Back-patch assorted latch-related fixes.

Fix a whole bunch of signal handlers that had been hacked to do things that
might change errno, without adding the necessary save/restore logic for
errno.  Also make some minor fixes in unix_latch.c, and clean up bizarre
and unsafe scheme for disowning the process's latch.  While at it, rename
the PGPROC latch field to procLatch for consistency with 9.2.

Issues noted while reviewing a patch by Peter Geoghegan.

src/backend/access/transam/xlog.c

@@ -9881,34 +9881,50 @@ startupproc_quickdie(SIGNAL_ARGS)
 static void
 StartupProcSigUsr1Handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	latch_sigusr1_handler();
+
+	errno = save_errno;
 }
 
 /* SIGUSR2: set flag to finish recovery */
 static void
 StartupProcTriggerHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	promote_triggered = true;
 	WakeupRecovery();
+
+	errno = save_errno;
 }
 
 /* SIGHUP: set flag to re-read config file at next convenient time */
 static void
 StartupProcSigHupHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	got_SIGHUP = true;
 	WakeupRecovery();
+
+	errno = save_errno;
 }
 
 /* SIGTERM: set flag to abort redo and exit */
 static void
 StartupProcShutdownHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	if (in_restore_command)
 		proc_exit(1);
 	else
 		shutdown_requested = true;
 	WakeupRecovery();
+
+	errno = save_errno;
 }
 
 /* Handle SIGHUP and SIGTERM signals of startup process */

src/backend/port/unix_latch.c

@@ -135,9 +135,12 @@ DisownLatch(volatile Latch *latch)
  * If the latch is already set, the function returns immediately.
  *
  * The 'timeout' is given in milliseconds, and -1 means wait forever.
- * On some platforms, signals cause the timeout to be restarted, so beware
+ * On some platforms, signals do not interrupt the wait, or even
- * that the function can sleep for several times longer than the specified
+ * cause the timeout to be restarted, so beware that the function can sleep
- * timeout.
+ * for several times longer than the requested timeout.  However, this
+ * difficulty is not so great as it seems, because the signal handlers for any
+ * signals that the caller should respond to ought to be programmed to end the
+ * wait by calling SetLatch.  Ideally, the timeout parameter is vestigial.
  *
  * The latch must be owned by the current process, ie. it must be a
  * backend-local latch initialized with InitLatch, or a shared latch
@@ -228,6 +231,7 @@ WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
 		{
 			if (errno == EINTR)
 				continue;
+			waiting = false;
 			ereport(ERROR,
 					(errcode_for_socket_access(),
 					 errmsg("select() failed: %m")));
@@ -255,6 +259,10 @@ WaitLatchOrSocket(volatile Latch *latch, pgsocket sock, bool forRead,
  * Sets a latch and wakes up anyone waiting on it.
  *
  * This is cheap if the latch is already set, otherwise not so much.
+ *
+ * NB: when calling this in a signal handler, be sure to save and restore
+ * errno around it.  (That's standard practice in most signal handlers, of
+ * course, but we used to omit it in handlers that only set a flag.)
  */
 void
 SetLatch(volatile Latch *latch)
@@ -300,7 +308,10 @@ SetLatch(volatile Latch *latch)
 	if (owner_pid == 0)
 		return;
 	else if (owner_pid == MyProcPid)
-		sendSelfPipeByte();
+	{
+		if (waiting)
+			sendSelfPipeByte();
+	}
 	else
 		kill(owner_pid, SIGUSR1);
 }
@@ -332,7 +343,11 @@ ResetLatch(volatile Latch *latch)
  * SetLatch uses SIGUSR1 to wake up the process waiting on the latch.
  *
  * Wake up WaitLatch, if we're waiting.  (We might not be, since SIGUSR1 is
- * overloaded for multiple purposes.)
+ * overloaded for multiple purposes; or we might not have reached WaitLatch
+ * yet, in which case we don't need to fill the pipe either.)
+ *
+ * NB: when calling this in a signal handler, be sure to save and restore
+ * errno around it.
  */
 void
 latch_sigusr1_handler(void)
@@ -396,13 +411,19 @@ retry:
 	}
 }
 
