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.

3 files changed, 15 insertions, 4 deletions

diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 57ee0f93dd4..51649380d51 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -13,7 +13,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/miscadmin.h,v 1.209 2009/01/05 02:27:45 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/miscadmin.h,v 1.210 2009/05/05 19:59:00 tgl Exp $
  *
  * NOTES
  *	  some of the information in this file should be moved to other files.
@@ -137,6 +137,7 @@ extern PGDLLIMPORT int MyProcPid;
 extern PGDLLIMPORT pg_time_t MyStartTime;
 extern PGDLLIMPORT struct Port *MyProcPort;
 extern long MyCancelKey;
+extern int	MyPMChildSlot;
 
 extern char OutputFileName[];
 extern PGDLLIMPORT char my_exec_path[];
diff --git a/src/include/postmaster/postmaster.h b/src/include/postmaster/postmaster.h
index 3abffa8b1c7..3b8061e4f7d 100644
--- a/src/include/postmaster/postmaster.h
+++ b/src/include/postmaster/postmaster.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/postmaster/postmaster.h,v 1.19 2009/01/01 17:24:01 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/postmaster/postmaster.h,v 1.20 2009/05/05 19:59:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -38,6 +38,8 @@ extern const char *progname;
 extern int	PostmasterMain(int argc, char *argv[]);
 extern void ClosePostmasterPorts(bool am_syslogger);
 
+extern int	MaxLivePostmasterChildren(void);
+
 #ifdef EXEC_BACKEND
 extern pid_t postmaster_forkexec(int argc, char *argv[]);
 extern int	SubPostmasterMain(int argc, char *argv[]);
diff --git a/src/include/storage/pmsignal.h b/src/include/storage/pmsignal.h
index bd6cff154ef..ebfc1b8aa65 100644
--- a/src/include/storage/pmsignal.h
+++ b/src/include/storage/pmsignal.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/pmsignal.h,v 1.23 2009/02/23 09:28:50 heikki Exp $
+ * $PostgreSQL: pgsql/src/include/storage/pmsignal.h,v 1.24 2009/05/05 19:59:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -33,12 +33,20 @@ typedef enum
 	NUM_PMSIGNALS				/* Must be last value of enum! */
 } PMSignalReason;
 
+/* PMSignalData is an opaque struct, details known only within pmsignal.c */
+typedef struct PMSignalData PMSignalData;
+
 /*
  * prototypes for functions in pmsignal.c
  */
-extern void PMSignalInit(void);
+extern Size PMSignalShmemSize(void);
+extern void PMSignalShmemInit(void);
 extern void SendPostmasterSignal(PMSignalReason reason);
 extern bool CheckPostmasterSignal(PMSignalReason reason);
+extern int	AssignPostmasterChildSlot(void);
+extern bool ReleasePostmasterChildSlot(int slot);
+extern void MarkPostmasterChildActive(void);
+extern void MarkPostmasterChildInactive(void);
 extern bool PostmasterIsAlive(bool amDirectChild);
 
 #endif   /* PMSIGNAL_H */