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

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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.
This commit is contained in:
Tom Lane
2009-05-05 19:59:00 +00:00
parent 8f348112f3
commit 969d7cd431
8 changed files with 280 additions and 92 deletions
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12
src/include/storage/pmsignal.h
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@@ -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 */