-/* Read all available data from the self-pipe */
+/*
+ * Read all available data from the self-pipe
+ *
+ * Note: this is only called when waiting = true.  If it fails and doesn't
+ * return, it must reset that flag first (though ideally, this will never
+ * happen).
+ */
 static void
 drainSelfPipe(void)
 {
 	/*
 	 * There shouldn't normally be more than one byte in the pipe, or maybe a
-	 * few more if multiple processes run SetLatch at the same instant.
+	 * few bytes if multiple processes run SetLatch at the same instant.
 	 */
 	char		buf[16];
 	int			rc;
@@ -417,9 +438,21 @@ drainSelfPipe(void)
 			else if (errno == EINTR)
 				continue;		/* retry */
 			else
+			{
+				waiting = false;
 				elog(ERROR, "read() on self-pipe failed: %m");
+			}
 		}
 		else if (rc == 0)
+		{
+			waiting = false;
 			elog(ERROR, "unexpected EOF on self-pipe");
+		}
+		else if (rc < sizeof(buf))
+		{
+			/* we successfully drained the pipe; no need to read() again */
+			break;
+		}
+		/* else buffer wasn't big enough, so read again */
 	}
 }

src/backend/replication/syncrep.c

@@ -111,9 +111,6 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 	Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
 	Assert(WalSndCtl != NULL);
 
-	/* Reset the latch before adding ourselves to the queue. */
-	ResetLatch(&MyProc->waitLatch);
-
 	LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
 	Assert(MyProc->syncRepState == SYNC_REP_NOT_WAITING);
 
@@ -167,7 +164,7 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		int			syncRepState;
 
 		/* Must reset the latch before testing state. */
-		ResetLatch(&MyProc->waitLatch);
+		ResetLatch(&MyProc->procLatch);
 
 		/*
 		 * Try checking the state without the lock first.  There's no
@@ -247,11 +244,11 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
 		}
 
 		/*
-		 * Wait on latch for up to 60 seconds. This allows us to check for
+		 * Wait on latch for up to 10 seconds. This allows us to check for
 		 * cancel/die signal or postmaster death regularly while waiting. Note
 		 * that timeout here does not necessarily release from loop.
 		 */
-		WaitLatch(&MyProc->waitLatch, 60000L);
+		WaitLatch(&MyProc->procLatch, 10000L);
 	}
 
 	/*
@@ -322,7 +319,7 @@ SyncRepCancelWait(void)
 }
 
 void
-SyncRepCleanupAtProcExit(int code, Datum arg)
+SyncRepCleanupAtProcExit(void)
 {
 	if (!SHMQueueIsDetached(&(MyProc->syncRepLinks)))
 	{
@@ -330,8 +327,6 @@ SyncRepCleanupAtProcExit(int code, Datum arg)
 		SHMQueueDelete(&(MyProc->syncRepLinks));
 		LWLockRelease(SyncRepLock);
 	}
-
-	DisownLatch(&MyProc->waitLatch);
 }
 
 /*
@@ -560,9 +555,7 @@ SyncRepWakeQueue(bool all)
 		/*
 		 * Wake only when we have set state and removed from queue.
 		 */
-		Assert(SHMQueueIsDetached(&(thisproc->syncRepLinks)));
+		SetLatch(&(thisproc->procLatch));
-		Assert(thisproc->syncRepState == SYNC_REP_WAIT_COMPLETE);
-		SetLatch(&(thisproc->waitLatch));
 
 		numprocs++;
 	}

src/backend/replication/walreceiver.c

@@ -373,11 +373,15 @@ WalRcvSigHupHandler(SIGNAL_ARGS)
 static void
 WalRcvShutdownHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	got_SIGTERM = true;
 
 	/* Don't joggle the elbow of proc_exit */
 	if (!proc_exit_inprogress && WalRcvImmediateInterruptOK)
 		ProcessWalRcvInterrupts();
+
+	errno = save_errno;
 }
 
 /*

src/backend/replication/walsender.c

@@ -1188,18 +1188,26 @@ XLogSend(char *msgbuf, bool *caughtup)
 static void
 WalSndSigHupHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	got_SIGHUP = true;
 	if (MyWalSnd)
 		SetLatch(&MyWalSnd->latch);
+
+	errno = save_errno;
 }
 
 /* SIGTERM: set flag to shut down */
 static void
 WalSndShutdownHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	walsender_shutdown_requested = true;
 	if (MyWalSnd)
 		SetLatch(&MyWalSnd->latch);
+
+	errno = save_errno;
 }
 
