async.c

关系型数据库 Postgresql 6.5.2

	bool		isnull;
	char	   *relname;
	int32		listenerPID;

	if (!pendingNotifies)
		return;					/* no NOTIFY statements in this
								 * transaction */

	/*
	 * 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())
	{
		ClearPendingNotifies();
		return;
	}

	TPRINTF(TRACE_NOTIFY, "AtCommit_Notify");

	lRel = heap_openr(ListenerRelationName);
	LockRelation(lRel, AccessExclusiveLock);
	tdesc = RelationGetDescr(lRel);
	sRel = heap_beginscan(lRel, 0, SnapshotNow, 0, (ScanKey) NULL);

	/* preset data to update notify column to MyProcPid */
	nulls[0] = nulls[1] = nulls[2] = ' ';
	repl[0] = repl[1] = repl[2] = ' ';
	repl[Anum_pg_listener_notify - 1] = 'r';
	value[0] = value[1] = value[2] = (Datum) 0;
	value[Anum_pg_listener_notify - 1] = Int32GetDatum(MyProcPid);

	while (HeapTupleIsValid(lTuple = heap_getnext(sRel, 0)))
	{
		d = heap_getattr(lTuple, Anum_pg_listener_relname, tdesc, &isnull);
		relname = (char *) DatumGetPointer(d);

		if (AsyncExistsPendingNotify(relname))
		{
			d = heap_getattr(lTuple, Anum_pg_listener_pid, tdesc, &isnull);
			listenerPID = DatumGetInt32(d);

			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.
				 */
				TPRINTF(TRACE_NOTIFY, "AtCommit_Notify: notifying self");
				NotifyMyFrontEnd(relname, listenerPID);
			}
			else
			{
				TPRINTF(TRACE_NOTIFY, "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...)
					 */
					heap_delete(lRel, &lTuple->t_self, NULL);
				}
				else
				{
					d = heap_getattr(lTuple, Anum_pg_listener_notify,
									 tdesc, &isnull);
					if (DatumGetInt32(d) == 0)
					{
						rTuple = heap_modifytuple(lTuple, lRel,
												  value, nulls, repl);
						heap_replace(lRel, &lTuple->t_self, rTuple, NULL);
					}
				}
			}
		}
	}

	heap_endscan(sRel);

	/*
	 * We do not do RelationUnsetLockForWrite(lRel) here, because the
	 * transaction is about to be committed anyway.
	 */
	heap_close(lRel);

	ClearPendingNotifies();

	TPRINTF(TRACE_NOTIFY, "AtCommit_Notify: done");
}

/*
 *--------------------------------------------------------------
 * AtAbort_Notify
 *
 *		This is called at transaction abort.
 *
 *		Gets rid of pending outbound notifies that we would have executed
 *		if the transaction got committed.
 *
 * Results:
 *		XXX
 *
 *--------------------------------------------------------------
 */
void
AtAbort_Notify()
{
	ClearPendingNotifies();
}

/*
 *--------------------------------------------------------------
 * Async_NotifyHandler
 *
 *		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.
 *
 * Results:
 *		none
 *
 * Side effects:
 *		per above
 *--------------------------------------------------------------
 */

void
Async_NotifyHandler(SIGNAL_ARGS)
{

	/*
	 * 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.
	 */

	if (notifyInterruptEnabled)
	{

		/*
		 * 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. */
				TPRINTF(TRACE_NOTIFY,
						"Async_NotifyHandler: perform async notify");
				ProcessIncomingNotify();
				TPRINTF(TRACE_NOTIFY, "Async_NotifyHandler: done");
			}
		}
	}
	else
	{

		/*
		 * In this path it is NOT SAFE to do much of anything, except
		 * this:
		 */
		notifyInterruptOccurred = 1;
	}
}

