Change the autovacuum launcher to read pg_database directly, rather than

via the "flat files" facility.  This requires making it enough like a backend
to be able to run transactions; it's no longer an "auxiliary process" but
more like the autovacuum worker processes.  Also, its signal handling has
to be brought into line with backends/workers.  In particular, since it
now has to handle procsignal.c processing, the special autovac-launcher-only
signal conditions are moved to SIGUSR2.

Alvaro, with some cleanup from Tom

src/backend/utils/init/globals.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/globals.c,v 1.109 2009/08/28 18:23:53 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/globals.c,v 1.110 2009/08/31 19:41:00 tgl Exp $
  *
  * NOTES
  *	  Globals used all over the place should be declared here and not
@@ -100,8 +100,8 @@ int			maintenance_work_mem = 16384;
 
 /*
  * Primary determinants of sizes of shared-memory structures.  MaxBackends is
- * MaxConnections + autovacuum_max_workers (it is computed by the GUC assign
- * hook):
+ * MaxConnections + autovacuum_max_workers + 1 (it is computed by the GUC
+ * assign hooks for those variables):
  */
 int			NBuffers = 1000;
 int			MaxBackends = 100;

src/backend/utils/init/postinit.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.195 2009/08/29 19:26:51 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.196 2009/08/31 19:41:00 tgl Exp $
  *
  *
  *-------------------------------------------------------------------------
@@ -460,7 +460,9 @@ BaseInit(void)
  * name can be returned to the caller in out_dbname.  If out_dbname isn't
  * NULL, it must point to a buffer of size NAMEDATALEN.
  *
- * In bootstrap mode no parameters are used.
+ * In bootstrap mode no parameters are used.  The autovacuum launcher process
+ * doesn't use any parameters either, because it only goes far enough to be
+ * able to read pg_database; it doesn't connect to any particular database.
  *
  * The return value indicates whether the userID is a superuser.  (That
  * can only be tested inside a transaction, so we want to do it during
@@ -537,6 +539,12 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	if (!bootstrap)
 		pgstat_initialize();
 
+	/*
+	 * Load relcache entries for the shared system catalogs.  This must
+	 * create at least an entry for pg_database.
+	 */
+	RelationCacheInitializePhase2();
+
 	/*
 	 * Set up process-exit callback to do pre-shutdown cleanup.  This has to
 	 * be after we've initialized all the low-level modules like the buffer
@@ -548,10 +556,17 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	 */
 	on_shmem_exit(ShutdownPostgres, 0);
 
+	/* The autovacuum launcher is done here */
+	if (IsAutoVacuumLauncherProcess())
+		return true;			/* result doesn't matter */
+
 	/*
 	 * Start a new transaction here before first access to db, and get a
 	 * snapshot.  We don't have a use for the snapshot itself, but we're
 	 * interested in the secondary effect that it sets RecentGlobalXmin.
+	 * (This is critical for anything that reads heap pages, because HOT
+	 * may decide to prune them even if the process doesn't attempt to
+	 * modify any tuples.)
 	 */
 	if (!bootstrap)
 	{
@@ -559,12 +574,6 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 		(void) GetTransactionSnapshot();
 	}
 
-	/*
-	 * Load relcache entries for the shared system catalogs.  This must
-	 * create at least an entry for pg_database.
-	 */
-	RelationCacheInitializePhase2();
-
 	/*
 	 * Set up the global variables holding database id and default tablespace.
 	 * But note we won't actually try to touch the database just yet.