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/utils/init/globals.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/globals.c,v 1.107 2009/01/01 17:23:51 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/globals.c,v 1.108 2009/05/05 19:59:00 tgl Exp $
  *
  * NOTES
  *	  Globals used all over the place should be declared here and not
@@ -36,6 +36,7 @@ int			MyProcPid;
 pg_time_t	MyStartTime;
 struct Port *MyProcPort;
 long		MyCancelKey;
+int			MyPMChildSlot;
 
 /*
  * DataDir is the absolute path to the top level of the PGDATA directory tree.