1 /*-------------------------------------------------------------------------
4 * Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
6 * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
7 * Portions Copyright (c) 1994, Regents of the University of California
10 * $Header: /cvsroot/pgsql/src/backend/commands/async.c,v 1.84 2002/05/05 00:03:28 tgl Exp $
12 *-------------------------------------------------------------------------
15 /*-------------------------------------------------------------------------
16 * New Async Notification Model:
17 * 1. Multiple backends on same machine. Multiple backends listening on
18 * one relation. (Note: "listening on a relation" is not really the
19 * right way to think about it, since the notify names need not have
20 * anything to do with the names of relations actually in the database.
21 * But this terminology is all over the code and docs, and I don't feel
22 * like trying to replace it.)
24 * 2. There is a tuple in relation "pg_listener" for each active LISTEN,
25 * ie, each relname/listenerPID pair. The "notification" field of the
26 * tuple is zero when no NOTIFY is pending for that listener, or the PID
27 * of the originating backend when a cross-backend NOTIFY is pending.
28 * (We skip writing to pg_listener when doing a self-NOTIFY, so the
29 * notification field should never be equal to the listenerPID field.)
31 * 3. The NOTIFY statement itself (routine Async_Notify) just adds the target
32 * relname to a list of outstanding NOTIFY requests. Actual processing
33 * happens if and only if we reach transaction commit. At that time (in
34 * routine AtCommit_Notify) we scan pg_listener for matching relnames.
35 * If the listenerPID in a matching tuple is ours, we just send a notify
36 * message to our own front end. If it is not ours, and "notification"
37 * is not already nonzero, we set notification to our own PID and send a
38 * SIGUSR2 signal to the receiving process (indicated by listenerPID).
39 * BTW: if the signal operation fails, we presume that the listener backend
40 * crashed without removing this tuple, and remove the tuple for it.
42 * 4. Upon receipt of a SIGUSR2 signal, the signal handler can call inbound-
43 * notify processing immediately if this backend is idle (ie, it is
44 * waiting for a frontend command and is not within a transaction block).
45 * Otherwise the handler may only set a flag, which will cause the
46 * processing to occur just before we next go idle.
48 * 5. Inbound-notify processing consists of scanning pg_listener for tuples
49 * matching our own listenerPID and having nonzero notification fields.
50 * For each such tuple, we send a message to our frontend and clear the
51 * notification field. BTW: this routine has to start/commit its own
52 * transaction, since by assumption it is only called from outside any
55 * Although we grab AccessExclusiveLock on pg_listener for any operation,
56 * the lock is never held very long, so it shouldn't cause too much of
57 * a performance problem.
59 * An application that listens on the same relname it notifies will get
60 * NOTIFY messages for its own NOTIFYs. These can be ignored, if not useful,
61 * by comparing be_pid in the NOTIFY message to the application's own backend's
62 * PID. (As of FE/BE protocol 2.0, the backend's PID is provided to the
63 * frontend during startup.) The above design guarantees that notifies from
64 * other backends will never be missed by ignoring self-notifies. Note,
65 * however, that we do *not* guarantee that a separate frontend message will
66 * be sent for every outside NOTIFY. Since there is only room for one
67 * originating PID in pg_listener, outside notifies occurring at about the
68 * same time may be collapsed into a single message bearing the PID of the
69 * first outside backend to perform the NOTIFY.
70 *-------------------------------------------------------------------------
78 #include <sys/types.h>
79 #include <netinet/in.h>
81 #include "access/heapam.h"
82 #include "catalog/catname.h"
83 #include "catalog/pg_listener.h"
84 #include "commands/async.h"
85 #include "libpq/libpq.h"
86 #include "libpq/pqformat.h"
87 #include "miscadmin.h"
88 #include "storage/ipc.h"
89 #include "tcop/tcopprot.h"
90 #include "utils/fmgroids.h"
91 #include "utils/ps_status.h"
92 #include "utils/syscache.h"
95 /* stuff that we really ought not be touching directly :-( */
96 extern TransactionState CurrentTransactionState;
100 * State for outbound notifies consists of a list of all relnames NOTIFYed
101 * in the current transaction. We do not actually perform a NOTIFY until
102 * and unless the transaction commits. pendingNotifies is NIL if no
103 * NOTIFYs have been done in the current transaction. The List nodes and
104 * referenced strings are all palloc'd in TopTransactionContext.
