Use _exit(2) for SIGQUIT during ProcessStartupPacket, too.

Bring the signal handling for startup-packet collection into line
with the policy established in commits bedadc73 and 8e19a826,
namely don't risk running atexit callbacks when handling SIGQUIT.

Ideally, we'd not do so for SIGTERM or timeout interrupts either,
but that change seems a bit too risky for the back branches.
For now, just improve the comments in this area to describe the risk.

Also relocate where BackendInitialize re-disables these interrupts,
to minimize the code span where they're active.  This doesn't buy
a whole lot of safety, but it can't hurt.

In passing, rename startup_die() to remove confusion about whether
it is for the startup process.

Like the previous commits, back-patch to all supported branches.

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

src/backend/postmaster/postmaster.c

@@ -112,6 +112,7 @@
 #include "postmaster/autovacuum.h"
 #include "postmaster/bgworker_internals.h"
 #include "postmaster/fork_process.h"
+#include "postmaster/interrupt.h"
 #include "postmaster/pgarch.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
@@ -405,7 +406,7 @@ static void SIGHUP_handler(SIGNAL_ARGS);
 static void pmdie(SIGNAL_ARGS);
 static void reaper(SIGNAL_ARGS);
 static void sigusr1_handler(SIGNAL_ARGS);
-static void startup_die(SIGNAL_ARGS);
+static void process_startup_packet_die(SIGNAL_ARGS);
 static void dummy_handler(SIGNAL_ARGS);
 static void StartupPacketTimeoutHandler(void);
 static void CleanupBackend(int pid, int exitstatus);
@@ -4340,22 +4341,30 @@ BackendInitialize(Port *port)
 	whereToSendOutput = DestRemote; /* now safe to ereport to client */
 
 	/*
-	 * We arrange for a simple exit(1) if we receive SIGTERM or SIGQUIT or
-	 * timeout while trying to collect the startup packet.  Otherwise the
-	 * postmaster cannot shutdown the database FAST or IMMED cleanly if a
-	 * buggy client fails to send the packet promptly.  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.
-	 */
-	pqsignal(SIGTERM, startup_die);
-	pqsignal(SIGQUIT, startup_die);
+	 * 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.
+	 *
+	 * 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.
+	 */
+	pqsignal(SIGTERM, process_startup_packet_die);
+	pqsignal(SIGQUIT, SignalHandlerForCrashExit);
 	InitializeTimeouts();		/* establishes SIGALRM handler */
 	PG_SETMASK(&StartupBlockSig);
 
@@ -4411,8 +4420,8 @@ BackendInitialize(Port *port)
 		port->remote_hostname = strdup(remote_host);
 
 	/*
-	 * 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
+	 * 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
 	 * indefinitely.  PreAuthDelay and any DNS interactions above don't count
 	 * against the time limit.
 	 *
@@ -4434,6 +4443,12 @@ BackendInitialize(Port *port)
 	 */
 	status = ProcessStartupPacket(port, false, false);
 
+	/*
+	 * Disable the timeout, and prevent SIGTERM/SIGQUIT again.
+	 */
+	disable_timeout(STARTUP_PACKET_TIMEOUT, false);
+	PG_SETMASK(&BlockSig);
+
 	/*
 	 * Stop here if it was bad or a cancel packet.  ProcessStartupPacket
 	 * already did any appropriate error reporting.
@@ -4459,12 +4474,6 @@ BackendInitialize(Port *port)
 	pfree(ps_data.data);
 
 	set_ps_display("initializing");
-
-	/*
-	 * Disable the timeout, and prevent SIGTERM/SIGQUIT again.
-	 */
-	disable_timeout(STARTUP_PACKET_TIMEOUT, false);
-	PG_SETMASK(&BlockSig);
 }
 
 
@@ -5359,16 +5368,22 @@ sigusr1_handler(SIGNAL_ARGS)
 }
 
 /*
- * SIGTERM or SIGQUIT while processing startup packet.
+ * SIGTERM while processing startup packet.
  * Clean up and exit(1).
  *
- * XXX: possible future improvement: try to send a message indicating
- * why we are disconnecting.  Problem is to be sure we don't block while
- * doing so, nor mess up SSL initialization.  In practice, if the client
- * has wedged here, it probably couldn't do anything with the message anyway.
+ * 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.
+ *
+ * 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.
  */
 static void
-startup_die(SIGNAL_ARGS)
+process_startup_packet_die(SIGNAL_ARGS)
 {
 	proc_exit(1);
 }
@@ -5389,7 +5404,11 @@ dummy_handler(SIGNAL_ARGS)
 
 /*
  * Timeout while processing startup packet.
- * As for startup_die(), we clean up and exit(1).
+ * 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.
  */
 static void
 StartupPacketTimeoutHandler(void)