Don't run atexit callbacks during signal exits from ProcessStartupPacket.

Although 58c6feccf fixed the case for SIGQUIT, we were still calling
proc_exit() from signal handlers for SIGTERM and timeout failures in
ProcessStartupPacket.  Fortunately, at the point where that code runs,
we haven't yet connected to shared memory in any meaningful way, so
there is nothing we need to undo in shared memory.  This means it
should be safe to use _exit(1) here, ie, not run any atexit handlers
but also inform the postmaster that it's not a crash exit.

To make sure nobody breaks the "nothing to undo" expectation, add
a cross-check that no on-shmem-exit or before-shmem-exit handlers
have been registered yet when we finish using these signal handlers.

This change is simple enough that maybe it could be back-patched,
but I won't risk that right now.

Discussion: https://postgr.es/m/1850884.1599601164@sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -4298,6 +4298,8 @@ report_fork_failure_to_client(Port *port, int errnum)
  * returns: nothing.  Will not return at all if there's any failure.
  *
  * Note: this code does not depend on having any access to shared memory.
+ * Indeed, our approach to SIGTERM/timeout handling *requires* that
+ * shared memory not have been touched yet; see comments within.
  * In the EXEC_BACKEND case, we are physically attached to shared memory
  * but have not yet set up most of our local pointers to shmem structures.
  */
@@ -4341,27 +4343,14 @@ BackendInitialize(Port *port)
 	whereToSendOutput = DestRemote; /* now safe to ereport to client */
 
 	/*
-	 * We arrange to do proc_exit(1) if we receive SIGTERM or timeout while
-	 * trying to collect the startup packet; while SIGQUIT results in
-	 * _exit(2).  Otherwise the postmaster cannot shutdown the database FAST
-	 * or IMMED cleanly if a buggy client fails to send the packet promptly.
+	 * We arrange to do _exit(1) if we receive SIGTERM or timeout while trying
+	 * to collect the startup packet; while SIGQUIT results in _exit(2).
+	 * Otherwise the postmaster cannot shutdown the database FAST or IMMED
+	 * cleanly if a buggy client fails to send the packet promptly.
 	 *
-	 * XXX this is pretty dangerous; signal handlers should not call anything
-	 * as complex as proc_exit() directly.  We minimize the hazard by not
-	 * keeping these handlers active for longer than we must.  However, it
-	 * seems necessary to be able to escape out of DNS lookups as well as the
-	 * startup packet reception proper, so we can't narrow the scope further
-	 * than is done here.
-	 *
-	 * XXX it follows that the remainder of this function must tolerate losing
-	 * control at any instant.  Likewise, any pg_on_exit_callback registered
-	 * before or during this function must be prepared to execute at any
-	 * instant between here and the end of this function.  Furthermore,
-	 * affected callbacks execute partially or not at all when a second
-	 * exit-inducing signal arrives after proc_exit_prepare() decrements
-	 * on_proc_exit_index.  (Thanks to that mechanic, callbacks need not
-	 * anticipate more than one call.)  This is fragile; it ought to instead
-	 * follow the norm of handling interrupts at selected, safe opportunities.
+	 * Exiting with _exit(1) is only possible because we have not yet touched
+	 * shared memory; therefore no outside-the-process state needs to get
+	 * cleaned up.
 	 */
 	pqsignal(SIGTERM, process_startup_packet_die);
 	pqsignal(SIGQUIT, SignalHandlerForCrashExit);
@@ -4420,8 +4409,8 @@ BackendInitialize(Port *port)
 		port->remote_hostname = strdup(remote_host);
 
 	/*
-	 * Ready to begin client interaction.  We will give up and proc_exit(1)
-	 * after a time delay, so that a broken client can't hog a connection
+	 * Ready to begin client interaction.  We will give up and _exit(1) after
+	 * a time delay, so that a broken client can't hog a connection
 	 * indefinitely.  PreAuthDelay and any DNS interactions above don't count
 	 * against the time limit.
 	 *
@@ -4449,6 +4438,17 @@ BackendInitialize(Port *port)
 	disable_timeout(STARTUP_PACKET_TIMEOUT, false);
 	PG_SETMASK(&BlockSig);
 