106 static List *pendingNotifies = NIL;
109 * State for inbound notifies consists of two flags: one saying whether
110 * the signal handler is currently allowed to call ProcessIncomingNotify
111 * directly, and one saying whether the signal has occurred but the handler
112 * was not allowed to call ProcessIncomingNotify at the time.
114 * NB: the "volatile" on these declarations is critical! If your compiler
115 * does not grok "volatile", you'd be best advised to compile this file
116 * with all optimization turned off.
118 static volatile int notifyInterruptEnabled = 0;
119 static volatile int notifyInterruptOccurred = 0;
121 /* True if we've registered an on_shmem_exit cleanup */
122 static bool unlistenExitRegistered = false;
124 bool Trace_notify = false;
127 static void Async_UnlistenAll(void);
128 static void Async_UnlistenOnExit(void);
129 static void ProcessIncomingNotify(void);
130 static void NotifyMyFrontEnd(char *relname, int32 listenerPID);
131 static bool AsyncExistsPendingNotify(const char *relname);
132 static void ClearPendingNotifies(void);
136 *--------------------------------------------------------------
139 * This is executed by the SQL notify command.
141 * Adds the relation to the list of pending notifies.
142 * Actual notification happens during transaction commit.
143 * ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
148 *--------------------------------------------------------------
151 Async_Notify(char *relname)
154 elog(LOG, "Async_Notify: %s", relname);
156 /* no point in making duplicate entries in the list ... */
157 if (!AsyncExistsPendingNotify(relname))
160 * The name list needs to live until end of transaction, so store
161 * it in the top transaction context.
163 MemoryContext oldcontext;
165 oldcontext = MemoryContextSwitchTo(TopTransactionContext);
167 pendingNotifies = lcons(pstrdup(relname), pendingNotifies);
169 MemoryContextSwitchTo(oldcontext);
174 *--------------------------------------------------------------
177 * This is executed by the SQL listen command.
179 * Register a backend (identified by its Unix PID) as listening
180 * on the specified relation.
186 * pg_listener is updated.
188 *--------------------------------------------------------------
191 Async_Listen(char *relname, int pid)
196 Datum values[Natts_pg_listener];
197 char nulls[Natts_pg_listener];
199 bool alreadyListener = false;
202 elog(LOG, "Async_Listen: %s", relname);
204 lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
206 /* Detect whether we are already listening on this relname */
207 scan = heap_beginscan(lRel, 0, SnapshotNow, 0, (ScanKey) NULL);
208 while (HeapTupleIsValid(tuple = heap_getnext(scan, 0)))
210 Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);
212 if (listener->listenerpid == pid &&
213 strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
215 alreadyListener = true;
216 /* No need to scan the rest of the table */
224 heap_close(lRel, AccessExclusiveLock);
225 elog(WARNING, "Async_Listen: We are already listening on %s", relname);
230 * OK to insert a new tuple
233 for (i = 0; i < Natts_pg_listener; i++)
236 values[i] = PointerGetDatum(NULL);
240 values[i++] = (Datum) relname;
241 values[i++] = (Datum) pid;
242 values[i++] = (Datum) 0; /* no notifies pending */
244 tuple = heap_formtuple(RelationGetDescr(lRel), values, nulls);
245 heap_insert(lRel, tuple);
247 #ifdef NOT_USED /* currently there are no indexes */
248 if (RelationGetForm(lRel)->relhasindex)
250 Relation idescs[Num_pg_listener_indices];
252 CatalogOpenIndices(Num_pg_listener_indices, Name_pg_listener_indices, idescs);
253 CatalogIndexInsert(idescs, Num_pg_listener_indices, lRel, tuple);
254 CatalogCloseIndices(Num_pg_listener_indices, idescs);
258 heap_freetuple(tuple);
260 heap_close(lRel, AccessExclusiveLock);
263 * now that we are listening, make sure we will unlisten before dying.
