Support condition variables.

Condition variables provide a flexible way to sleep until a
cooperating process causes an arbitrary condition to become true.  In
simple cases, this can be accomplished with a WaitLatch/ResetLatch
loop; the cooperating process can call SetLatch after performing work
that might cause the condition to be satisfied, and the waiting
process can recheck the condition each time.  However, if the process
performing the work doesn't have an easy way to identify which
processes might be waiting, this doesn't work, because it can't
identify which latches to set.  Condition variables solve that problem
by internally maintaining a list of waiters; a process that may have
caused some waiter's condition to be satisfied must "signal" or
"broadcast" on the condition variable.

Robert Haas and Thomas Munro

src/backend/storage/lmgr/proc.c

@@ -43,6 +43,7 @@
 #include "postmaster/autovacuum.h"
 #include "replication/slot.h"
 #include "replication/syncrep.h"
+#include "storage/condition_variable.h"
 #include "storage/standby.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
@@ -802,6 +803,9 @@ ProcKill(int code, Datum arg)
 	 */
 	LWLockReleaseAll();
 
+	/* Cancel any pending condition variable sleep, too */
+	ConditionVariableCancelSleep();
+
 	/* Make sure active replication slots are released */
 	if (MyReplicationSlot != NULL)
 		ReplicationSlotRelease();
@@ -907,6 +911,9 @@ AuxiliaryProcKill(int code, Datum arg)
 	/* Release any LW locks I am holding (see notes above) */
 	LWLockReleaseAll();
 
+	/* Cancel any pending condition variable sleep, too */
+	ConditionVariableCancelSleep();
+
 	/*
 	 * Reset MyLatch to the process local one.  This is so that signal
 	 * handlers et al can continue using the latch after the shared latch