+	/*
+	 * As a safety check that nothing in startup has yet performed
+	 * shared-memory modifications that would need to be undone if we had
+	 * exited through SIGTERM or timeout above, check that no on_shmem_exit
+	 * handlers have been registered yet.  (This isn't terribly bulletproof,
+	 * since someone might misuse an on_proc_exit handler for shmem cleanup,
+	 * but it's a cheap and helpful check.  We cannot disallow on_proc_exit
+	 * handlers unfortunately, since pq_init() already registered one.)
+	 */
+	check_on_shmem_exit_lists_are_empty();
+
 	/*
 	 * Stop here if it was bad or a cancel packet.  ProcessStartupPacket
 	 * already did any appropriate error reporting.
@@ -5369,23 +5369,21 @@ sigusr1_handler(SIGNAL_ARGS)
 
 /*
  * SIGTERM while processing startup packet.
- * Clean up and exit(1).
  *
- * Running proc_exit() from a signal handler is pretty unsafe, since we
- * can't know what code we've interrupted.  But the alternative of using
- * _exit(2) is also unpalatable, since it'd mean that a "fast shutdown"
- * would cause a database crash cycle (forcing WAL replay at restart)
- * if any sessions are in authentication.  So we live with it for now.
+ * Running proc_exit() from a signal handler would be quite unsafe.
+ * However, since we have not yet touched shared memory, we can just
+ * pull the plug and exit without running any atexit handlers.
  *
- * One might be tempted to try to send a message indicating why we are
- * disconnecting.  However, that would make this even more unsafe.  Also,
- * it seems undesirable to provide clues about the database's state to
- * a client that has not yet completed authentication.
+ * One might be tempted to try to send a message, or log one, indicating
+ * why we are disconnecting.  However, that would be quite unsafe in itself.
+ * Also, it seems undesirable to provide clues about the database's state
+ * to a client that has not yet completed authentication, or even sent us
+ * a startup packet.
  */
 static void
 process_startup_packet_die(SIGNAL_ARGS)
 {
-	proc_exit(1);
+	_exit(1);
 }
 
 /*
@@ -5404,16 +5402,12 @@ dummy_handler(SIGNAL_ARGS)
 
 /*
  * Timeout while processing startup packet.
- * As for process_startup_packet_die(), we clean up and exit(1).
- *
- * This is theoretically just as hazardous as in process_startup_packet_die(),
- * although in practice we're almost certainly waiting for client input,
- * which greatly reduces the risk.
+ * As for process_startup_packet_die(), we exit via _exit(1).
  */
 static void
 StartupPacketTimeoutHandler(void)
 {
-	proc_exit(1);
+	_exit(1);
 }
 
 

src/backend/storage/ipc/ipc.c

@@ -416,3 +416,20 @@ on_exit_reset(void)
 	on_proc_exit_index = 0;
 	reset_on_dsm_detach();
 }
+
+/* ----------------------------------------------------------------
+ *		check_on_shmem_exit_lists_are_empty
+ *
+ *		Debugging check that no shmem cleanup handlers have been registered
+ *		prematurely in the current process.
+ * ----------------------------------------------------------------
+ */
+void
+check_on_shmem_exit_lists_are_empty(void)
+{
+	if (before_shmem_exit_index)
+		elog(FATAL, "before_shmem_exit has been called prematurely");
+	if (on_shmem_exit_index)
+		elog(FATAL, "on_shmem_exit has been called prematurely");
+	/* Checking DSM detach state seems unnecessary given the above */
+}

src/include/storage/ipc.h

@@ -72,6 +72,7 @@ extern void on_shmem_exit(pg_on_exit_callback function, Datum arg);
 extern void before_shmem_exit(pg_on_exit_callback function, Datum arg);
 extern void cancel_before_shmem_exit(pg_on_exit_callback function, Datum arg);
 extern void on_exit_reset(void);
+extern void check_on_shmem_exit_lists_are_empty(void);
 
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;