Provide support for multiplexing SIGUSR1 signal. The upcoming synchronous

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

/*-------------------------------------------------------------------------
 *
 * async.c
 *        Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */

/*-------------------------------------------------------------------------
 * New Async Notification Model:
 * 1. Multiple backends on same machine.  Multiple backends listening on
 *        one relation.  (Note: "listening on a relation" is not really the
 *        right way to think about it, since the notify names need not have
 *        anything to do with the names of relations actually in the database.
 *        But this terminology is all over the code and docs, and I don't feel
 *        like trying to replace it.)
 *
 * 2. There is a tuple in relation "pg_listener" for each active LISTEN,
 *        ie, each relname/listenerPID pair.  The "notification" field of the
 *        tuple is zero when no NOTIFY is pending for that listener, or the PID
 *        of the originating backend when a cross-backend NOTIFY is pending.
 *        (We skip writing to pg_listener when doing a self-NOTIFY, so the
 *        notification field should never be equal to the listenerPID field.)
 *
 * 3. The NOTIFY statement itself (routine Async_Notify) just adds the target
 *        relname to a list of outstanding NOTIFY requests.  Actual processing
 *        happens if and only if we reach transaction commit.  At that time (in
 *        routine AtCommit_Notify) we scan pg_listener for matching relnames.
 *        If the listenerPID in a matching tuple is ours, we just send a notify
 *        message to our own front end.  If it is not ours, and "notification"
 *        is not already nonzero, we set notification to our own PID and send a
 *        SIGUSR2 signal to the receiving process (indicated by listenerPID).
 *        BTW: if the signal operation fails, we presume that the listener backend
 *        crashed without removing this tuple, and remove the tuple for it.
 *
 * 4. Upon receipt of a SIGUSR2 signal, the signal handler can call inbound-
 *        notify processing immediately if this backend is idle (ie, it is
 *        waiting for a frontend command and is not within a transaction block).
 *        Otherwise the handler may only set a flag, which will cause the
 *        processing to occur just before we next go idle.
 *
 * 5. Inbound-notify processing consists of scanning pg_listener for tuples
 *        matching our own listenerPID and having nonzero notification fields.
 *        For each such tuple, we send a message to our frontend and clear the
 *        notification field.  BTW: this routine has to start/commit its own
 *        transaction, since by assumption it is only called from outside any
 *        transaction.
 *
 * Like NOTIFY, LISTEN and UNLISTEN just add the desired action to a list
 * of pending actions.  If we reach transaction commit, the changes are
 * applied to pg_listener just before executing any pending NOTIFYs.  This
 * method is necessary because to avoid race conditions, we must hold lock
 * on pg_listener from when we insert a new listener tuple until we commit.
 * To do that and not create undue hazard of deadlock, we don't want to
 * touch pg_listener until we are otherwise done with the transaction;
 * in particular it'd be uncool to still be taking user-commanded locks
 * while holding the pg_listener lock.
 *
 * Although we grab ExclusiveLock on pg_listener for any operation,
 * the lock is never held very long, so it shouldn't cause too much of
 * a performance problem.  (Previously we used AccessExclusiveLock, but
 * there's no real reason to forbid concurrent reads.)
 *
 * An application that listens on the same relname 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.  Note,
 * however, that we do *not* guarantee that a separate frontend message will
 * be sent for every outside NOTIFY.  Since there is only room for one
 * originating PID in pg_listener, outside notifies occurring at about the
 * same time may be collapsed into a single message bearing the PID of the
 * first outside backend to perform the NOTIFY.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

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

#include "access/heapam.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "catalog/pg_listener.h"
#include "commands/async.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/sinval.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/tqual.h"


/*
 * 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 modify pg_listener 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            condname[1];                            /* actually, as long as needed */
} 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 relnames 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.
 */
static List *pendingNotifies = NIL;                             /* list of C strings */

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

/*
 * State for inbound notifies consists of two flags: one saying whether
 * the signal handler is currently allowed to call ProcessIncomingNotify
 * directly, and one saying whether the signal has occurred but the handler
 * was not allowed to call ProcessIncomingNotify at the time.
 *
 * NB: the "volatile" on these declarations is critical!  If your compiler
 * does not grok "volatile", you'd be best advised to compile this file
 * with all optimization turned off.
 */
static volatile sig_atomic_t notifyInterruptEnabled = 0;
static volatile sig_atomic_t notifyInterruptOccurred = 0;

