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Commitac1dfc3

Browse files
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 parent1e67817 commitac1dfc3

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5 files changed

+62
-1
lines changed

5 files changed

+62
-1
lines changed

‎src/backend/postmaster/startup.c

Lines changed: 16 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -19,6 +19,8 @@
1919
*/
2020
#include"postgres.h"
2121

22+
#include<unistd.h>
23+
2224
#include"access/xlog.h"
2325
#include"libpq/pqsignal.h"
2426
#include"miscadmin.h"
@@ -91,7 +93,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
9193
intsave_errno=errno;
9294

9395
if (in_restore_command)
94-
proc_exit(1);
96+
{
97+
/*
98+
* If we are in a child process (e.g., forked by system() in
99+
* RestoreArchivedFile()), we don't want to call any exit callbacks.
100+
* The parent will take care of that.
101+
*/
102+
if (MyProcPid== (int)getpid())
103+
proc_exit(1);
104+
else
105+
{
106+
write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n");
107+
_exit(1);
108+
}
109+
}
95110
else
96111
shutdown_requested= true;
97112
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
@@ -821,6 +821,10 @@ ProcKill(int code, Datum arg)
821821

822822
Assert(MyProc!=NULL);
823823

824+
/* not safe if forked by system(), etc. */
825+
if (MyProc->pid!= (int)getpid())
826+
elog(PANIC,"ProcKill() called in child process");
827+
824828
/* Make sure we're out of the sync rep lists */
825829
SyncRepCleanupAtProcExit();
826830

@@ -945,6 +949,10 @@ AuxiliaryProcKill(int code, Datum arg)
945949

946950
Assert(proctype >=0&&proctype<NUM_AUXILIARY_PROCS);
947951

952+
/* not safe if forked by system(), etc. */
953+
if (MyProc->pid!= (int)getpid())
954+
elog(PANIC,"AuxiliaryProcKill() called in child process");
955+
948956
auxproc=&AuxiliaryProcs[proctype];
949957

950958
Assert(MyProc==auxproc);

‎src/backend/utils/error/elog.c

Lines changed: 28 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3457,6 +3457,34 @@ write_stderr(const char *fmt,...)
34573457
}
34583458

34593459

3460+
/*
3461+
* Write a message to STDERR using only async-signal-safe functions. This can
3462+
* be used to safely emit a message from a signal handler.
3463+
*
3464+
* TODO: It is likely possible to safely do a limited amount of string
3465+
* interpolation (e.g., %s and %d), but that is not presently supported.
3466+
*/
3467+
void
3468+
write_stderr_signal_safe(constchar*str)
3469+
{
3470+
intnwritten=0;
3471+
intntotal=strlen(str);
3472+
3473+
while (nwritten<ntotal)
3474+
{
3475+
intrc;
3476+
3477+
rc=write(STDERR_FILENO,str+nwritten,ntotal-nwritten);
3478+
3479+
/* Just give up on error. There isn't much else we can do. */
3480+
if (rc==-1)
3481+
return;
3482+
3483+
nwritten+=rc;
3484+
}
3485+
}
3486+
3487+
34603488
/*
34613489
* is_log_level_output -- is elevel logically >= log_min_level?
34623490
*

‎src/include/utils/elog.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -436,4 +436,10 @@ extern void set_syslog_parameters(const char *ident, int facility);
436436
*/
437437
externvoidwrite_stderr(constchar*fmt,...)pg_attribute_printf(1,2);
438438

439+
/*
440+
* Write a message to STDERR using only async-signal-safe functions. This can
441+
* be used to safely emit a message from a signal handler.
442+
*/
443+
externvoidwrite_stderr_signal_safe(constchar*fmt);
444+
439445
#endif/* ELOG_H */

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