Extend the ProcSignal mechanism to support barriers.

A new function EmitProcSignalBarrier() can be used to emit a global
barrier which all backends that participate in the ProcSignal
mechanism must absorb, and a new function WaitForProcSignalBarrier()
can be used to wait until all relevant backends have in fact
absorbed the barrier.

This can be used to coordinate global state changes, such as turning
checksums on while the system is running.

There's no real client of this mechanism yet, although two are
proposed, but an enum has to have at least one element, so this
includes a placeholder type (PROCSIGNAL_BARRIER_PLACEHOLDER) which
should be replaced by the first real client of this mechanism to
get committed.

Andres Freund and Robert Haas, reviewed by Daniel Gustafsson and,
in earlier versions, by Magnus Hagander.

Discussion: http://postgr.es/m/CA+TgmoZwDk=BguVDVa+qdA6SBKef=PKbaKDQALTC_9qoz1mJqg@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -1473,7 +1473,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting to apply WAL at recovery because it is delayed.</entry>
         </row>
         <row>
-         <entry morerows="66"><literal>IO</literal></entry>
+         <entry morerows="67"><literal>IO</literal></entry>
          <entry><literal>BufFileRead</literal></entry>
          <entry>Waiting for a read from a buffered file.</entry>
         </row>
@@ -1593,6 +1593,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>LogicalRewriteWrite</literal></entry>
          <entry>Waiting for a write of logical rewrite mappings.</entry>
         </row>
+        <row>
+         <entry><literal>ProcSignalBarrier</literal></entry>
+         <entry>Waiting for a barrier event to be processed by all backends.</entry>
+        </row>
         <row>
          <entry><literal>RelationMapRead</literal></entry>
          <entry>Waiting for a read of the relation map file.</entry>

src/backend/postmaster/autovacuum.c

@@ -820,6 +820,10 @@ HandleAutoVacLauncherInterrupts(void)
 		rebuild_database_list(InvalidOid);
 	}
 
+	/* Process barrier events */
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
+
 	/* Process sinval catchup interrupts that happened while sleeping */
 	ProcessCatchupInterrupt();
 }

src/backend/postmaster/checkpointer.c

@@ -524,6 +524,9 @@ CheckpointerMain(void)
 static void
 HandleCheckpointerInterrupts(void)
 {
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
+
 	if (ConfigReloadPending)
 	{
 		ConfigReloadPending = false;
@@ -710,6 +713,10 @@ CheckpointWriteDelay(int flags, double progress)
 		AbsorbSyncRequests();
 		absorb_counter = WRITES_PER_ABSORB;
 	}
+
+	/* Check for barrier events. */
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
 }
 
 /*

src/backend/postmaster/interrupt.c

@@ -20,6 +20,7 @@
 #include "postmaster/interrupt.h"
 #include "storage/ipc.h"
 #include "storage/latch.h"
+#include "storage/procsignal.h"
 #include "utils/guc.h"
 
 volatile sig_atomic_t ConfigReloadPending = false;
@@ -31,6 +32,9 @@ volatile sig_atomic_t ShutdownRequestPending = false;
 void
 HandleMainLoopInterrupts(void)
 {
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
+
 	if (ConfigReloadPending)
 	{
 		ConfigReloadPending = false;

src/backend/postmaster/pgstat.c

@@ -3988,6 +3988,9 @@ pgstat_get_wait_io(WaitEventIO w)
 		case WAIT_EVENT_LOGICAL_REWRITE_WRITE:
 			event_name = "LogicalRewriteWrite";
 			break;
+		case WAIT_EVENT_PROC_SIGNAL_BARRIER:
+			event_name = "ProcSignalBarrier";
+			break;
 		case WAIT_EVENT_RELATION_MAP_READ:
 			event_name = "RelationMapRead";
 			break;

src/backend/postmaster/startup.c

@@ -96,7 +96,7 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
 	errno = save_errno;
 }
 
-/* Handle SIGHUP and SIGTERM signals of startup process */
+/* Handle various signals that might be sent to the startup process */
 void
 HandleStartupProcInterrupts(void)
 {
@@ -121,6 +121,10 @@ HandleStartupProcInterrupts(void)
 	 */
 	if (IsUnderPostmaster && !PostmasterIsAlive())
 		exit(1);
+
+	/* Process barrier events */
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
 }
 
 

src/backend/replication/walreceiver.c

@@ -148,7 +148,8 @@ ProcessWalRcvInterrupts(void)
 	/*
 	 * Although walreceiver interrupt handling doesn't use the same scheme as
 	 * regular backends, call CHECK_FOR_INTERRUPTS() to make sure we receive
-	 * any incoming signals on Win32.
+	 * any incoming signals on Win32, and also to make sure we process any
+	 * barrier events.
 	 */
 	CHECK_FOR_INTERRUPTS();
 

src/backend/storage/buffer/bufmgr.c

@@ -1852,6 +1852,10 @@ BufferSync(int flags)
 		}
 
 		UnlockBufHdr(bufHdr, buf_state);
+
+		/* Check for barrier events in case NBuffers is large. */
+		if (ProcSignalBarrierPending)
+			ProcessProcSignalBarrier();
 	}
 
 	if (num_to_scan == 0)
@@ -1930,6 +1934,10 @@ BufferSync(int flags)
 		}
 
 		s->num_to_scan++;
+
+		/* Check for barrier events. */
+		if (ProcSignalBarrierPending)
+			ProcessProcSignalBarrier();
 	}
 
 	Assert(num_spaces > 0);
@@ -2018,6 +2026,8 @@ BufferSync(int flags)
 
 		/*
 		 * Sleep to throttle our I/O rate.
+		 *
+		 * (This will check for barrier events even if it doesn't sleep.)
 		 */
 		CheckpointWriteDelay(flags, (double) num_processed / num_to_scan);
 	}