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

bool            Trace_notify = false;


static void queue_listen(ListenActionKind action, const char *condname);
static void Async_UnlistenOnExit(int code, Datum arg);
static void Exec_Listen(Relation lRel, const char *relname);
static void Exec_Unlisten(Relation lRel, const char *relname);
static void Exec_UnlistenAll(Relation lRel);
static void Send_Notify(Relation lRel);
static void ProcessIncomingNotify(void);
static void NotifyMyFrontEnd(char *relname, int32 listenerPID);
static bool AsyncExistsPendingNotify(const char *relname);
static void ClearPendingActionsAndNotifies(void);


/*
 * Async_Notify
 *
 *              This is executed by the SQL notify command.
 *
 *              Adds the relation to the list of pending notifies.
 *              Actual notification happens during transaction commit.
 *              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 */
void
Async_Notify(const char *relname)
{
        if (Trace_notify)
                elog(DEBUG1, "Async_Notify(%s)", relname);

        /* no point in making duplicate entries in the list ... */
        if (!AsyncExistsPendingNotify(relname))
        {
                /*
                 * The name list needs to live until end of transaction, so store it
                 * in the transaction context.
                 */
                MemoryContext oldcontext;

                oldcontext = MemoryContextSwitchTo(CurTransactionContext);

                /*
                 * Ordering of the list isn't important.  We choose to put new
                 * entries on the front, as this might make duplicate-elimination
                 * a tad faster when the same condition is signaled many times in
                 * a row.
                 */
                pendingNotifies = lcons(pstrdup(relname), pendingNotifies);

                MemoryContextSwitchTo(oldcontext);
        }
}

/*
 * queue_listen
 *              Common code for listen, unlisten, unlisten all commands.
 *
 *              Adds the request to the list of pending actions.
 *              Actual update of pg_listener happens during transaction commit.
 *              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 */
static void
queue_listen(ListenActionKind action, const char *condname)
{
        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(sizeof(ListenAction) + strlen(condname));
        actrec->action = action;
        strcpy(actrec->condname, condname);

        pendingActions = lappend(pendingActions, actrec);

        MemoryContextSwitchTo(oldcontext);
}

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

        queue_listen(LISTEN_LISTEN, relname);
}

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

        queue_listen(LISTEN_UNLISTEN, relname);
}

/*
 * 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);

        queue_listen(LISTEN_UNLISTEN_ALL, "");
}

/*
 * Async_UnlistenOnExit
 *
 *              Clean up the pg_listener table at backend exit.
 *
 *              This is executed 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)
{
        /*
         * We need to start/commit a transaction for the unlisten, but if there is
         * already an active transaction we had better abort that one first.
         * Otherwise we'd end up committing changes that probably ought to be
         * discarded.
         */
        AbortOutOfAnyTransaction();
        /* Now we can do the unlisten */
        StartTransactionCommand();
        Async_UnlistenAll();
        CommitTransactionCommand();
}

/*
 * 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)
{
        ListCell   *p;

        /* It's not sensible to have any pending LISTEN/UNLISTEN actions */
        if (pendingActions)
                ereport(ERROR,
                                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                 errmsg("cannot PREPARE a transaction that has executed LISTEN or UNLISTEN")));

        /* We can deal with pending NOTIFY though */
        foreach(p, pendingNotifies)
        {
                const char *relname = (const char *) lfirst(p);

                RegisterTwoPhaseRecord(TWOPHASE_RM_NOTIFY_ID, 0,
                                                           relname, strlen(relname) + 1);
        }

        /*
         * We can clear the state immediately, rather than needing a separate
         * PostPrepare call, because if the transaction fails we'd just discard
         * the state anyway.
         */
        ClearPendingActionsAndNotifies();
}

/*
 * AtCommit_Notify
 *
 *              This is called at transaction commit.
 *
 *              If there are pending LISTEN/UNLISTEN actions, insert or delete
 *              tuples in pg_listener accordingly.
 *
 *              If there are outbound notify requests in the pendingNotifies list,
 *              scan pg_listener for matching tuples, and either signal the other
 *              backend or send a message to our own frontend.
 *
 *              NOTE: we are still inside the current transaction, therefore can
 *              piggyback on its committing of changes.
 */
void
AtCommit_Notify(void)
{
        Relation        lRel;
        ListCell   *p;

