Avoid possible hang during smart shutdown.

If a smart shutdown occurs just as a child is starting up, and the
child subsequently becomes a walsender, there is a race condition:
the postmaster might count the exstant backends, determine that there
is one normal backend, and wait for it to die off.  Had the walsender
transition already occurred before the postmaster counted, it would
have proceeded with the shutdown.

To fix this, have each child that transforms into a walsender kick
the postmaster just after doing so, so that the state machine is
certain to advance.

Fujii Masao

src/backend/postmaster/postmaster.c

@@ -2890,8 +2890,8 @@ LogChildExit(int lev, const char *procname, int pid, int exitstatus)
 /*
  * Advance the postmaster's state machine and take actions as appropriate
  *
- * This is common code for pmdie() and reaper(), which receive the signals
- * that might mean we need to change state.
+ * This is common code for pmdie(), reaper() and sigusr1_handler(), which
+ * receive the signals that might mean we need to change state.
  */
 static void
 PostmasterStateMachine(void)
@@ -4284,6 +4284,13 @@ sigusr1_handler(SIGNAL_ARGS)
 		WalReceiverPID = StartWalReceiver();
 	}
 
+	if (CheckPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE) &&
+		(pmState == PM_WAIT_BACKUP || pmState == PM_WAIT_BACKENDS))
+	{
+		/* Advance postmaster's state machine */
+		PostmasterStateMachine();
+	}
+
 	if (CheckPromoteSignal() && StartupPID != 0 &&
 		(pmState == PM_STARTUP || pmState == PM_RECOVERY ||
 		 pmState == PM_HOT_STANDBY || pmState == PM_WAIT_READONLY))

src/backend/replication/walsender.c

@@ -361,6 +361,7 @@ StartReplication(StartReplicationCmd * cmd)
 	 * after this.
 	 */
 	MarkPostmasterChildWalSender();
+	SendPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE);
 
 	/*
 	 * Check that we're logging enough information in the WAL for

src/include/storage/pmsignal.h

@@ -29,6 +29,7 @@ typedef enum
 	PMSIGNAL_START_AUTOVAC_LAUNCHER,	/* start an autovacuum launcher */
 	PMSIGNAL_START_AUTOVAC_WORKER,		/* start an autovacuum worker */
 	PMSIGNAL_START_WALRECEIVER, /* start a walreceiver */
+	PMSIGNAL_ADVANCE_STATE_MACHINE,	/* advance postmaster's state machine */
 
 	NUM_PMSIGNALS				/* Must be last value of enum! */
 } PMSignalReason;