * The reason that this is not done in AtCommit_Notify is that there is
 * a nonzero chance of errors here (for example, encoding conversion errors
 * while trying to format messages to our frontend).  An error during
 * AtCommit_Notify would be a PANIC condition.  The timing is also arranged
 * to ensure that a transaction's self-notifies are delivered to the frontend
 * before it gets the terminating ReadyForQuery message.
 *
 * Note that we send signals and process the queue even if the transaction
 * eventually aborted.  This is because we need to clean out whatever got
 * added to the queue.
 *
 * NOTE: we are outside of any transaction here.
 */
void
ProcessCompletedNotifies(void)
{
        MemoryContext caller_context;
        bool            signalled;

        /* Nothing to do if we didn't send any notifications */
        if (!backendHasSentNotifications)
                return;

        /*
         * We reset the flag immediately; otherwise, if any sort of error occurs
         * below, we'd be locked up in an infinite loop, because control will come
         * right back here after error cleanup.
         */
        backendHasSentNotifications = false;

        /*
         * We must preserve the caller's memory context (probably MessageContext)
         * across the transaction we do here.
         */
        caller_context = CurrentMemoryContext;

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

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

        /* Send signals to other backends */
        signalled = SignalBackends();

        if (listenChannels != NIL)
        {
                /* Read the queue ourselves, and send relevant stuff to the frontend */
                asyncQueueReadAllNotifications();
        }
        else if (!signalled)
        {
                /*
                 * If we found no other listening backends, and we aren't listening
                 * ourselves, then we must execute asyncQueueAdvanceTail to flush the
                 * queue, because ain't nobody else gonna do it.  This prevents queue
                 * overflow when we're sending useless notifies to nobody. (A new
                 * listener could have joined since we looked, but if so this is
                 * harmless.)
                 */
                asyncQueueAdvanceTail();
        }

        CommitTransactionCommand();

        MemoryContextSwitchTo(caller_context);

        /* We don't need pq_flush() here since postgres.c will do one shortly */
}

/*
 * 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)
{
        bool            advanceTail;

        Assert(listenChannels == NIL);          /* else caller error */

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

        LWLockAcquire(AsyncQueueLock, LW_SHARED);
        /* check if entry is valid and oldest ... */
        advanceTail = (MyProcPid == QUEUE_BACKEND_PID(MyBackendId)) &&
                QUEUE_POS_EQUAL(QUEUE_BACKEND_POS(MyBackendId), QUEUE_TAIL);
        /* ... then mark it invalid */
        QUEUE_BACKEND_PID(MyBackendId) = InvalidPid;
        QUEUE_BACKEND_DBOID(MyBackendId) = InvalidOid;
        LWLockRelease(AsyncQueueLock);

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

        /* If we were the laziest backend, try to advance the tail pointer */
        if (advanceTail)
                asyncQueueAdvanceTail();
}

/*
 * Test whether there is room to insert more notification messages.
 *
 * Caller must hold at least shared AsyncQueueLock.
 */
static bool
asyncQueueIsFull(void)
{
        int                     nexthead;
        int                     boundary;

        /*
         * The queue is full if creating a new head page would create a page that
         * logically precedes the current global tail pointer, ie, the head
         * pointer would wrap around compared to the tail.  We cannot create such
         * a head page for fear of confusing slru.c.  For safety we round the tail
         * pointer back to a segment boundary (compare the truncation logic in
         * asyncQueueAdvanceTail).
         *
         * Note that this test is *not* dependent on how much space there is on
         * the current head page.  This is necessary because asyncQueueAddEntries
         * might try to create the next head page in any case.
         */
        nexthead = QUEUE_POS_PAGE(QUEUE_HEAD) + 1;
        if (nexthead > QUEUE_MAX_PAGE)
                nexthead = 0;                   /* wrap around */
        boundary = QUEUE_POS_PAGE(QUEUE_TAIL);
        boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
        return asyncQueuePagePrecedes(nexthead, boundary);
}

/*
 * 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)
{
        int                     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++;
                if (pageno > QUEUE_MAX_PAGE)
                        pageno = 0;                     /* wrap around */
                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 = strlen(n->channel);
        size_t          payloadlen = strlen(n->payload);
        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->channel, channellen + 1);
        memcpy(qe->data + channellen + 1, n->payload, payloadlen + 1);
}

