Prevent possibility of panics during shutdown checkpoint.

When the checkpointer writes the shutdown checkpoint, it checks
afterwards whether any WAL has been written since it started and
throws a PANIC if so.  At that point, only walsenders are still
active, so one might think this could not happen, but walsenders can
also generate WAL, for instance in BASE_BACKUP and logical decoding
related commands (e.g. via hint bits).  So they can trigger this panic
if such a command is run while the shutdown checkpoint is being
written.

To fix this, divide the walsender shutdown into two phases.  First,
checkpointer, itself triggered by postmaster, sends a
PROCSIG_WALSND_INIT_STOPPING signal to all walsenders.  If the backend
is idle or runs an SQL query this causes the backend to shutdown, if
logical replication is in progress all existing WAL records are
processed followed by a shutdown.  Otherwise this causes the walsender
to switch to the "stopping" state. In this state, the walsender will
reject any further replication commands. The checkpointer begins the
shutdown checkpoint once all walsenders are confirmed as
stopping. When the shutdown checkpoint finishes, the postmaster sends
us SIGUSR2. This instructs walsender to send any outstanding WAL,
including the shutdown checkpoint record, wait for it to be replicated
to the standby, and then exit.

Author: Andres Freund, based on an earlier patch by Michael Paquier
Reported-By: Fujii Masao, Andres Freund
Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/20170602002912.tqlwn4gymzlxpvs2@alap3.anarazel.de
Backpatch: 9.4, where logical decoding was introduced

doc/src/sgml/monitoring.sgml

@@ -1690,6 +1690,11 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
            <literal>backup</>: This WAL sender is sending a backup.
           </para>
          </listitem>
+         <listitem>
+          <para>
+           <literal>stopping</>: This WAL sender is stopping.
+          </para>
+         </listitem>
        </itemizedlist>
      </entry>
     </row>

src/backend/access/transam/xlog.c

@@ -8324,6 +8324,17 @@ ShutdownXLOG(int code, Datum arg)
 	ereport(IsPostmasterEnvironment ? LOG : NOTICE,
 			(errmsg("shutting down")));
 
+	/*
+	 * Signal walsenders to move to stopping state.
+	 */
+	WalSndInitStopping();
+
+	/*
+	 * Wait for WAL senders to be in stopping state.  This prevents commands
+	 * from writing new WAL.
+	 */
+	WalSndWaitStopping();
+
 	if (RecoveryInProgress())
 		CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
 	else

src/backend/replication/walsender.c

@@ -24,11 +24,17 @@
  * are treated as not a crash but approximately normal termination;
  * the walsender will exit quickly without sending any more XLOG records.
  *
- * If the server is shut down, postmaster sends us SIGUSR2 after all
- * regular backends have exited and the shutdown checkpoint has been written.
- * This instructs walsender to send any outstanding WAL, including the
- * shutdown checkpoint record, wait for it to be replicated to the standby,
- * and then exit.
+ * If the server is shut down, checkpointer sends us
+ * PROCSIG_WALSND_INIT_STOPPING after all regular backends have exited.  If
+ * the backend is idle or runs an SQL query this causes the backend to
+ * shutdown, if logical replication is in progress all existing WAL records
+ * are processed followed by a shutdown.  Otherwise this causes the walsender
+ * to switch to the "stopping" state. In this state, the walsender will reject
+ * any further replication commands. The checkpointer begins the shutdown
+ * checkpoint once all walsenders are confirmed as stopping. When the shutdown
+ * checkpoint finishes, the postmaster sends us SIGUSR2. This instructs
+ * walsender to send any outstanding WAL, including the shutdown checkpoint
+ * record, wait for it to be replicated to the standby, and then exit.
  *
  *
  * Portions Copyright (c) 2010-2017, PostgreSQL Global Development Group
@@ -177,13 +183,14 @@ static bool WalSndCaughtUp = false;
 