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

        /*
         * NOTIFY is disabled if not normal processing mode. This test used to be
         * in xact.c, but it seems cleaner to do it here.
         */
        if (!IsNormalProcessingMode())
        {
                ClearPendingActionsAndNotifies();
                return;
        }

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

        /* Acquire ExclusiveLock on pg_listener */
        lRel = heap_open(ListenerRelationId, ExclusiveLock);

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

                switch (actrec->action)
                {
                        case LISTEN_LISTEN:
                                Exec_Listen(lRel, actrec->condname);
                                break;
                        case LISTEN_UNLISTEN:
                                Exec_Unlisten(lRel, actrec->condname);
                                break;
                        case LISTEN_UNLISTEN_ALL:
                                Exec_UnlistenAll(lRel);
                                break;
                }

                /* We must CCI after each action in case of conflicting actions */
                CommandCounterIncrement();
        }

        /* Perform any pending notifies */
        if (pendingNotifies)
                Send_Notify(lRel);

        /*
         * We do NOT release the lock on pg_listener here; we need to hold it
         * until end of transaction (which is about to happen, anyway) to ensure
         * that notified backends see our tuple updates when they look. Else they
         * might disregard the signal, which would make the application programmer
         * very unhappy.  Also, this prevents race conditions when we have just
         * inserted a listening tuple.
         */
        heap_close(lRel, NoLock);

        ClearPendingActionsAndNotifies();

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

/*
 * Exec_Listen --- subroutine for AtCommit_Notify
 *
 *              Register the current backend as listening on the specified relation.
 */
static void
Exec_Listen(Relation lRel, const char *relname)
{
        HeapScanDesc scan;
        HeapTuple       tuple;
        Datum           values[Natts_pg_listener];
        bool            nulls[Natts_pg_listener];
        NameData        condname;
        bool            alreadyListener = false;

        if (Trace_notify)
                elog(DEBUG1, "Exec_Listen(%s,%d)", relname, MyProcPid);

        /* Detect whether we are already listening on this relname */
        scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);
        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);

                if (listener->listenerpid == MyProcPid &&
                        strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
                {
                        alreadyListener = true;
                        /* No need to scan the rest of the table */
                        break;
                }
        }
        heap_endscan(scan);

        if (alreadyListener)
                return;

        /*
         * OK to insert a new tuple
         */
        memset(nulls, false, sizeof(nulls));

        namestrcpy(&condname, relname);
        values[Anum_pg_listener_relname - 1] = NameGetDatum(&condname);
        values[Anum_pg_listener_pid - 1] = Int32GetDatum(MyProcPid);
        values[Anum_pg_listener_notify - 1] = Int32GetDatum(0); /* no notifies pending */

        tuple = heap_form_tuple(RelationGetDescr(lRel), values, nulls);

        simple_heap_insert(lRel, tuple);

#ifdef NOT_USED                                 /* currently there are no indexes */
        CatalogUpdateIndexes(lRel, tuple);
#endif

        heap_freetuple(tuple);

        /*
         * now that we are listening, make sure we will unlisten before dying.
         */
        if (!unlistenExitRegistered)
        {
                on_shmem_exit(Async_UnlistenOnExit, 0);
                unlistenExitRegistered = true;
        }
}

/*
 * Exec_Unlisten --- subroutine for AtCommit_Notify
 *
 *              Remove the current backend from the list of listening backends
 *              for the specified relation.
 */
static void
Exec_Unlisten(Relation lRel, const char *relname)
{
        HeapScanDesc scan;
        HeapTuple       tuple;

        if (Trace_notify)
                elog(DEBUG1, "Exec_Unlisten(%s,%d)", relname, MyProcPid);

        scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);
        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);

                if (listener->listenerpid == MyProcPid &&
                        strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
                {
                        /* Found the matching tuple, delete it */
                        simple_heap_delete(lRel, &tuple->t_self);

                        /*
                         * We assume there can be only one match, so no need to scan the
                         * rest of the table
                         */
                        break;
                }
        }
        heap_endscan(scan);

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

/*
 * Exec_UnlistenAll --- subroutine for AtCommit_Notify
 *
 *              Update pg_listener to unlisten all relations for this backend.
 */
static void
Exec_UnlistenAll(Relation lRel)
{
        HeapScanDesc scan;
        HeapTuple       lTuple;
        ScanKeyData key[1];