/*
 * 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 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 AsyncQueueLock already from the caller and grab AsyncCtlLock
 * locally in this function.
 */
static ListCell *
asyncQueueAddEntries(ListCell *nextNotify)
{
        AsyncQueueEntry qe;
        QueuePosition queue_head;
        int                     pageno;
        int                     offset;
        int                     slotno;

        /* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
        LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);

        /*
         * 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;

        /* Fetch the current page */
        pageno = QUEUE_POS_PAGE(queue_head);
        slotno = SimpleLruReadPage(AsyncCtl, pageno, true, InvalidTransactionId);
        /* Note we mark the page dirty before writing in it */
        AsyncCtl->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(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(AsyncCtl->shared->page_buffer[slotno] + offset,
                           &qe,
                           qe.length);

                /* Advance queue_head appropriately, and detect if page is full */
                if (asyncQueueAdvance(&(queue_head), qe.length))
                {
                        /*
                         * 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(AsyncCtl, QUEUE_POS_PAGE(queue_head));
                        /* And exit the loop */
                        break;
                }
        }

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

        LWLockRelease(AsyncCtlLock);

        return nextNotify;
}

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

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

        PG_RETURN_FLOAT8(usage);
}

/*
 * Return the fraction of the queue that is currently occupied.
 *
 * The caller must hold AsyncQueueLock in (at least) shared mode.
 */
static double
asyncQueueUsage(void)
{
        int                     headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
        int                     tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
        int                     occupied;

        occupied = headPage - tailPage;

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

        if (occupied < 0)
        {
                /* head has wrapped around, tail not yet */
                occupied += QUEUE_MAX_PAGE + 1;
        }

        return (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
}

/*
 * 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 AsyncQueueLock.
 */
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;
                int                     i;

                for (i = 1; i <= MaxBackends; i++)
                {
                        if (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 all listening backends (except our own).
 *
 * Returns true if we sent at least one signal.
 *
 * Since we need EXCLUSIVE lock anyway we also check the position of the other
 * backends and in case one is already up-to-date we don't signal it.
 * This can happen if concurrent notifying transactions have sent a signal and
 * the signaled backend has read the other notifications and ours in the same
 * step.
 *
 * Since we know the BackendId and the Pid the signalling is quite cheap.
 */
static bool
SignalBackends(void)
{
        bool            signalled = false;
        int32      *pids;
        BackendId  *ids;
        int                     count;
        int                     i;
        int32           pid;

        /*
         * Identify all backends that are listening and not already up-to-date. We
         * don't want to send signals while holding the AsyncQueueLock, so we just
         * build a list of target PIDs.
         *
         * XXX in principle these pallocs could fail, which would be bad. Maybe
         * preallocate the arrays?      But in practice this is only run in trivial
         * transactions, so there should surely be space available.
         */
        pids = (int32 *) palloc(MaxBackends * sizeof(int32));
        ids = (BackendId *) palloc(MaxBackends * sizeof(BackendId));
        count = 0;

        LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
        for (i = 1; i <= MaxBackends; i++)
        {
                pid = QUEUE_BACKEND_PID(i);
                if (pid != InvalidPid && pid != MyProcPid)
                {
                        QueuePosition pos = QUEUE_BACKEND_POS(i);

                        if (!QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
                        {
                                pids[count] = pid;
                                ids[count] = i;
                                count++;
                        }
                }
        }
        LWLockRelease(AsyncQueueLock);

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

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

        pfree(pids);
        pfree(ids);

        return signalled;
}

/*
 * 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();
}

/*
 * AtSubStart_Notify() --- Take care of subtransaction start.
 *
 * Push empty state for the new subtransaction.
 */
void
AtSubStart_Notify(void)
{
        MemoryContext old_cxt;

        /* Keep the list-of-lists in TopTransactionContext for simplicity */
        old_cxt = MemoryContextSwitchTo(TopTransactionContext);

        upperPendingActions = lcons(pendingActions, upperPendingActions);

        Assert(list_length(upperPendingActions) ==
                   GetCurrentTransactionNestLevel() - 1);

        pendingActions = NIL;

        upperPendingNotifies = lcons(pendingNotifies, upperPendingNotifies);

        Assert(list_length(upperPendingNotifies) ==
                   GetCurrentTransactionNestLevel() - 1);

        pendingNotifies = NIL;