 /* Flags set by signal handlers for later service in main loop */
 static volatile sig_atomic_t got_SIGHUP = false;
-static volatile sig_atomic_t walsender_ready_to_stop = false;
+static volatile sig_atomic_t got_SIGUSR2 = false;
+static volatile sig_atomic_t got_STOPPING = false;
 
 /*
- * This is set while we are streaming. When not set, SIGUSR2 signal will be
- * handled like SIGTERM. When set, the main loop is responsible for checking
- * walsender_ready_to_stop and terminating when it's set (after streaming any
- * remaining WAL).
+ * This is set while we are streaming. When not set
+ * PROCSIG_WALSND_INIT_STOPPING signal will be handled like SIGTERM. When set,
+ * the main loop is responsible for checking got_STOPPING and terminating when
+ * it's set (after streaming any remaining WAL).
  */
 static volatile sig_atomic_t replication_active = false;
 
@@ -300,7 +307,8 @@ WalSndErrorCleanup(void)
 	ReplicationSlotCleanup();
 
 	replication_active = false;
-	if (walsender_ready_to_stop)
+
+	if (got_STOPPING || got_SIGUSR2)
 		proc_exit(0);
 
 	/* Revert back to startup state */
@@ -677,7 +685,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndLoop(XLogSendPhysical);
 
 		replication_active = false;
-		if (walsender_ready_to_stop)
+		if (got_STOPPING)
 			proc_exit(0);
 		WalSndSetState(WALSNDSTATE_STARTUP);
 
@@ -1055,7 +1063,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	{
 		ereport(LOG,
 				(errmsg("terminating walsender process after promotion")));
-		walsender_ready_to_stop = true;
+		got_STOPPING = true;
 	}
 
 	WalSndSetState(WALSNDSTATE_CATCHUP);
@@ -1106,7 +1114,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	ReplicationSlotRelease();
 
 	replication_active = false;
-	if (walsender_ready_to_stop)
+	if (got_STOPPING)
 		proc_exit(0);
 	WalSndSetState(WALSNDSTATE_STARTUP);
 
@@ -1311,6 +1319,14 @@ WalSndWaitForWal(XLogRecPtr loc)
 		/* Check for input from the client */
 		ProcessRepliesIfAny();
 
+		/*
+		 * If we're shutting down, trigger pending WAL to be written out,
+		 * otherwise we'd possibly end up waiting for WAL that never gets
+		 * written, because walwriter has shut down already.
+		 */
+		if (got_STOPPING)
+			XLogBackgroundFlush();
+
 		/* Update our idea of the currently flushed position. */
 		if (!RecoveryInProgress())
 			RecentFlushPtr = GetFlushRecPtr();
@@ -1326,7 +1342,7 @@ WalSndWaitForWal(XLogRecPtr loc)
 		 * RecentFlushPtr, so we can send all remaining data before shutting
 		 * down.
 		 */
-		if (walsender_ready_to_stop)
+		if (got_STOPPING)
 			break;
 
 		/*
@@ -1400,6 +1416,22 @@ exec_replication_command(const char *cmd_string)
 	MemoryContext cmd_context;
 	MemoryContext old_context;
 
+	/*
+	 * If WAL sender has been told that shutdown is getting close, switch its
+	 * status accordingly to handle the next replication commands correctly.
+	 */
+	if (got_STOPPING)
+		WalSndSetState(WALSNDSTATE_STOPPING);
+
+	/*
+	 * Throw error if in stopping mode.  We need prevent commands that could
+	 * generate WAL while the shutdown checkpoint is being written.  To be
+	 * safe, we just prohibit all new commands.
+	 */
+	if (MyWalSnd->state == WALSNDSTATE_STOPPING)
+		ereport(ERROR,
+				(errmsg("cannot execute new commands while WAL sender is in stopping mode")));
+
 	/*
 	 * CREATE_REPLICATION_SLOT ... LOGICAL exports a snapshot until the next
 	 * command arrives. Clean up the old stuff if there's anything.
@@ -2128,7 +2160,7 @@ WalSndLoop(WalSndSendDataCallback send_data)
 			 * normal termination at shutdown, or a promotion, the walsender
 			 * is not sure which.
 			 */
-			if (walsender_ready_to_stop)
+			if (got_SIGUSR2)
 				WalSndDone(send_data);
 		}
 
