Minor adjustments to make failures in startup/shutdown behave more cleanly.

StartupXLOG and ShutdownXLOG no longer need to be critical sections, because
in all contexts where they are invoked, elog(ERROR) would be translated to
elog(FATAL) anyway.  (One change in bgwriter.c is needed to make this true:
set ExitOnAnyError before trying to exit.  This is a good fix anyway since
the existing code would have gone into an infinite loop on elog(ERROR) during
shutdown.)  That avoids a misleading report of PANIC during semi-orderly
failures.  Modify the postmaster to include the startup process in the set of
processes that get SIGTERM when a fast shutdown is requested, and also fix it
to not try to restart the bgwriter if the bgwriter fails while trying to write
the shutdown checkpoint.  Net result is that "pg_ctl stop -m fast" does
something reasonable for a system in warm standby mode, and so should Unix
system shutdown (ie, universal SIGTERM).  Per gripe from Stephen Harris and
some corner-case testing of my own.

src/backend/access/transam/xlog.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.257 2006/11/21 20:59:52 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.258 2006/11/30 18:29:11 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4651,8 +4651,6 @@ StartupXLOG(void)
 	uint32		freespace;
 	TransactionId oldestActiveXID;
 
-	CritSectionCount++;
-
 	/*
 	 * Read control file and check XLOG status looks valid.
 	 *
@@ -5188,7 +5186,6 @@ StartupXLOG(void)
 
 	ereport(LOG,
 			(errmsg("database system is ready")));
-	CritSectionCount--;
 
 	/* Shut down readFile facility, free space */
 	if (readFile >= 0)
@@ -5426,12 +5423,10 @@ ShutdownXLOG(int code, Datum arg)
 	ereport(LOG,
 			(errmsg("shutting down")));
 
-	CritSectionCount++;
 	CreateCheckPoint(true, true);
 	ShutdownCLOG();
 	ShutdownSUBTRANS();
 	ShutdownMultiXact();
-	CritSectionCount--;
 
 	ereport(LOG,
 			(errmsg("database system is shut down")));
@@ -5605,10 +5600,7 @@ CreateCheckPoint(bool shutdown, bool force)
 	 *
 	 * This I/O could fail for various reasons.  If so, we will fail to
 	 * complete the checkpoint, but there is no reason to force a system
-	 * panic. Accordingly, exit critical section while doing it.  (If we are
-	 * doing a shutdown checkpoint, we probably *should* panic --- but that
-	 * will happen anyway because we'll still be inside the critical section
-	 * established by ShutdownXLOG.)
+	 * panic. Accordingly, exit critical section while doing it.
 	 */
 	END_CRIT_SECTION();
 

src/backend/postmaster/bgwriter.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.31 2006/11/21 20:59:52 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/bgwriter.c,v 1.32 2006/11/30 18:29:12 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -350,6 +350,12 @@ BackgroundWriterMain(void)
 		}
 		if (shutdown_requested)
 		{
+			/*
+			 * From here on, elog(ERROR) should end with exit(1), not send
+			 * control back to the sigsetjmp block above
+			 */
+			ExitOnAnyError = true;
+			/* Close down the database */
 			ShutdownXLOG(0, 0);
 			DumpFreeSpaceMap(0, 0);
 			/* Normal exit from the bgwriter is here */

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.504 2006/11/28 12:54:41 petere Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.505 2006/11/30 18:29:12 tgl Exp $
  *
  * NOTES
  *
@@ -1934,8 +1934,13 @@ pmdie(SIGNAL_ARGS)
 			 * Note: if we previously got SIGTERM then we may send SIGUSR2 to
 			 * the bgwriter a second time here.  This should be harmless.
 			 */
-			if (StartupPID != 0 || FatalError)
-				break;			/* let reaper() handle this */
+			if (StartupPID != 0)
+			{
+				signal_child(StartupPID, SIGTERM);
+				break;			/* let reaper() do the rest */
+			}
+			if (FatalError)
+				break;			/* let reaper() handle this case */
 			/* Start the bgwriter if not running */
 			if (BgWriterPID == 0)
 				BgWriterPID = StartBackgroundWriter();
@@ -2108,6 +2113,21 @@ reaper(SIGNAL_ARGS)
 			 */
 			HandleChildCrash(pid, exitstatus,
 							 _("background writer process"));
+
+			/*
+			 * If the bgwriter crashed while trying to write the shutdown
+			 * checkpoint, we may as well just stop here; any recovery
+			 * required will happen on next postmaster start.
+			 */
+			if (Shutdown > NoShutdown &&
+				!DLGetHead(BackendList) && AutoVacPID == 0)
+			{
+				ereport(LOG,
+						(errmsg("abnormal database system shutdown")));
+				ExitPostmaster(1);
+			}
+
+			/* Else, proceed as in normal crash recovery */
 			continue;
 		}