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

5 files changed, 23 insertions, 1 deletions

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 7fe9e1d440a..e919317bab9 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/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();
 }
diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index 014b9e5289c..9b2277d4aee 100644
--- a/src/backend/postmaster/checkpointer.c
+++ b/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();
 }
 
 /*
diff --git a/src/backend/postmaster/interrupt.c b/src/backend/postmaster/interrupt.c
index 6900cd02f6f..214ccaf34be 100644
--- a/src/backend/postmaster/interrupt.c
+++ b/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;
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index e9315122033..7410b2ff5e9 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/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;
diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c
index 4f59c71f733..11f7052e788 100644
--- a/src/backend/postmaster/startup.c
+++ b/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();
 }