Make archiver process handle barrier events.

Commit d75288f made WAL archiver process an auxiliary process.
An auxiliary process needs to handle barrier events but the commit
forgot to make archiver process do that.

Reported-by: Thomas Munro
Author: Fujii Masao
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CA+hUKGLah2w1pWKHonZP_+EQw69=q56AHYwCgEN8GDzsRG_Hgw@mail.gmail.com

Author: MasaoFujii

src/backend/postmaster/pgarch.c

@@ -102,6 +102,7 @@ static bool pgarch_archiveXlog(char *xlog);
 static bool pgarch_readyXlog(char *xlog);
 static void pgarch_archiveDone(char *xlog);
 static void pgarch_die(int code, Datum arg);
+static void HandlePgArchInterrupts(void);
 
 /* Report shared memory space needed by PgArchShmemInit */
 Size
@@ -257,12 +258,8 @@ pgarch_MainLoop(void)
 		/* When we get SIGUSR2, we do one more archive cycle, then exit */
 		time_to_stop = ready_to_stop;
 
-		/* Check for config update */
-		if (ConfigReloadPending)
-		{
-			ConfigReloadPending = false;
-			ProcessConfigFile(PGC_SIGHUP);
-		}
+		/* Check for barrier events and config update */
+		HandlePgArchInterrupts();
 
 		/*
 		 * If we've gotten SIGTERM, we normally just sit and do nothing until
@@ -355,15 +352,11 @@ pgarch_ArchiverCopyLoop(void)
 				return;
 
 			/*
-			 * Check for config update.  This is so that we'll adopt a new
-			 * setting for archive_command as soon as possible, even if there
-			 * is a backlog of files to be archived.
+			 * Check for barrier events and config update.  This is so that
+			 * we'll adopt a new setting for archive_command as soon as
+			 * possible, even if there is a backlog of files to be archived.
 			 */
-			if (ConfigReloadPending)
-			{
-				ConfigReloadPending = false;
-				ProcessConfigFile(PGC_SIGHUP);
-			}
+			HandlePgArchInterrupts();
 
 			/* can't do anything if no command ... */
 			if (!XLogArchiveCommandSet())
@@ -703,3 +696,23 @@ pgarch_die(int code, Datum arg)
 {
 	PgArch->pgprocno = INVALID_PGPROCNO;
 }
+
+/*
+ * Interrupt handler for WAL archiver process.
+ *
+ * This is called in the loops pgarch_MainLoop and pgarch_ArchiverCopyLoop.
+ * It checks for barrier events and config update, but not shutdown request
+ * because how to handle shutdown request is different between those loops.
+ */
+static void
+HandlePgArchInterrupts(void)
+{
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
+
+	if (ConfigReloadPending)
+	{
+		ConfigReloadPending = false;
+		ProcessConfigFile(PGC_SIGHUP);
+	}
+}