 /*
@@ -1238,16 +1246,24 @@ WalSndQuickDieHandler(SIGNAL_ARGS)
 static void
 WalSndXLogSendHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	latch_sigusr1_handler();
+
+	errno = save_errno;
 }
 
 /* SIGUSR2: set flag to do a last cycle and shut down afterwards */
 static void
 WalSndLastCycleHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	walsender_ready_to_stop = true;
 	if (MyWalSnd)
 		SetLatch(&MyWalSnd->latch);
+
+	errno = save_errno;
 }
 
 /* Set up signal handlers */

src/backend/storage/lmgr/proc.c

@@ -187,7 +187,8 @@ InitProcGlobal(void)
 	ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;
 
 	/*
-	 * Pre-create the PGPROC structures and create a semaphore for each.
+	 * Pre-create the PGPROC structures and create a semaphore and latch
+	 * for each.
 	 */
 	procs = (PGPROC *) ShmemAlloc((MaxConnections) * sizeof(PGPROC));
 	if (!procs)
@@ -198,9 +199,9 @@ InitProcGlobal(void)
 	for (i = 0; i < MaxConnections; i++)
 	{
 		PGSemaphoreCreate(&(procs[i].sem));
+		InitSharedLatch(&procs[i].procLatch);
 		procs[i].links.next = (SHM_QUEUE *) ProcGlobal->freeProcs;
 		ProcGlobal->freeProcs = &procs[i];
-		InitSharedLatch(&procs[i].waitLatch);
 	}
 
 	/*
@@ -217,9 +218,9 @@ InitProcGlobal(void)
 	for (i = 0; i < autovacuum_max_workers + 1; i++)
 	{
 		PGSemaphoreCreate(&(procs[i].sem));
+		InitSharedLatch(&procs[i].procLatch);
 		procs[i].links.next = (SHM_QUEUE *) ProcGlobal->autovacFreeProcs;
 		ProcGlobal->autovacFreeProcs = &procs[i];
-		InitSharedLatch(&procs[i].waitLatch);
 	}
 
 	/*
@@ -230,7 +231,7 @@ InitProcGlobal(void)
 	{
 		AuxiliaryProcs[i].pid = 0;		/* marks auxiliary proc as not in use */
 		PGSemaphoreCreate(&(AuxiliaryProcs[i].sem));
-		InitSharedLatch(&procs[i].waitLatch);
+		InitSharedLatch(&AuxiliaryProcs[i].procLatch);
 	}
 
 	/* Create ProcStructLock spinlock, too */
@@ -306,8 +307,8 @@ InitProcess(void)
 		MarkPostmasterChildActive();
 
 	/*
-	 * Initialize all fields of MyProc, except for the semaphore which was
+	 * Initialize all fields of MyProc, except for the semaphore and latch,
-	 * prepared for us by InitProcGlobal.
+	 * which were prepared for us by InitProcGlobal.
 	 */
 	SHMQueueElemInit(&(MyProc->links));
 	MyProc->waitStatus = STATUS_OK;
@@ -333,12 +334,17 @@ InitProcess(void)
 		SHMQueueInit(&(MyProc->myProcLocks[i]));
 	MyProc->recoveryConflictPending = false;
 
-	/* Initialise for sync rep */
+	/* Initialize fields for sync rep */
 	MyProc->waitLSN.xlogid = 0;
 	MyProc->waitLSN.xrecoff = 0;
 	MyProc->syncRepState = SYNC_REP_NOT_WAITING;
 	SHMQueueElemInit(&(MyProc->syncRepLinks));
-	OwnLatch(&MyProc->waitLatch);
+
+	/*
+	 * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
+	 * Note that there's no particular need to do ResetLatch here.
+	 */
+	OwnLatch(&MyProc->procLatch);
 
 	/*
 	 * We might be reusing a semaphore that belonged to a failed process. So
@@ -379,7 +385,6 @@ InitProcessPhase2(void)
 	/*
 	 * Arrange to clean that up at backend exit.
 	 */
-	on_shmem_exit(SyncRepCleanupAtProcExit, 0);
 	on_shmem_exit(RemoveProcFromArray, 0);
 }
 
@@ -454,8 +459,8 @@ InitAuxiliaryProcess(void)
 	SpinLockRelease(ProcStructLock);
 
