Avoid calling proc_exit() in processes forked by system().

The SIGTERM handler for the startup process immediately calls
proc_exit() for the duration of the restore_command, i.e., a call
to system().  This system() call forks a new process to execute the
shell command, and this child process inherits the parent's signal
handlers.  If both the parent and child processes receive SIGTERM,
both will attempt to call proc_exit().  This can end badly.  For
example, both processes will try to remove themselves from the
PGPROC shared array.

To fix this problem, this commit adds a check in
StartupProcShutdownHandler() 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 we
just 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(), and
AuxiliaryProcKill() that verify they are not being called within
such child processes.

Suggested-by: Andres Freund
Reviewed-by: Thomas Munro, Andres Freund
Discussion: https://postgr.es/m/Y9nGDSgIm83FHcad%40paquier.xyz
Discussion: https://postgr.es/m/20230223231503.GA743455%40nathanxps13
Backpatch-through: 11

diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c
index a00ae68b5614f1f556666deff4bd127aceba71dc..567e9e33e558ab3453cd85fefb51ee7a6a0a1b51 100644 (file)
--- a/src/backend/postmaster/startup.c
+++ b/src/backend/postmaster/startup.c
@@ -132,7 +132,20 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
        int                     save_errno = errno;
 
        if (in_restore_command)
-               proc_exit(1);
+       {
+               /*
+                * If we are in a child process (e.g., forked by system() in
+                * RestoreArchivedFile()), we don't want to call any exit callbacks.
+                * The parent will take care of that.
+                */
+               if (MyProcPid == (int) getpid())
+                       proc_exit(1);
+               else
+               {
+                       write_stderr_signal_safe("StartupProcShutdownHandler() called in child process\n");
+                       _exit(1);
+               }
+       }
        else
                shutdown_requested = true;
        WakeupRecovery();

diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c
index 05d02c23f55449c127fbcd0e3638ab032692e61a..ae132358abbbcfe896c2a30ff5d60be70143701d 100644 (file)
--- a/src/backend/storage/ipc/ipc.c
+++ b/src/backend/storage/ipc/ipc.c
@@ -103,6 +103,10 @@ static int on_proc_exit_index,
 void
 proc_exit(int code)
 {
+       /* not safe if forked by system(), etc. */
+       if (MyProcPid != (int) getpid())
+               elog(PANIC, "proc_exit() called in child process");
+
        /* Clean up everything that must be cleaned up */
        proc_exit_prepare(code);
 

diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index bc8d6e8c01d1563cf6b176b5b2a9c787f4747a7a..80a8b48c3cd9331739a6969c4116d460beba2831 100644 (file)
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -822,6 +822,10 @@ ProcKill(int code, Datum arg)
 
        Assert(MyProc != NULL);
 
+       /* not safe if forked by system(), etc. */
+       if (MyProc->pid != (int) getpid())
+               elog(PANIC, "ProcKill() called in child process");
+
        /* Make sure we're out of the sync rep lists */
        SyncRepCleanupAtProcExit();
 
@@ -946,6 +950,10 @@ AuxiliaryProcKill(int code, Datum arg)
 
        Assert(proctype >= 0 && proctype < NUM_AUXILIARY_PROCS);
 
+       /* not safe if forked by system(), etc. */
+       if (MyProc->pid != (int) getpid())
+               elog(PANIC, "AuxiliaryProcKill() called in child process");
+
        auxproc = &AuxiliaryProcs[proctype];
 
        Assert(MyProc == auxproc);

diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index e164f5bfee49b7660a2efde7bdc91f61a752db80..bca06ca331517fb1e4d82a773df17603652eb58c 100644 (file)
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3416,6 +3416,34 @@ write_stderr(const char *fmt,...)
 }
 
 
+/*
+ * Write a message to STDERR using only async-signal-safe functions.  This can
+ * be used to safely emit a message from a signal handler.
+ *
+ * TODO: It is likely possible to safely do a limited amount of string
+ * interpolation (e.g., %s and %d), but that is not presently supported.
+ */
+void
+write_stderr_signal_safe(const char *str)
+{
+       int                     nwritten = 0;
+       int                     ntotal = strlen(str);
+
+       while (nwritten < ntotal)
+       {
+               int                     rc;
+
+               rc = write(STDERR_FILENO, str + nwritten, ntotal - nwritten);
+
+               /* Just give up on error.  There isn't much else we can do. */
+               if (rc == -1)
+                       return;
+
+               nwritten += rc;
+       }
+}
+
+
 /*
  * is_log_level_output -- is elevel logically >= log_min_level?
  *

diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index cb8d5e919018b4faf0fa099be61b73c43f7dfee3..31fbdab842c0128807b897ed4686874e4488218b 100644 (file)
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -433,4 +433,10 @@ extern void set_syslog_parameters(const char *ident, int facility);
  */
 extern void write_stderr(const char *fmt,...) pg_attribute_printf(1, 2);
 
+/*
+ * Write a message to STDERR using only async-signal-safe functions.  This can
+ * be used to safely emit a message from a signal handler.
+ */
+extern void write_stderr_signal_safe(const char *fmt);
+
 #endif                                                 /* ELOG_H */