Improve autovacuum launcher's ability to detect a problem in worker startup,

by having the postmaster signal it when certain failures occur.  This requires
the postmaster setting a flag in shared memory, but should be as safe as the
pmsignal.c code is.

Also make sure the launcher honor's a postgresql.conf change turning it off
on SIGHUP.

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.527 2007/03/22 19:53:30 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.528 2007/06/25 16:09:03 alvherre Exp $
  *
  * NOTES
  *
@@ -3830,14 +3830,8 @@ StartAutovacuumWorker(void)
 		return;
 
 	bn = (Backend *) malloc(sizeof(Backend));
-	if (!bn)
+	if (bn)
 	{
-		ereport(LOG,
-				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("out of memory")));
-		return;
-	}
-
 		bn->pid = StartAutoVacWorker();
 		bn->is_autovacuum = true;
 		/* we don't need a cancel key */
@@ -3848,14 +3842,23 @@ StartAutovacuumWorker(void)
 #ifdef EXEC_BACKEND
 			ShmemBackendArrayAdd(bn);
 #endif
+			/* all OK */
+			return;
 		}
-	else
-	{
-		/* not much we can do */
-		ereport(LOG,
-				(errmsg("could not fork new process for autovacuum: %m")));
+
+		/*
+		 * fork failed, fall through to report -- actual error message was
+		 * logged by StartAutoVacWorker
+		 */
 		free(bn);
 	}
+	else
+		elog(LOG, "out of memory");
+
+	/* report the failure to the launcher */
+	AutoVacWorkerFailed();
+	if (AutoVacPID != 0)
+		kill(AutoVacPID, SIGUSR1);
 }
 
 /*

src/include/postmaster/autovacuum.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/postmaster/autovacuum.h,v 1.10 2007/04/18 16:44:18 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/postmaster/autovacuum.h,v 1.11 2007/06/25 16:09:03 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,6 +42,8 @@ extern bool IsAutoVacuumWorkerProcess(void);
 extern void autovac_init(void);
 extern int	StartAutoVacLauncher(void);
 extern int	StartAutoVacWorker(void);
+/* called from postmaster when a worker could not be forked */
+extern void AutoVacWorkerFailed(void);
 
 /* autovacuum cost-delay balancer */
 extern void AutoVacuumUpdateDelay(void);