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Commitd0e7f95

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Avoid calling proc_exit() in processes forked by system().
The SIGTERM handler for the startup process immediately callsproc_exit() for the duration of the restore_command, i.e., a callto system(). This system() call forks a new process to execute theshell command, and this child process inherits the parent's signalhandlers. If both the parent and child processes receive SIGTERM,both will attempt to call proc_exit(). This can end badly. Forexample, both processes will try to remove themselves from thePGPROC shared array.To fix this problem, this commit adds a check inStartupProcShutdownHandler() to see whether MyProcPid == getpid().If they match, this is the parent process, and we can proc_exit()like before. If they do not match, this is a child process, and wejust emit a message to STDERR (in a signal safe manner) and_exit(), thereby skipping any problematic exit callbacks.This commit also adds checks in proc_exit(), ProcKill(), andAuxiliaryProcKill() that verify they are not being called withinsuch child processes.Suggested-by: Andres FreundReviewed-by: Thomas Munro, Andres FreundDiscussion:https://postgr.es/m/Y9nGDSgIm83FHcad%40paquier.xyzDiscussion:https://postgr.es/m/20230223231503.GA743455%40nathanxps13Backpatch-through: 11
1 parenta295684 commitd0e7f95

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5 files changed

+60
-1
lines changed

5 files changed

+60
-1
lines changed

‎src/backend/postmaster/startup.c

Lines changed: 14 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -132,7 +132,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
132132
intsave_errno=errno;
133133

134134
if (in_restore_command)
135-
proc_exit(1);
135+
{
136+
/*
137+
* If we are in a child process (e.g., forked by system() in
138+
* RestoreArchivedFile()), we don't want to call any exit callbacks.
139+
* The parent will take care of that.
140+
*/
141+
if (MyProcPid== (int)getpid())
142+
proc_exit(1);
143+
else
144+
{
145+
write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n");
146+
_exit(1);
147+
}
148+
}
136149
else
137150
shutdown_requested= true;
138151
WakeupRecovery();

‎src/backend/storage/ipc/ipc.c

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -103,6 +103,10 @@ static inton_proc_exit_index,
103103
void
104104
proc_exit(intcode)
105105
{
106+
/* not safe if forked by system(), etc. */
107+
if (MyProcPid!= (int)getpid())
108+
elog(PANIC,"proc_exit() called in child process");
109+
106110
/* Clean up everything that must be cleaned up */
107111
proc_exit_prepare(code);
108112

‎src/backend/storage/lmgr/proc.c

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -805,6 +805,10 @@ ProcKill(int code, Datum arg)
805805

806806
Assert(MyProc!=NULL);
807807

808+
/* not safe if forked by system(), etc. */
809+
if (MyProc->pid!= (int)getpid())
810+
elog(PANIC,"ProcKill() called in child process");
811+
808812
/* Make sure we're out of the sync rep lists */
809813
SyncRepCleanupAtProcExit();
810814

@@ -929,6 +933,10 @@ AuxiliaryProcKill(int code, Datum arg)
929933

930934
Assert(proctype >=0&&proctype<NUM_AUXILIARY_PROCS);
931935

936+
/* not safe if forked by system(), etc. */
937+
if (MyProc->pid!= (int)getpid())
938+
elog(PANIC,"AuxiliaryProcKill() called in child process");
939+
932940
auxproc=&AuxiliaryProcs[proctype];
933941

934942
Assert(MyProc==auxproc);

‎src/backend/utils/error/elog.c

Lines changed: 28 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3703,6 +3703,34 @@ write_stderr(const char *fmt,...)
37033703
}
37043704

37053705

3706+
/*
3707+
* Write a message to STDERR using only async-signal-safe functions. This can
3708+
* be used to safely emit a message from a signal handler.
3709+
*
3710+
* TODO: It is likely possible to safely do a limited amount of string
3711+
* interpolation (e.g., %s and %d), but that is not presently supported.
3712+
*/
3713+
void
3714+
write_stderr_signal_safe(constchar*str)
3715+
{
3716+
intnwritten=0;
3717+
intntotal=strlen(str);
3718+
3719+
while (nwritten<ntotal)
3720+
{
3721+
intrc;
3722+
3723+
rc=write(STDERR_FILENO,str+nwritten,ntotal-nwritten);
3724+
3725+
/* Just give up on error. There isn't much else we can do. */
3726+
if (rc==-1)
3727+
return;
3728+
3729+
nwritten+=rc;
3730+
}
3731+
}
3732+
3733+
37063734
/*
37073735
* is_log_level_output -- is elevel logically >= log_min_level?
37083736
*

‎src/include/utils/elog.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -413,4 +413,10 @@ extern void set_syslog_parameters(const char *ident, int facility);
413413
*/
414414
externvoidwrite_stderr(constchar*fmt,...)pg_attribute_printf(1,2);
415415

416+
/*
417+
* Write a message to STDERR using only async-signal-safe functions. This can
418+
* be used to safely emit a message from a signal handler.
419+
*/
420+
externvoidwrite_stderr_signal_safe(constchar*fmt);
421+
416422
#endif/* ELOG_H */

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