@@ -2443,6 +2475,10 @@ XLogSendPhysical(void)
 	XLogRecPtr	endptr;
 	Size		nbytes;
 
+	/* If requested switch the WAL sender to the stopping state. */
+	if (got_STOPPING)
+		WalSndSetState(WALSNDSTATE_STOPPING);
+
 	if (streamingDoneSending)
 	{
 		WalSndCaughtUp = true;
@@ -2733,7 +2769,16 @@ XLogSendLogical(void)
 		 * point, then we're caught up.
 		 */
 		if (logical_decoding_ctx->reader->EndRecPtr >= GetFlushRecPtr())
+		{
 			WalSndCaughtUp = true;
+
+			/*
+			 * Have WalSndLoop() terminate the connection in an orderly
+			 * manner, after writing out all the pending data.
+			 */
+			if (got_STOPPING)
+				got_SIGUSR2 = true;
+		}
 	}
 
 	/* Update shared memory status */
@@ -2843,6 +2888,26 @@ WalSndRqstFileReload(void)
 	}
 }
 
+/*
+ * Handle PROCSIG_WALSND_INIT_STOPPING signal.
+ */
+void
+HandleWalSndInitStopping(void)
+{
+	Assert(am_walsender);
+
+	/*
+	 * If replication has not yet started, die like with SIGTERM. If
+	 * replication is active, only set a flag and wake up the main loop. It
+	 * will send any outstanding WAL, wait for it to be replicated to the
+	 * standby, and then exit gracefully.
+	 */
+	if (!replication_active)
+		kill(MyProcPid, SIGTERM);
+	else
+		got_STOPPING = true;
+}
+
 /* SIGHUP: set flag to re-read config file at next convenient time */
 static void
 WalSndSigHupHandler(SIGNAL_ARGS)
@@ -2856,22 +2921,17 @@ WalSndSigHupHandler(SIGNAL_ARGS)
 	errno = save_errno;
 }
 
-/* SIGUSR2: set flag to do a last cycle and shut down afterwards */
+/*
+ * SIGUSR2: set flag to do a last cycle and shut down afterwards. The WAL
+ * sender should already have been switched to WALSNDSTATE_STOPPING at
+ * this point.
+ */
 static void
 WalSndLastCycleHandler(SIGNAL_ARGS)
 {
 	int			save_errno = errno;
 
-	/*
-	 * If replication has not yet started, die like with SIGTERM. If
-	 * replication is active, only set a flag and wake up the main loop. It
-	 * will send any outstanding WAL, wait for it to be replicated to the
-	 * standby, and then exit gracefully.
-	 */
-	if (!replication_active)
-		kill(MyProcPid, SIGTERM);
-
-	walsender_ready_to_stop = true;
+	got_SIGUSR2 = true;
 	SetLatch(MyLatch);
 
