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Commit54fc9dc

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 parentf6e1ee3 commit54fc9dc

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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"
@@ -102,7 +104,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
102104
intsave_errno=errno;
103105

104106
if (in_restore_command)
105-
proc_exit(1);
107+
{
108+
/*
109+
* If we are in a child process (e.g., forked by system() in
110+
* RestoreArchivedFile()), we don't want to call any exit callbacks.
111+
* The parent will take care of that.
112+
*/
113+
if (MyProcPid== (int)getpid())
114+
proc_exit(1);
115+
else
116+
{
117+
write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n");
118+
_exit(1);
119+
}
120+
}
106121
else
107122
shutdown_requested= true;
108123
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
@@ -826,6 +826,10 @@ ProcKill(int code, Datum arg)
826826

827827
Assert(MyProc!=NULL);
828828

829+
/* not safe if forked by system(), etc. */
830+
if (MyProc->pid!= (int)getpid())
831+
elog(PANIC,"ProcKill() called in child process");
832+
829833
/* Make sure we're out of the sync rep lists */
830834
SyncRepCleanupAtProcExit();
831835

@@ -955,6 +959,10 @@ AuxiliaryProcKill(int code, Datum arg)
955959

956960
Assert(proctype >=0&&proctype<NUM_AUXILIARY_PROCS);
957961

962+
/* not safe if forked by system(), etc. */
963+
if (MyProc->pid!= (int)getpid())
964+
elog(PANIC,"AuxiliaryProcKill() called in child process");
965+
958966
auxproc=&AuxiliaryProcs[proctype];
959967

960968
Assert(MyProc==auxproc);

‎src/backend/utils/error/elog.c

Lines changed: 28 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3590,6 +3590,34 @@ write_stderr(const char *fmt,...)
35903590
}
35913591

35923592

3593+
/*
3594+
* Write a message to STDERR using only async-signal-safe functions. This can
3595+
* be used to safely emit a message from a signal handler.
3596+
*
3597+
* TODO: It is likely possible to safely do a limited amount of string
3598+
* interpolation (e.g., %s and %d), but that is not presently supported.
3599+
*/
3600+
void
3601+
write_stderr_signal_safe(constchar*str)
3602+
{
3603+
intnwritten=0;
3604+
intntotal=strlen(str);
3605+
3606+
while (nwritten<ntotal)
3607+
{
3608+
intrc;
3609+
3610+
rc=write(STDERR_FILENO,str+nwritten,ntotal-nwritten);
3611+
3612+
/* Just give up on error. There isn't much else we can do. */
3613+
if (rc==-1)
3614+
return;
3615+
3616+
nwritten+=rc;
3617+
}
3618+
}
3619+
3620+
35933621
/*
35943622
* Adjust the level of a recovery-related message per trace_recovery_messages.
35953623
*

‎src/include/utils/elog.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -453,4 +453,10 @@ extern void set_syslog_parameters(const char *ident, int facility);
453453
*/
454454
externvoidwrite_stderr(constchar*fmt,...)pg_attribute_printf(1,2);
455455

456+
/*
457+
* Write a message to STDERR using only async-signal-safe functions. This can
458+
* be used to safely emit a message from a signal handler.
459+
*/
460+
externvoidwrite_stderr_signal_safe(constchar*fmt);
461+
456462
#endif/* ELOG_H */

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