265 if (!unlistenExitRegistered)
267 on_shmem_exit(Async_UnlistenOnExit, 0);
268 unlistenExitRegistered = true;
273 *--------------------------------------------------------------
276 * This is executed by the SQL unlisten command.
278 * Remove the backend from the list of listening backends
279 * for the specified relation.
285 * pg_listener is updated.
287 *--------------------------------------------------------------
290 Async_Unlisten(char *relname, int pid)
296 /* Handle specially the `unlisten "*"' command */
297 if ((!relname) || (*relname == '\0') || (strcmp(relname, "*") == 0))
304 elog(LOG, "Async_Unlisten %s", relname);
306 lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
308 scan = heap_beginscan(lRel, 0, SnapshotNow, 0, (ScanKey) NULL);
309 while (HeapTupleIsValid(tuple = heap_getnext(scan, 0)))
311 Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);
313 if (listener->listenerpid == pid &&
314 strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
316 /* Found the matching tuple, delete it */
317 simple_heap_delete(lRel, &tuple->t_self);
320 * We assume there can be only one match, so no need to scan
321 * the rest of the table
328 heap_close(lRel, AccessExclusiveLock);
331 * We do not complain about unlistening something not being listened;
337 *--------------------------------------------------------------
340 * Unlisten all relations for this backend.
342 * This is invoked by UNLISTEN "*" command, and also at backend exit.
348 * pg_listener is updated.
350 *--------------------------------------------------------------
353 Async_UnlistenAll(void)
362 elog(LOG, "Async_UnlistenAll");
364 lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
365 tdesc = RelationGetDescr(lRel);
367 /* Find and delete all entries with my listenerPID */
368 ScanKeyEntryInitialize(&key[0], 0,
369 Anum_pg_listener_pid,
371 Int32GetDatum(MyProcPid));
372 scan = heap_beginscan(lRel, 0, SnapshotNow, 1, key);
374 while (HeapTupleIsValid(lTuple = heap_getnext(scan, 0)))
375 simple_heap_delete(lRel, &lTuple->t_self);
378 heap_close(lRel, AccessExclusiveLock);
382 *--------------------------------------------------------------
383 * Async_UnlistenOnExit
385 * Clean up the pg_listener table at backend exit.
387 * This is executed if we have done any LISTENs in this backend.
388 * It might not be necessary anymore, if the user UNLISTENed everything,
389 * but we don't try to detect that case.
395 * pg_listener is updated if necessary.
397 *--------------------------------------------------------------
400 Async_UnlistenOnExit(void)
403 * We need to start/commit a transaction for the unlisten, but if
404 * there is already an active transaction we had better abort that one
405 * first. Otherwise we'd end up committing changes that probably
406 * ought to be discarded.
408 AbortOutOfAnyTransaction();
409 /* Now we can do the unlisten */
410 StartTransactionCommand();
412 CommitTransactionCommand();
416 *--------------------------------------------------------------
419 * This is called at transaction commit.
421 * If there are outbound notify requests in the pendingNotifies list,
422 * scan pg_listener for matching tuples, and either signal the other
423 * backend or send a message to our own frontend.
425 * NOTE: we are still inside the current transaction, therefore can
426 * piggyback on its committing of changes.
432 * Tuples in pg_listener that have matching relnames and other peoples'
433 * listenerPIDs are updated with a nonzero notification field.
435 *--------------------------------------------------------------
438 AtCommit_Notify(void)
445 Datum value[Natts_pg_listener];
446 char repl[Natts_pg_listener],
447 nulls[Natts_pg_listener];
449 if (pendingNotifies == NIL)
450 return; /* no NOTIFY statements in this
454 * NOTIFY is disabled if not normal processing mode. This test used to
455 * be in xact.c, but it seems cleaner to do it here.