 	/*
-	 * Initialize all fields of MyProc, except for the semaphore which was
+	 * Initialize all fields of MyProc, except for the semaphore and latch,
-	 * prepared for us by InitProcGlobal.
+	 * which were prepared for us by InitProcGlobal.
 	 */
 	SHMQueueElemInit(&(MyProc->links));
 	MyProc->waitStatus = STATUS_OK;
@@ -475,6 +480,12 @@ InitAuxiliaryProcess(void)
 	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
 		SHMQueueInit(&(MyProc->myProcLocks[i]));
 
+	/*
+	 * Acquire ownership of the PGPROC's latch, so that we can use WaitLatch.
+	 * Note that there's no particular need to do ResetLatch here.
+	 */
+	OwnLatch(&MyProc->procLatch);
+
 	/*
 	 * We might be reusing a semaphore that belonged to a failed process. So
 	 * be careful and reinitialize its value here.	(This is not strictly
@@ -677,6 +688,9 @@ ProcKill(int code, Datum arg)
 
 	Assert(MyProc != NULL);
 
+	/* Make sure we're out of the sync rep lists */
+	SyncRepCleanupAtProcExit();
+
 	/*
 	 * Release any LW locks I am holding.  There really shouldn't be any, but
 	 * it's cheap to check again before we cut the knees off the LWLock
@@ -684,6 +698,9 @@ ProcKill(int code, Datum arg)
 	 */
 	LWLockReleaseAll();
 
+	/* Release ownership of the process's latch, too */
+	DisownLatch(&MyProc->procLatch);
+
 	SpinLockAcquire(ProcStructLock);
 
 	/* Return PGPROC structure (and semaphore) to appropriate freelist */
@@ -739,6 +756,9 @@ AuxiliaryProcKill(int code, Datum arg)
 	/* Release any LW locks I am holding (see notes above) */
 	LWLockReleaseAll();
 
+	/* Release ownership of the process's latch, too */
+	DisownLatch(&MyProc->procLatch);
+
 	SpinLockAcquire(ProcStructLock);
 
 	/* Mark auxiliary proc no longer in use */

src/backend/tcop/postgres.c

@@ -2643,11 +2643,12 @@ die(SIGNAL_ARGS)
 			InterruptHoldoffCount--;
 			ProcessInterrupts();
 		}
-
-		/* Interrupt any sync rep wait which is currently in progress. */
-		SetLatch(&(MyProc->waitLatch));
 	}
 
+	/* If we're still here, waken anything waiting on the process latch */
+	if (MyProc)
+		SetLatch(&MyProc->procLatch);
+
 	errno = save_errno;
 }
 
@@ -2684,11 +2685,12 @@ StatementCancelHandler(SIGNAL_ARGS)
 			InterruptHoldoffCount--;
 			ProcessInterrupts();
 		}
-
-		/* Interrupt any sync rep wait which is currently in progress. */
-		SetLatch(&(MyProc->waitLatch));
 	}
 
+	/* If we're still here, waken anything waiting on the process latch */
+	if (MyProc)
+		SetLatch(&MyProc->procLatch);
+
 	errno = save_errno;
 }
 

src/include/replication/syncrep.h

@@ -33,8 +33,8 @@ extern char *SyncRepStandbyNames;
 /* called by user backend */
 extern void SyncRepWaitForLSN(XLogRecPtr XactCommitLSN);
 
-/* callback at backend exit */
+/* called at backend exit */
-extern void SyncRepCleanupAtProcExit(int code, Datum arg);
+extern void SyncRepCleanupAtProcExit(void);
 
 /* called by wal sender */
 extern void SyncRepInitConfig(void);

src/include/storage/proc.h

@@ -118,13 +118,14 @@ struct PGPROC
 	LOCKMASK	heldLocks;		/* bitmask for lock types already held on this
 								 * lock object by this backend */
 
+	Latch		procLatch;		/* generic latch for process */
+
 	/*
 	 * Info to allow us to wait for synchronous replication, if needed.
 	 * waitLSN is InvalidXLogRecPtr if not waiting; set only by user backend.
 	 * syncRepState must not be touched except by owning process or WALSender.
 	 * syncRepLinks used only while holding SyncRepLock.
 	 */
-	Latch		waitLatch;		/* allow us to wait for sync rep */
 	XLogRecPtr	waitLSN;		/* waiting for this LSN or higher */
 	int			syncRepState;	/* wait state for sync rep */
 	SHM_QUEUE	syncRepLinks;	/* list link if process is in syncrep queue */