        MemoryContextSwitchTo(old_cxt);
}

/*
 * AtSubCommit_Notify() --- Take care of subtransaction commit.
 *
 * Reassign all items in the pending lists to the parent transaction.
 */
void
AtSubCommit_Notify(void)
{
        List       *parentPendingActions;
        List       *parentPendingNotifies;

        parentPendingActions = (List *) linitial(upperPendingActions);
        upperPendingActions = list_delete_first(upperPendingActions);

        Assert(list_length(upperPendingActions) ==
                   GetCurrentTransactionNestLevel() - 2);

        /*
         * Mustn't try to eliminate duplicates here --- see queue_listen()
         */
        pendingActions = list_concat(parentPendingActions, pendingActions);

        parentPendingNotifies = (List *) linitial(upperPendingNotifies);
        upperPendingNotifies = list_delete_first(upperPendingNotifies);

        Assert(list_length(upperPendingNotifies) ==
                   GetCurrentTransactionNestLevel() - 2);

        /*
         * We could try to eliminate duplicates here, but it seems not worthwhile.
         */
        pendingNotifies = list_concat(parentPendingNotifies, pendingNotifies);
}

/*
 * 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.
         *
         * This routine could be called more than once at a given nesting level if
         * there is trouble during subxact abort.  Avoid dumping core by using
         * GetCurrentTransactionNestLevel as the indicator of how far we need to
         * prune the list.
         */
        while (list_length(upperPendingActions) > my_level - 2)
        {
                pendingActions = (List *) linitial(upperPendingActions);
                upperPendingActions = list_delete_first(upperPendingActions);
        }

        while (list_length(upperPendingNotifies) > my_level - 2)
        {
                pendingNotifies = (List *) linitial(upperPendingNotifies);
                upperPendingNotifies = list_delete_first(upperPendingNotifies);
        }
}

/*
 * 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 just after waiting for a frontend command.  If a
 *              interrupt arrives (via HandleNotifyInterrupt()) while reading, the
 *              read will 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.
 */
void
ProcessNotifyInterrupt(void)
{
        if (IsTransactionOrTransactionBlock())
                return;                                 /* not really idle */

        while (notifyInterruptPending)
                ProcessIncomingNotify();
}


/*
 * 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 oldpos;
        QueuePosition head;
        bool            advanceTail;

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

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

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

        /*----------
         * Note that we deliver everything that we see in the queue and that
         * matches our _current_ listening state.
         * Especially we do not take into account different commit times.
         * Consider the following example:
         *
         * Backend 1:                                    Backend 2:
         *
         * transaction starts
         * NOTIFY foo;
         * commit starts
         *                                                               transaction starts
         *                                                               LISTEN foo;
         *                                                               commit starts
         * commit to clog
         *                                                               commit to clog
         *
         * It could happen that backend 2 sees the notification from backend 1 in
         * the queue.  Even though the notifying transaction committed before
         * the listening transaction, we still deliver the notification.
         *
         * The idea is that an additional notification does not do any harm, we
         * just need to make sure that we do not miss a notification.
         *
         * 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 backend position before we lose
         * control to an error.  (We could alternatively retake AsyncQueueLock
         * and move the position before handling each individual message, but
         * that seems like too much lock traffic.)
         *----------
         */
        PG_TRY();
        {
                bool            reachedStop;

                do
                {
                        int                     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 AsyncCtlLock 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(AsyncCtl, 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,
                                   AsyncCtl->shared->page_buffer[slotno] + curoffset,
                                   copysize);
                        /* Release lock that we got from SimpleLruReadPage_ReadOnly() */
                        LWLockRelease(AsyncCtlLock);

                        /*
                         * 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 AsyncQueueLock 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);
                } while (!reachedStop);
        }
        PG_CATCH();
        {
                /* Update shared state */
                LWLockAcquire(AsyncQueueLock, LW_SHARED);
                QUEUE_BACKEND_POS(MyBackendId) = pos;
                advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
                LWLockRelease(AsyncQueueLock);

                /* If we were the laziest backend, try to advance the tail pointer */
                if (advanceTail)
                        asyncQueueAdvanceTail();

                PG_RE_THROW();
        }
        PG_END_TRY();

        /* Update shared state */
        LWLockAcquire(AsyncQueueLock, LW_SHARED);
        QUEUE_BACKEND_POS(MyBackendId) = pos;
        advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
        LWLockRelease(AsyncQueueLock);