 	errno = save_errno;
@@ -2969,6 +3029,77 @@ WalSndWakeup(void)
 	}
 }
 
+/*
+ * Signal all walsenders to move to stopping state.
+ *
+ * This will trigger walsenders to move to a state where no further WAL can be
+ * generated. See this file's header for details.
+ */
+void
+WalSndInitStopping(void)
+{
+	int			i;
+
+	for (i = 0; i < max_wal_senders; i++)
+	{
+		WalSnd	   *walsnd = &WalSndCtl->walsnds[i];
+		pid_t		pid;
+
+		SpinLockAcquire(&walsnd->mutex);
+		pid = walsnd->pid;
+		SpinLockRelease(&walsnd->mutex);
+
+		if (pid == 0)
+			continue;
+
+		SendProcSignal(pid, PROCSIG_WALSND_INIT_STOPPING, InvalidBackendId);
+	}
+}
+
+/*
+ * Wait that all the WAL senders have quit or reached the stopping state. This
+ * is used by the checkpointer to control when the shutdown checkpoint can
+ * safely be performed.
+ */
+void
+WalSndWaitStopping(void)
+{
+	for (;;)
+	{
+		int			i;
+		bool		all_stopped = true;
+
+		for (i = 0; i < max_wal_senders; i++)
+		{
+			WalSndState state;
+			WalSnd	   *walsnd = &WalSndCtl->walsnds[i];
+
+			SpinLockAcquire(&walsnd->mutex);
+
+			if (walsnd->pid == 0)
+			{
+				SpinLockRelease(&walsnd->mutex);
+				continue;
+			}
+
+			state = walsnd->state;
+			SpinLockRelease(&walsnd->mutex);
+
+			if (state != WALSNDSTATE_STOPPING)
+			{
+				all_stopped = false;
+				break;
+			}
+		}
+
+		/* safe to leave if confirmation is done for all WAL senders */
+		if (all_stopped)
+			return;
+
+		pg_usleep(10000L);		/* wait for 10 msec */
+	}
+}
+
 /* Set state for current walsender (only called in walsender) */
 void
 WalSndSetState(WalSndState state)
@@ -3002,6 +3133,8 @@ WalSndGetStateString(WalSndState state)
 			return "catchup";
 		case WALSNDSTATE_STREAMING:
 			return "streaming";
+		case WALSNDSTATE_STOPPING:
+			return "stopping";
 	}
 	return "UNKNOWN";
 }

src/backend/storage/ipc/procsignal.c

@@ -20,6 +20,7 @@
 #include "access/parallel.h"
 #include "commands/async.h"
 #include "miscadmin.h"
+#include "replication/walsender.h"
 #include "storage/latch.h"
 #include "storage/ipc.h"
 #include "storage/proc.h"
@@ -270,6 +271,9 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 	if (CheckProcSignal(PROCSIG_PARALLEL_MESSAGE))
 		HandleParallelMessageInterrupt();
 
+	if (CheckProcSignal(PROCSIG_WALSND_INIT_STOPPING))
+		HandleWalSndInitStopping();
+
 	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_DATABASE))
 		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_DATABASE);
 

src/include/replication/walsender.h

@@ -44,6 +44,9 @@ extern void WalSndSignals(void);
 extern Size WalSndShmemSize(void);
 extern void WalSndShmemInit(void);
 extern void WalSndWakeup(void);
+extern void WalSndInitStopping(void);
+extern void WalSndWaitStopping(void);
+extern void HandleWalSndInitStopping(void);
 extern void WalSndRqstFileReload(void);
 
 /*

src/include/replication/walsender_private.h

@@ -24,7 +24,8 @@ typedef enum WalSndState
 	WALSNDSTATE_STARTUP = 0,
 	WALSNDSTATE_BACKUP,
 	WALSNDSTATE_CATCHUP,
-	WALSNDSTATE_STREAMING
+	WALSNDSTATE_STREAMING,
+	WALSNDSTATE_STOPPING
 } WalSndState;
 
 /*

src/include/storage/procsignal.h

@@ -32,6 +32,7 @@ typedef enum
 	PROCSIG_CATCHUP_INTERRUPT,	/* sinval catchup interrupt */
 	PROCSIG_NOTIFY_INTERRUPT,	/* listen/notify interrupt */
 	PROCSIG_PARALLEL_MESSAGE,	/* message from cooperating parallel backend */
+	PROCSIG_WALSND_INIT_STOPPING,	/* ask walsenders to prepare for shutdown  */
 
 	/* Recovery conflict reasons */
 	PROCSIG_RECOVERY_CONFLICT_DATABASE,