/*
 * --------------------------------------------------------------
 * 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 (CurrentTransactionState->blockState != TRANS_DEFAULT)
		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)
		{
			TPRINTF(TRACE_NOTIFY,
					"EnableNotifyInterrupt: perform async notify");
			ProcessIncomingNotify();
			TPRINTF(TRACE_NOTIFY, "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.
 * --------------------------------------------------------------
 */

void
DisableNotifyInterrupt(void)
{
	notifyInterruptEnabled = 0;
}

/*
 * --------------------------------------------------------------
 * 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.
 *
 * Results:
 *		XXX
 *
 * --------------------------------------------------------------
 */
static void
ProcessIncomingNotify(void)
{
	Relation	lRel;
	TupleDesc	tdesc;
	ScanKeyData key[1];
	HeapScanDesc sRel;
	HeapTuple	lTuple,
				rTuple;
	Datum		d,
				value[Natts_pg_listener];
	char		repl[Natts_pg_listener],
				nulls[Natts_pg_listener];
	bool		isnull;
	char	   *relname;
	int32		sourcePID;

	TPRINTF(TRACE_NOTIFY, "ProcessIncomingNotify");
	PS_SET_STATUS("async_notify");

	notifyInterruptOccurred = 0;

	StartTransactionCommand();

	lRel = heap_openr(ListenerRelationName);
	LockRelation(lRel, AccessExclusiveLock);
	tdesc = RelationGetDescr(lRel);

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

	/* Prepare data for rewriting 0 into notification field */
	nulls[0] = nulls[1] = nulls[2] = ' ';
	repl[0] = repl[1] = repl[2] = ' ';
	repl[Anum_pg_listener_notify - 1] = 'r';
	value[0] = value[1] = value[2] = (Datum) 0;
	value[Anum_pg_listener_notify - 1] = Int32GetDatum(0);

	while (HeapTupleIsValid(lTuple = heap_getnext(sRel, 0)))
	{
		d = heap_getattr(lTuple, Anum_pg_listener_notify, tdesc, &isnull);
		sourcePID = DatumGetInt32(d);
		if (sourcePID != 0)
		{
			d = heap_getattr(lTuple, Anum_pg_listener_relname, tdesc, &isnull);
			relname = (char *) DatumGetPointer(d);
			/* Notify the frontend */
			TPRINTF(TRACE_NOTIFY, "ProcessIncomingNotify: received %s from %d",
					relname, (int) sourcePID);
			NotifyMyFrontEnd(relname, sourcePID);
			/* Rewrite the tuple with 0 in notification column */
			rTuple = heap_modifytuple(lTuple, lRel, value, nulls, repl);
			heap_replace(lRel, &lTuple->t_self, rTuple, NULL);
		}
	}
	heap_endscan(sRel);

	/*
	 * We do not do RelationUnsetLockForWrite(lRel) here, because the
	 * transaction is about to be committed anyway.
	 */
	heap_close(lRel);

	CommitTransactionCommand();

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

	PS_SET_STATUS("idle");
	TPRINTF(TRACE_NOTIFY, "ProcessIncomingNotify: done");
}

/* Send NOTIFY message to my front end. */

static void
NotifyMyFrontEnd(char *relname, int32 listenerPID)
{
	if (whereToSendOutput == Remote)
	{
		StringInfoData buf;

		pq_beginmessage(&buf);
		pq_sendbyte(&buf, 'A');
		pq_sendint(&buf, listenerPID, sizeof(int32));
		pq_sendstring(&buf, relname);
		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(NOTICE, "NOTIFY for %s", relname);
}

/* Does pendingNotifies include the given relname?
 *
 * NB: not called unless pendingNotifies != NULL.
 */

static int
AsyncExistsPendingNotify(char *relname)
{
	Dlelem	   *p;

	for (p = DLGetHead(pendingNotifies);
		 p != NULL;
		 p = DLGetSucc(p))
	{
		/* Use NAMEDATALEN for relname comparison.	  DZ - 26-08-1996 */
		if (!strncmp((const char *) DLE_VAL(p), relname, NAMEDATALEN))
			return 1;
	}

	return 0;
}

/* Clear the pendingNotifies list. */

static void
ClearPendingNotifies()
{
	Dlelem	   *p;

	if (pendingNotifies)
	{

		/*
		 * Since the referenced strings are malloc'd, we have to scan the
		 * list and delete them individually.  If we used palloc for the
		 * strings then we could just do DLFreeList to get rid of both the
		 * list nodes and the list base...
		 */
		while ((p = DLRemHead(pendingNotifies)) != NULL)
		{
			free(DLE_VAL(p));
			DLFreeElem(p);
		}
		DLFreeList(pendingNotifies);
		pendingNotifies = NULL;
	}
}