457 if (!IsNormalProcessingMode())
459 ClearPendingNotifies();
464 elog(LOG, "AtCommit_Notify");
466 /* preset data to update notify column to MyProcPid */
467 nulls[0] = nulls[1] = nulls[2] = ' ';
468 repl[0] = repl[1] = repl[2] = ' ';
469 repl[Anum_pg_listener_notify - 1] = 'r';
470 value[0] = value[1] = value[2] = (Datum) 0;
471 value[Anum_pg_listener_notify - 1] = Int32GetDatum(MyProcPid);
473 lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
474 tdesc = RelationGetDescr(lRel);
475 scan = heap_beginscan(lRel, 0, SnapshotNow, 0, (ScanKey) NULL);
477 while (HeapTupleIsValid(lTuple = heap_getnext(scan, 0)))
479 Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
480 char *relname = NameStr(listener->relname);
481 int32 listenerPID = listener->listenerpid;
483 if (!AsyncExistsPendingNotify(relname))
486 if (listenerPID == MyProcPid)
489 * Self-notify: no need to bother with table update. Indeed,
490 * we *must not* clear the notification field in this path, or
491 * we could lose an outside notify, which'd be bad for
492 * applications that ignore self-notify messages.
496 elog(LOG, "AtCommit_Notify: notifying self");
498 NotifyMyFrontEnd(relname, listenerPID);
503 elog(LOG, "AtCommit_Notify: notifying pid %d",
507 * If someone has already notified this listener, we don't
508 * bother modifying the table, but we do still send a SIGUSR2
509 * signal, just in case that backend missed the earlier signal
510 * for some reason. It's OK to send the signal first, because
511 * the other guy can't read pg_listener until we unlock it.
513 if (kill(listenerPID, SIGUSR2) < 0)
516 * Get rid of pg_listener entry if it refers to a PID that
517 * no longer exists. Presumably, that backend crashed
518 * without deleting its pg_listener entries. This code
519 * used to only delete the entry if errno==ESRCH, but as
520 * far as I can see we should just do it for any failure
521 * (certainly at least for EPERM too...)
523 simple_heap_delete(lRel, &lTuple->t_self);
525 else if (listener->notification == 0)
527 rTuple = heap_modifytuple(lTuple, lRel,
529 simple_heap_update(lRel, &lTuple->t_self, rTuple);
531 #ifdef NOT_USED /* currently there are no indexes */
532 if (RelationGetForm(lRel)->relhasindex)
534 Relation idescs[Num_pg_listener_indices];
536 CatalogOpenIndices(Num_pg_listener_indices, Name_pg_listener_indices, idescs);
537 CatalogIndexInsert(idescs, Num_pg_listener_indices, lRel, rTuple);
538 CatalogCloseIndices(Num_pg_listener_indices, idescs);
548 * We do NOT release the lock on pg_listener here; we need to hold it
549 * until end of transaction (which is about to happen, anyway) to
550 * ensure that notified backends see our tuple updates when they look.
551 * Else they might disregard the signal, which would make the
552 * application programmer very unhappy.
554 heap_close(lRel, NoLock);
556 ClearPendingNotifies();
559 elog(LOG, "AtCommit_Notify: done");
563 *--------------------------------------------------------------
566 * This is called at transaction abort.
568 * Gets rid of pending outbound notifies that we would have executed
569 * if the transaction got committed.
574 *--------------------------------------------------------------
579 ClearPendingNotifies();
583 *--------------------------------------------------------------
584 * Async_NotifyHandler
586 * This is the signal handler for SIGUSR2.