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

        /* Find and delete all entries with my listenerPID */
        ScanKeyInit(&key[0],
                                Anum_pg_listener_pid,
                                BTEqualStrategyNumber, F_INT4EQ,
                                Int32GetDatum(MyProcPid));
        scan = heap_beginscan(lRel, SnapshotNow, 1, key);

        while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
                simple_heap_delete(lRel, &lTuple->t_self);

        heap_endscan(scan);
}

/*
 * Send_Notify --- subroutine for AtCommit_Notify
 *
 *              Scan pg_listener for tuples matching our pending notifies, and
 *              either signal the other backend or send a message to our own frontend.
 */
static void
Send_Notify(Relation lRel)
{
        TupleDesc       tdesc = RelationGetDescr(lRel);
        HeapScanDesc scan;
        HeapTuple       lTuple,
                                rTuple;
        Datum           value[Natts_pg_listener];
        bool            repl[Natts_pg_listener],
                                nulls[Natts_pg_listener];

        /* preset data to update notify column to MyProcPid */
        memset(nulls, false, sizeof(nulls));
        memset(repl, false, sizeof(repl));
        repl[Anum_pg_listener_notify - 1] = true;
        memset(value, 0, sizeof(value));
        value[Anum_pg_listener_notify - 1] = Int32GetDatum(MyProcPid);

        scan = heap_beginscan(lRel, SnapshotNow, 0, NULL);

        while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
                char       *relname = NameStr(listener->relname);
                int32           listenerPID = listener->listenerpid;

                if (!AsyncExistsPendingNotify(relname))
                        continue;

                if (listenerPID == MyProcPid)
                {
                        /*
                         * Self-notify: no need to bother with table update. Indeed, we
                         * *must not* clear the notification field in this path, or we
                         * could lose an outside notify, which'd be bad for applications
                         * that ignore self-notify messages.
                         */
                        if (Trace_notify)
                                elog(DEBUG1, "AtCommit_Notify: notifying self");

                        NotifyMyFrontEnd(relname, listenerPID);
                }
                else
                {
                        if (Trace_notify)
                                elog(DEBUG1, "AtCommit_Notify: notifying pid %d",
                                         listenerPID);

                        /*
                         * If someone has already notified this listener, we don't bother
                         * modifying the table, but we do still send a SIGUSR2 signal,
                         * just in case that backend missed the earlier signal for some
                         * reason.      It's OK to send the signal first, because the other
                         * guy can't read pg_listener until we unlock it.
                         */
                        if (kill(listenerPID, SIGUSR2) < 0)
                        {
                                /*
                                 * Get rid of pg_listener entry if it refers to a PID that no
                                 * longer exists.  Presumably, that backend crashed without
                                 * deleting its pg_listener entries. This code used to only
                                 * delete the entry if errno==ESRCH, but as far as I can see
                                 * we should just do it for any failure (certainly at least
                                 * for EPERM too...)
                                 */
                                simple_heap_delete(lRel, &lTuple->t_self);
                        }
                        else if (listener->notification == 0)
                        {
                                /* Rewrite the tuple with my PID in notification column */
                                rTuple = heap_modify_tuple(lTuple, tdesc, value, nulls, repl);
                                simple_heap_update(lRel, &lTuple->t_self, rTuple);

#ifdef NOT_USED                                 /* currently there are no indexes */
                                CatalogUpdateIndexes(lRel, rTuple);
#endif
                        }
                }
        }

        heap_endscan(scan);
}

/*
 * 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)
{
        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);
        }
}

/*
 * NotifyInterruptHandler
 *
 *              This is the signal handler for SIGUSR2.
 *
 *              If we are idle (notifyInterruptEnabled is set), we can safely invoke
 *              ProcessIncomingNotify directly.  Otherwise, just set a flag
 *              to do it later.
 */
void
NotifyInterruptHandler(SIGNAL_ARGS)
{
        int                     save_errno = errno;

        /*
         * Note: this is a SIGNAL HANDLER.      You must be very wary what you do
         * here. Some helpful soul had this routine sprinkled with TPRINTFs, which
         * would likely lead to corruption of stdio buffers if they were ever
         * turned on.
         */

        /* Don't joggle the elbow of proc_exit */
        if (proc_exit_inprogress)
                return;

        if (notifyInterruptEnabled)
        {
                bool            save_ImmediateInterruptOK = ImmediateInterruptOK;