        /* If we were the laziest backend, try to advance the tail pointer */
        if (advanceTail)
                asyncQueueAdvanceTail();
}

/*
 * 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 AsyncCtlLock 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)
{
        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 (TransactionIdIsInProgress(qe->xid))
                        {
                                /*
                                 * 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 TransactionIdIsInProgress 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 tqual.c.
                                 */
                                *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.
 */
static void
asyncQueueAdvanceTail(void)
{
        QueuePosition min;
        int                     i;
        int                     oldtailpage;
        int                     newtailpage;
        int                     boundary;

        LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
        min = QUEUE_HEAD;
        for (i = 1; i <= MaxBackends; i++)
        {
                if (QUEUE_BACKEND_PID(i) != InvalidPid)
                        min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
        }
        oldtailpage = QUEUE_POS_PAGE(QUEUE_TAIL);
        QUEUE_TAIL = min;
        LWLockRelease(AsyncQueueLock);

        /*
         * 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 AsyncCtlLock but will also release
                 * the lock again.
                 */
                SimpleLruTruncate(AsyncCtl, newtailpage);
        }
}

/*
 * ProcessIncomingNotify
 *
 *              Deal with arriving NOTIFYs from other backends as soon as it's safe to
 *              do so. This used to be called from the PROCSIG_NOTIFY_INTERRUPT
 *              signal handler, but isn't anymore.
 *
 *              Scan the queue for arriving notifications and report them to my front
 *              end.
 *
 *              NOTE: since we are outside any transaction, we must create our own.
 */
static void
ProcessIncomingNotify(void)
{
        /* 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", false);

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

        asyncQueueReadAllNotifications();

        CommitTransactionCommand();

        /*
         * Must flush the notify messages to ensure frontend gets them promptly.
         */
        pq_flush();

        set_ps_display("idle", false);

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

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

                pq_beginmessage(&buf, 'A');
                pq_sendint(&buf, srcPid, sizeof(int32));
                pq_sendstring(&buf, channel);
                if (PG_PROTOCOL_MAJOR(FrontendProtocol) >= 3)
                        pq_sendstring(&buf, payload);
                pq_endmessage(&buf);

                /*
                 * NOTE: we do not do pq_flush() here.  For a self-notify, it will
                 * happen at the end of the transaction, and for incoming notifies
                 * ProcessIncomingNotify will do it after finding all the notifies.
                 */
        }
        else
                elog(INFO, "NOTIFY for \"%s\" payload \"%s\"", channel, payload);
}

/* Does pendingNotifies include the given channel/payload? */
static bool
AsyncExistsPendingNotify(const char *channel, const char *payload)
{
        ListCell   *p;
        Notification *n;

        if (pendingNotifies == NIL)
                return false;

        if (payload == NULL)
                payload = "";

        /*----------
         * We need to append new elements to the end of the list in order to keep
         * the order. However, on the other hand we'd like to check the list
         * backwards in order to make duplicate-elimination a tad faster when the
         * same condition is signaled many times in a row. So as a compromise we
         * check the tail element first which we can access directly. If this
         * doesn't match, we check the whole list.
         *
         * As we are not checking our parents' lists, we can still get duplicates
         * in combination with subtransactions, like in:
         *
         * begin;
         * notify foo '1';
         * savepoint foo;
         * notify foo '1';
         * commit;
         *----------
         */
        n = (Notification *) llast(pendingNotifies);
        if (strcmp(n->channel, channel) == 0 &&
                strcmp(n->payload, payload) == 0)
                return true;

        foreach(p, pendingNotifies)
        {
                n = (Notification *) lfirst(p);

                if (strcmp(n->channel, channel) == 0 &&
                        strcmp(n->payload, payload) == 0)
                        return true;
        }

        return false;
}

/* Clear the pendingActions and pendingNotifies lists. */
static void
ClearPendingActionsAndNotifies(void)
{
        /*
         * We used to have to explicitly deallocate the list members and nodes,
         * because they were malloc'd.  Now, since we know they are palloc'd in
         * CurTransactionContext, we need not do that --- they'll go away
         * automatically at transaction exit.  We need only reset the list head
         * pointers.
         */
        pendingActions = NIL;
        pendingNotifies = NIL;
}