588 * If we are idle (notifyInterruptEnabled is set), we can safely invoke
589 * ProcessIncomingNotify directly. Otherwise, just set a flag
597 *--------------------------------------------------------------
600 Async_NotifyHandler(SIGNAL_ARGS)
602 int save_errno = errno;
605 * Note: this is a SIGNAL HANDLER. You must be very wary what you do
606 * here. Some helpful soul had this routine sprinkled with TPRINTFs,
607 * which would likely lead to corruption of stdio buffers if they were
611 if (notifyInterruptEnabled)
614 * I'm not sure whether some flavors of Unix might allow another
615 * SIGUSR2 occurrence to recursively interrupt this routine. To
616 * cope with the possibility, we do the same sort of dance that
617 * EnableNotifyInterrupt must do --- see that routine for
620 notifyInterruptEnabled = 0; /* disable any recursive signal */
621 notifyInterruptOccurred = 1; /* do at least one iteration */
624 notifyInterruptEnabled = 1;
625 if (!notifyInterruptOccurred)
627 notifyInterruptEnabled = 0;
628 if (notifyInterruptOccurred)
630 /* Here, it is finally safe to do stuff. */
632 elog(LOG, "Async_NotifyHandler: perform async notify");
634 ProcessIncomingNotify();
637 elog(LOG, "Async_NotifyHandler: done");
644 * In this path it is NOT SAFE to do much of anything, except
647 notifyInterruptOccurred = 1;
654 * --------------------------------------------------------------
655 * EnableNotifyInterrupt
657 * This is called by the PostgresMain main loop just before waiting
658 * for a frontend command. If we are truly idle (ie, *not* inside
659 * a transaction block), then process any pending inbound notifies,
660 * and enable the signal handler to process future notifies directly.
662 * NOTE: the signal handler starts out disabled, and stays so until
663 * PostgresMain calls this the first time.
664 * --------------------------------------------------------------
667 EnableNotifyInterrupt(void)
669 if (CurrentTransactionState->blockState != TRANS_DEFAULT)
670 return; /* not really idle */
673 * This code is tricky because we are communicating with a signal
674 * handler that could interrupt us at any point. If we just checked
675 * notifyInterruptOccurred and then set notifyInterruptEnabled, we
676 * could fail to respond promptly to a signal that happens in between
677 * those two steps. (A very small time window, perhaps, but Murphy's
678 * Law says you can hit it...) Instead, we first set the enable flag,
679 * then test the occurred flag. If we see an unserviced interrupt has
680 * occurred, we re-clear the enable flag before going off to do the
681 * service work. (That prevents re-entrant invocation of
682 * ProcessIncomingNotify() if another interrupt occurs.) If an
683 * interrupt comes in between the setting and clearing of
684 * notifyInterruptEnabled, then it will have done the service work and
685 * left notifyInterruptOccurred zero, so we have to check again after
686 * clearing enable. The whole thing has to be in a loop in case
687 * another interrupt occurs while we're servicing the first. Once we
688 * get out of the loop, enable is set and we know there is no
689 * unserviced interrupt.
691 * NB: an overenthusiastic optimizing compiler could easily break this
692 * code. Hopefully, they all understand what "volatile" means these
697 notifyInterruptEnabled = 1;
698 if (!notifyInterruptOccurred)
700 notifyInterruptEnabled = 0;
701 if (notifyInterruptOccurred)
704 elog(LOG, "EnableNotifyInterrupt: perform async notify");
706 ProcessIncomingNotify();
709 elog(LOG, "EnableNotifyInterrupt: done");
715 * --------------------------------------------------------------
716 * DisableNotifyInterrupt
718 * This is called by the PostgresMain main loop just after receiving
719 * a frontend command. Signal handler execution of inbound notifies
720 * is disabled until the next EnableNotifyInterrupt call.
721 * --------------------------------------------------------------
724 DisableNotifyInterrupt(void)
726 notifyInterruptEnabled = 0;
730 * --------------------------------------------------------------
731 * ProcessIncomingNotify
733 * Deal with arriving NOTIFYs from other backends.
734 * This is called either directly from the SIGUSR2 signal handler,
735 * or the next time control reaches the outer idle loop.
736 * Scan pg_listener for arriving notifies, report them to my front end,
737 * and clear the notification field in pg_listener until next time.
739 * NOTE: since we are outside any transaction, we must create our own.