                /*
                 * We may be called while ImmediateInterruptOK is true; turn it off
                 * while messing with the NOTIFY state.  (We would have to save and
                 * restore it anyway, because PGSemaphore operations inside
                 * ProcessIncomingNotify() might reset it.)
                 */
                ImmediateInterruptOK = false;

                /*
                 * I'm not sure whether some flavors of Unix might allow another
                 * SIGUSR2 occurrence to recursively interrupt this routine. To cope
                 * with the possibility, we do the same sort of dance that
                 * EnableNotifyInterrupt must do --- see that routine for comments.
                 */
                notifyInterruptEnabled = 0;             /* disable any recursive signal */
                notifyInterruptOccurred = 1;    /* do at least one iteration */
                for (;;)
                {
                        notifyInterruptEnabled = 1;
                        if (!notifyInterruptOccurred)
                                break;
                        notifyInterruptEnabled = 0;
                        if (notifyInterruptOccurred)
                        {
                                /* Here, it is finally safe to do stuff. */
                                if (Trace_notify)
                                        elog(DEBUG1, "NotifyInterruptHandler: perform async notify");

                                ProcessIncomingNotify();

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

                /*
                 * Restore ImmediateInterruptOK, and check for interrupts if needed.
                 */
                ImmediateInterruptOK = save_ImmediateInterruptOK;
                if (save_ImmediateInterruptOK)
                        CHECK_FOR_INTERRUPTS();
        }
        else
        {
                /*
                 * In this path it is NOT SAFE to do much of anything, except this:
                 */
                notifyInterruptOccurred = 1;
        }

        errno = save_errno;
}

/*
 * EnableNotifyInterrupt
 *
 *              This is called by the PostgresMain main loop just before waiting
 *              for a frontend command.  If we are truly idle (ie, *not* inside
 *              a transaction block), then process any pending inbound notifies,
 *              and enable the signal handler to process future notifies directly.
 *
 *              NOTE: the signal handler starts out disabled, and stays so until
 *              PostgresMain calls this the first time.
 */
void
EnableNotifyInterrupt(void)
{
        if (IsTransactionOrTransactionBlock())
                return;                                 /* not really idle */

        /*
         * This code is tricky because we are communicating with a signal handler
         * that could interrupt us at any point.  If we just checked
         * notifyInterruptOccurred and then set notifyInterruptEnabled, we could
         * fail to respond promptly to a signal that happens in between those two
         * steps.  (A very small time window, perhaps, but Murphy's Law says you
         * can hit it...)  Instead, we first set the enable flag, then test the
         * occurred flag.  If we see an unserviced interrupt has occurred, we
         * re-clear the enable flag before going off to do the service work. (That
         * prevents re-entrant invocation of ProcessIncomingNotify() if another
         * interrupt occurs.) If an interrupt comes in between the setting and
         * clearing of notifyInterruptEnabled, then it will have done the service
         * work and left notifyInterruptOccurred zero, so we have to check again
         * after clearing enable.  The whole thing has to be in a loop in case
         * another interrupt occurs while we're servicing the first. Once we get
         * out of the loop, enable is set and we know there is no unserviced
         * interrupt.
         *
         * NB: an overenthusiastic optimizing compiler could easily break this
         * code. Hopefully, they all understand what "volatile" means these days.
         */
        for (;;)
        {
                notifyInterruptEnabled = 1;
                if (!notifyInterruptOccurred)
                        break;
                notifyInterruptEnabled = 0;
                if (notifyInterruptOccurred)
                {
                        if (Trace_notify)
                                elog(DEBUG1, "EnableNotifyInterrupt: perform async notify");

                        ProcessIncomingNotify();

