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

src/backend/postmaster/startup.c

@@ -19,6 +19,8 @@
  */
 #include "postgres.h"
 
+#include <unistd.h>
+
 #include "access/xlog.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
@@ -102,7 +104,20 @@ 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();

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);
 

src/backend/storage/lmgr/proc.c

@@ -826,6 +826,10 @@ 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();
 
@@ -955,6 +959,10 @@ 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);

src/backend/utils/error/elog.c

@@ -3590,6 +3590,34 @@ 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;
+	}
+}
+
+
 /*
  * Adjust the level of a recovery-related message per trace_recovery_messages.
  *

src/include/utils/elog.h

@@ -453,4 +453,10 @@ 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 */