Install a "dead man switch" to allow the postmaster to detect cases where

a backend has done exit(0) or exit(1) without having disengaged itself
from shared memory.  We are at risk for this whenever third-party code is
loaded into a backend, since such code might not know it's supposed to go
through proc_exit() instead.  Also, it is reported that under Windows
there are ways to externally kill a process that cause the status code
returned to the postmaster to be indistinguishable from a voluntary exit
(thank you, Microsoft).  If this does happen then the system is probably
hosed --- for instance, the dead session might still be holding locks.
So the best recovery method is to treat this like a backend crash.

The dead man switch is armed for a particular child process when it
acquires a regular PGPROC, and disarmed when the PGPROC is released;
these should be the first and last touches of shared memory resources
in a backend, or close enough anyway.  This choice means there is no
coverage for auxiliary processes, but I doubt we need that, since they
shouldn't be executing any user-provided code anyway.

This patch also improves the management of the EXEC_BACKEND
ShmemBackendArray array a bit, by reducing search costs.

Although this problem is of long standing, the lack of field complaints
seems to mean it's not critical enough to risk back-patching; at least
not till we get some more testing of this mechanism.

src/backend/storage/lmgr/proc.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/proc.c,v 1.205 2009/01/01 17:23:48 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/proc.c,v 1.206 2009/05/05 19:59:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -41,6 +41,7 @@
 #include "postmaster/autovacuum.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
+#include "storage/pmsignal.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "storage/spin.h"
@@ -274,6 +275,14 @@ InitProcess(void)
 				 errmsg("sorry, too many clients already")));
 	}
 
+	/*
+	 * Now that we have a PGPROC, mark ourselves as an active postmaster
+	 * child; this is so that the postmaster can detect it if we exit
+	 * without cleaning up.
+	 */
+	if (IsUnderPostmaster)
+		MarkPostmasterChildActive();
+
 	/*
 	 * Initialize all fields of MyProc, except for the semaphore which was
 	 * prepared for us by InitProcGlobal.
@@ -614,6 +623,13 @@ ProcKill(int code, Datum arg)
 
 	SpinLockRelease(ProcStructLock);
 
+	/*
+	 * This process is no longer present in shared memory in any meaningful
+	 * way, so tell the postmaster we've cleaned up acceptably well.
+	 */
+	if (IsUnderPostmaster)
+		MarkPostmasterChildInactive();
+
 	/* wake autovac launcher if needed -- see comments in FreeWorkerInfo */
 	if (AutovacuumLauncherPid != 0)
 		kill(AutovacuumLauncherPid, SIGUSR1);