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

/*
 * DisableNotifyInterrupt
 *
 *              This is called by the PostgresMain main loop just after receiving
 *              a frontend command.  Signal handler execution of inbound notifies
 *              is disabled until the next EnableNotifyInterrupt call.
 *
 *              This also needs to be called when SIGUSR1 with 
 *              PROCSIG_CATCHUP_INTERRUPT is received, so as to prevent conflicts 
 *              if one signal interrupts the other.  So we must return the previous 
 *              state of the flag.
 */
bool
DisableNotifyInterrupt(void)
{
        bool            result = (notifyInterruptEnabled != 0);

        notifyInterruptEnabled = 0;

        return result;
}

/*
 * ProcessIncomingNotify
 *
 *              Deal with arriving NOTIFYs from other backends.
 *              This is called either directly from the SIGUSR2 signal handler,
 *              or the next time control reaches the outer idle loop.
 *              Scan pg_listener for arriving notifies, report them to my front end,
 *              and clear the notification field in pg_listener until next time.
 *
 *              NOTE: since we are outside any transaction, we must create our own.
 */
static void
ProcessIncomingNotify(void)
{
        Relation        lRel;
        TupleDesc       tdesc;
        ScanKeyData key[1];
        HeapScanDesc scan;
        HeapTuple       lTuple,
                                rTuple;
        Datum           value[Natts_pg_listener];
        bool            repl[Natts_pg_listener],
                                nulls[Natts_pg_listener];
        bool            catchup_enabled;

        /* Must prevent catchup interrupt while I am running */
        catchup_enabled = DisableCatchupInterrupt();

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

        set_ps_display("notify interrupt", false);

        notifyInterruptOccurred = 0;

        StartTransactionCommand();

        lRel = heap_open(ListenerRelationId, ExclusiveLock);
        tdesc = RelationGetDescr(lRel);

        /* Scan only entries with my listenerPID */
        ScanKeyInit(&key[0],
                                Anum_pg_listener_pid,
                                BTEqualStrategyNumber, F_INT4EQ,
                                Int32GetDatum(MyProcPid));
        scan = heap_beginscan(lRel, SnapshotNow, 1, key);

        /* Prepare data for rewriting 0 into notification field */
        memset(nulls, false, sizeof(nulls));
        memset(repl, false, sizeof(repl));
        repl[Anum_pg_listener_notify - 1] = true;
        memset(value, 0, sizeof(value));
        value[Anum_pg_listener_notify - 1] = Int32GetDatum(0);

        while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
                Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
                char       *relname = NameStr(listener->relname);
                int32           sourcePID = listener->notification;

                if (sourcePID != 0)
                {
                        /* Notify the frontend */

                        if (Trace_notify)
                                elog(DEBUG1, "ProcessIncomingNotify: received %s from %d",
                                         relname, (int) sourcePID);

                        NotifyMyFrontEnd(relname, sourcePID);

                        /*
                         * Rewrite the tuple with 0 in notification column.
                         */
                        rTuple = heap_modify_tuple(lTuple, tdesc, value, nulls, repl);
                        simple_heap_update(lRel, &lTuple->t_self, rTuple);

#ifdef NOT_USED                                 /* currently there are no indexes */
                        CatalogUpdateIndexes(lRel, rTuple);
#endif
                }
        }
        heap_endscan(scan);

        /*
         * We do NOT release the lock on pg_listener here; we need to hold it
         * until end of transaction (which is about to happen, anyway) to ensure
         * that other backends see our tuple updates when they look. Otherwise, a
         * transaction started after this one might mistakenly think it doesn't
         * need to send this backend a new NOTIFY.
         */
        heap_close(lRel, NoLock);

        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");

        if (catchup_enabled)
                EnableCatchupInterrupt();
}

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

                pq_beginmessage(&buf, 'A');
                pq_sendint(&buf, listenerPID, sizeof(int32));
                pq_sendstring(&buf, relname);
                if (PG_PROTOCOL_MAJOR(FrontendProtocol) >= 3)
                {
                        /* XXX Add parameter string here later */
                        pq_sendstring(&buf, "");
                }
                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", relname);
}

/* Does pendingNotifies include the given relname? */
static bool
AsyncExistsPendingNotify(const char *relname)
{
        ListCell   *p;

        foreach(p, pendingNotifies)
        {
                const char *prelname = (const char *) lfirst(p);

                if (strcmp(prelname, relname) == 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;
}

/*
 * 2PC processing routine for COMMIT PREPARED case.
 *
 * (We don't have to do anything for ROLLBACK PREPARED.)
 */
void
notify_twophase_postcommit(TransactionId xid, uint16 info,
                                                   void *recdata, uint32 len)
{
        /*
         * Set up to issue the NOTIFY at the end of my own current transaction.
         * (XXX this has some issues if my own transaction later rolls back, or if
         * there is any significant delay before I commit.      OK for now because we
         * disallow COMMIT PREPARED inside a transaction block.)
         */
        Async_Notify((char *) recdata);
}