Add logging for excessive ProcSignalBarrier waits.

To enable diagnosis of systems that are not processing ProcSignalBarrier
requests promptly, add a LOG message every 5 seconds if we seem to be
wedged.  Although you could already see this state as a wait event in
pg_stat_activity, the log message also shows the PID of the process that
is preventing progress.

Also add DEBUG1 logging around the whole wait loop.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA%2BTgmoYJ03r5359gQutRGP9BtigYCg3_UskcmnVjBf-QO3-0pQ%40mail.gmail.com

src/backend/storage/ipc/procsignal.c

@@ -393,6 +393,11 @@ WaitForProcSignalBarrier(uint64 generation)
 {
 	Assert(generation <= pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration));
 
+	elog(DEBUG1,
+		 "waiting for all backends to process ProcSignalBarrier generation "
+		 UINT64_FORMAT,
+		 generation);
+
 	for (int i = NumProcSignalSlots - 1; i >= 0; i--)
 	{
 		ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
@@ -407,13 +412,22 @@ WaitForProcSignalBarrier(uint64 generation)
 		oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
 		while (oldval < generation)
 		{
-			ConditionVariableSleep(&slot->pss_barrierCV,
-								   WAIT_EVENT_PROC_SIGNAL_BARRIER);
+			if (ConditionVariableTimedSleep(&slot->pss_barrierCV,
+											5000,
+											WAIT_EVENT_PROC_SIGNAL_BARRIER))
+				ereport(LOG,
+						(errmsg("still waiting for backend with PID %lu to accept ProcSignalBarrier",
+								(unsigned long) slot->pss_pid)));
 			oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
 		}
 		ConditionVariableCancelSleep();
 	}
 
+	elog(DEBUG1,
+		 "finished waiting for all backends to process ProcSignalBarrier generation "
+		 UINT64_FORMAT,
+		 generation);
+
 	/*
 	 * The caller is probably calling this function because it wants to read
 	 * the shared state or perform further writes to shared state once all