Make postmaster restart archiver soon after it dies, even during recovery.

After the archiver dies, postmaster tries to start a new one immediately.
But previously this could happen only while server was running normally
even though archiving was enabled always (i.e., archive_mode was set to
always). So the archiver running during recovery could not restart soon
after it died. This is an oversight in commit ffd37740.

This commit changes reaper(), postmaster's signal handler to cleanup
after a child process dies, so that it tries to a new archiver even during
recovery if necessary.

Patch by me. Review by Alvaro Herrera.

src/backend/postmaster/postmaster.c

@@ -406,6 +406,17 @@ static pid_t StartChildProcess(AuxProcType type);
 static void StartAutovacuumWorker(void);
 static void InitPostmasterDeathWatchHandle(void);
 
+/*
+ * Archiver is allowed to start up at the current postmaster state?
+ *
+ * If WAL archiving is enabled always, we are allowed to start archiver
+ * even during recovery.
+ */
+#define PgArchStartupAllowed()	\
+	((XLogArchivingActive() && pmState == PM_RUN) ||	\
+	 (XLogArchivingAlways() &&	\
+	  (pmState == PM_RECOVERY || pmState == PM_HOT_STANDBY)))
+
 #ifdef EXEC_BACKEND
 
 #ifdef WIN32
@@ -1649,21 +1660,9 @@ ServerLoop(void)
 		if (PgStatPID == 0 && pmState == PM_RUN)
 			PgStatPID = pgstat_start();
 
-		/*
-		 * If we have lost the archiver, try to start a new one.
-		 *
-		 * If WAL archiving is enabled always, we try to start a new archiver
-		 * even during recovery.
-		 */
-		if (PgArchPID == 0 && wal_level >= WAL_LEVEL_ARCHIVE)
-		{
-			if ((pmState == PM_RUN && XLogArchiveMode > ARCHIVE_MODE_OFF) ||
-				((pmState == PM_RECOVERY || pmState == PM_HOT_STANDBY) &&
-				 XLogArchiveMode == ARCHIVE_MODE_ALWAYS))
-			{
+		/* If we have lost the archiver, try to start a new one. */
+		if (PgArchPID == 0 && PgArchStartupAllowed())
 				PgArchPID = pgarch_start();
-			}
-		}
 
 		/* If we need to signal the autovacuum launcher, do so now */
 		if (avlauncher_needs_signal)
@@ -2669,7 +2668,7 @@ reaper(SIGNAL_ARGS)
 			 */
 			if (!IsBinaryUpgrade && AutoVacuumingActive() && AutoVacPID == 0)
 				AutoVacPID = StartAutoVacLauncher();
-			if (XLogArchivingActive() && PgArchPID == 0)
+			if (PgArchStartupAllowed() && PgArchPID == 0)
 				PgArchPID = pgarch_start();
 			if (PgStatPID == 0)
 				PgStatPID = pgstat_start();
@@ -2810,7 +2809,7 @@ reaper(SIGNAL_ARGS)
 			if (!EXIT_STATUS_0(exitstatus))
 				LogChildExit(LOG, _("archiver process"),
 							 pid, exitstatus);
-			if (XLogArchivingActive() && pmState == PM_RUN)
+			if (PgArchStartupAllowed())
 				PgArchPID = pgarch_start();
 			continue;
 		}
@@ -4833,11 +4832,8 @@ sigusr1_handler(SIGNAL_ARGS)
 		 * files.
 		 */
 		Assert(PgArchPID == 0);
-		if (wal_level >= WAL_LEVEL_ARCHIVE &&
-			XLogArchiveMode == ARCHIVE_MODE_ALWAYS)
-		{
+		if (XLogArchivingAlways())
 			PgArchPID = pgarch_start();
-		}
 
 		pmState = PM_RECOVERY;
 	}

src/include/access/xlog.h

@@ -126,8 +126,12 @@ typedef enum WalLevel
 } WalLevel;
 extern int	wal_level;
 
+/* Is WAL archiving enabled (always or only while server is running normally)? */
 #define XLogArchivingActive() \
 	(XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level >= WAL_LEVEL_ARCHIVE)
+/* Is WAL archiving enabled always (even during recovery)? */
+#define XLogArchivingAlways() \
+	(XLogArchiveMode == ARCHIVE_MODE_ALWAYS && wal_level >= WAL_LEVEL_ARCHIVE)
 #define XLogArchiveCommandSet() (XLogArchiveCommand[0] != '\0')
 
 /*