744 * --------------------------------------------------------------
747 ProcessIncomingNotify(void)
755 Datum value[Natts_pg_listener];
756 char repl[Natts_pg_listener],
757 nulls[Natts_pg_listener];
760 elog(LOG, "ProcessIncomingNotify");
762 set_ps_display("async_notify");
764 notifyInterruptOccurred = 0;
766 StartTransactionCommand();
768 lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
769 tdesc = RelationGetDescr(lRel);
771 /* Scan only entries with my listenerPID */
772 ScanKeyEntryInitialize(&key[0], 0,
773 Anum_pg_listener_pid,
775 Int32GetDatum(MyProcPid));
776 scan = heap_beginscan(lRel, 0, SnapshotNow, 1, key);
778 /* Prepare data for rewriting 0 into notification field */
779 nulls[0] = nulls[1] = nulls[2] = ' ';
780 repl[0] = repl[1] = repl[2] = ' ';
781 repl[Anum_pg_listener_notify - 1] = 'r';
782 value[0] = value[1] = value[2] = (Datum) 0;
783 value[Anum_pg_listener_notify - 1] = Int32GetDatum(0);
785 while (HeapTupleIsValid(lTuple = heap_getnext(scan, 0)))
787 Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
788 char *relname = NameStr(listener->relname);
789 int32 sourcePID = listener->notification;
793 /* Notify the frontend */
796 elog(LOG, "ProcessIncomingNotify: received %s from %d",
797 relname, (int) sourcePID);
799 NotifyMyFrontEnd(relname, sourcePID);
800 /* Rewrite the tuple with 0 in notification column */
801 rTuple = heap_modifytuple(lTuple, lRel, value, nulls, repl);
802 simple_heap_update(lRel, &lTuple->t_self, rTuple);
804 #ifdef NOT_USED /* currently there are no indexes */
805 if (RelationGetForm(lRel)->relhasindex)
807 Relation idescs[Num_pg_listener_indices];
809 CatalogOpenIndices(Num_pg_listener_indices, Name_pg_listener_indices, idescs);
810 CatalogIndexInsert(idescs, Num_pg_listener_indices, lRel, rTuple);
811 CatalogCloseIndices(Num_pg_listener_indices, idescs);
819 * We do NOT release the lock on pg_listener here; we need to hold it
820 * until end of transaction (which is about to happen, anyway) to
821 * ensure that other backends see our tuple updates when they look.
822 * Otherwise, a transaction started after this one might mistakenly
823 * think it doesn't need to send this backend a new NOTIFY.
825 heap_close(lRel, NoLock);
827 CommitTransactionCommand();
830 * Must flush the notify messages to ensure frontend gets them
835 set_ps_display("idle");
838 elog(LOG, "ProcessIncomingNotify: done");
842 * Send NOTIFY message to my front end.
845 NotifyMyFrontEnd(char *relname, int32 listenerPID)
847 if (whereToSendOutput == Remote)
851 pq_beginmessage(&buf);
852 pq_sendbyte(&buf, 'A');
853 pq_sendint(&buf, listenerPID, sizeof(int32));
854 pq_sendstring(&buf, relname);
858 * NOTE: we do not do pq_flush() here. For a self-notify, it will
859 * happen at the end of the transaction, and for incoming notifies
860 * ProcessIncomingNotify will do it after finding all the
865 elog(INFO, "NOTIFY for %s", relname);
868 /* Does pendingNotifies include the given relname? */
870 AsyncExistsPendingNotify(const char *relname)
874 foreach(p, pendingNotifies)
876 /* Use NAMEDATALEN for relname comparison. DZ - 26-08-1996 */
877 if (strncmp((const char *) lfirst(p), relname, NAMEDATALEN) == 0)
884 /* Clear the pendingNotifies list. */
886 ClearPendingNotifies(void)
889 * We used to have to explicitly deallocate the list members and
890 * nodes, because they were malloc'd. Now, since we know they are
891 * palloc'd in TopTransactionContext, we need not do that --- they'll
892 * go away automatically at transaction exit. We need only reset the
895 pendingNotifies = NIL;