Centralize logic for restoring errno in signal handlers.

Presently, we rely on each individual signal handler to save the
initial value of errno and then restore it before returning if
needed.  This is easily forgotten and, if missed, often goes
undetected for a long time.

In commit 3b00fdba9f, we introduced a wrapper signal handler
function that checks whether MyProcPid matches getpid().  This
commit moves the aforementioned errno restoration code from the
individual signal handlers to the new wrapper handler so that we no
longer need to worry about missing it.

Reviewed-by: Andres Freund, Noah Misch
Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13

doc/src/sgml/sources.sgml

@@ -1007,18 +1007,10 @@ MemoryContextSwitchTo(MemoryContext context)
 static void
 handle_sighup(SIGNAL_ARGS)
 {
-    int         save_errno = errno;
-
     got_SIGHUP = true;
     SetLatch(MyLatch);
-
-    errno = save_errno;
 }
 </programlisting>
-     <varname>errno</varname> is saved and restored because
-     <function>SetLatch()</function> might change it. If that were not done
-     interrupted code that's currently inspecting <varname>errno</varname> might see the wrong
-     value.
     </para>
    </simplesect>
 

src/backend/postmaster/autovacuum.c

@@ -1410,12 +1410,8 @@ AutoVacWorkerFailed(void)
 static void
 avl_sigusr2_handler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	got_SIGUSR2 = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 

src/backend/postmaster/checkpointer.c

@@ -852,15 +852,11 @@ IsCheckpointOnSchedule(double progress)
 static void
 ReqCheckpointHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	/*
 	 * The signaling process should have set ckpt_flags nonzero, so all we
 	 * need do is ensure that our main loop gets kicked out of any wait.
 	 */
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 

src/backend/postmaster/interrupt.c

@@ -60,12 +60,8 @@ HandleMainLoopInterrupts(void)
 void
 SignalHandlerForConfigReload(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	ConfigReloadPending = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*
@@ -108,10 +104,6 @@ SignalHandlerForCrashExit(SIGNAL_ARGS)
 void
 SignalHandlerForShutdownRequest(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	ShutdownRequestPending = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }

src/backend/postmaster/pgarch.c

@@ -283,13 +283,9 @@ PgArchWakeup(void)
 static void
 pgarch_waken_stop(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	/* set flag to do a final cycle and shut down afterwards */
 	ready_to_stop = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*

src/backend/postmaster/postmaster.c

@@ -2612,12 +2612,8 @@ InitProcessGlobals(void)
 static void
 handle_pm_pmsignal_signal(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	pending_pm_pmsignal = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*
@@ -2626,12 +2622,8 @@ handle_pm_pmsignal_signal(SIGNAL_ARGS)
 static void
 handle_pm_reload_request_signal(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	pending_pm_reload_request = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*
@@ -2711,8 +2703,6 @@ process_pm_reload_request(void)
 static void
 handle_pm_shutdown_request_signal(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	switch (postgres_signal_arg)
 	{
 		case SIGTERM:
@@ -2729,8 +2719,6 @@ handle_pm_shutdown_request_signal(SIGNAL_ARGS)
 			break;
 	}
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*
@@ -2890,12 +2878,8 @@ process_pm_shutdown_request(void)
 static void
 handle_pm_child_exit_signal(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	pending_pm_child_exit = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /*

src/backend/postmaster/startup.c

@@ -95,32 +95,22 @@ static void StartupProcExit(int code, Datum arg);
 static void
 StartupProcTriggerHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	promote_signaled = true;
 	WakeupRecovery();
-
-	errno = save_errno;
 }
 
 /* SIGHUP: set flag to re-read config file at next convenient time */
 static void
 StartupProcSigHupHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	got_SIGHUP = true;
 	WakeupRecovery();
-
-	errno = save_errno;
 }
 
 /* SIGTERM: set flag to abort redo and exit */
 static void
 StartupProcShutdownHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	if (in_restore_command)
 	{
 		/*
@@ -139,8 +129,6 @@ StartupProcShutdownHandler(SIGNAL_ARGS)
 	else
 		shutdown_requested = true;
 	WakeupRecovery();
-
-	errno = save_errno;
 }
 
 /*

src/backend/postmaster/syslogger.c

@@ -1642,10 +1642,6 @@ RemoveLogrotateSignalFiles(void)
 static void
 sigUsr1Handler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	rotation_requested = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }

src/backend/replication/walsender.c

@@ -3476,12 +3476,8 @@ HandleWalSndInitStopping(void)
 static void
 WalSndLastCycleHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	got_SIGUSR2 = true;
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /* Set up signal handlers */

src/backend/storage/ipc/latch.c

@@ -2243,12 +2243,8 @@ GetNumRegisteredWaitEvents(WaitEventSet *set)
 static void
 latch_sigurg_handler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	if (waiting)
 		sendSelfPipeByte();
-
-	errno = save_errno;
 }
 
 /* Send one byte to the self-pipe, to wake up WaitLatch */

src/backend/storage/ipc/procsignal.c

@@ -638,8 +638,6 @@ CheckProcSignal(ProcSignalReason reason)
 void
 procsignal_sigusr1_handler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	if (CheckProcSignal(PROCSIG_CATCHUP_INTERRUPT))
 		HandleCatchupInterrupt();
 
@@ -683,6 +681,4 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 		HandleRecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
 
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }

src/backend/tcop/postgres.c

@@ -2970,8 +2970,6 @@ quickdie(SIGNAL_ARGS)
 void
 die(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	/* Don't joggle the elbow of proc_exit */
 	if (!proc_exit_inprogress)
 	{
@@ -2993,8 +2991,6 @@ die(SIGNAL_ARGS)
 	 */
 	if (DoingCommandRead && whereToSendOutput != DestRemote)
 		ProcessInterrupts();
-
-	errno = save_errno;
 }
 
 /*
@@ -3004,8 +3000,6 @@ die(SIGNAL_ARGS)
 void
 StatementCancelHandler(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	/*
 	 * Don't joggle the elbow of proc_exit
 	 */
@@ -3017,8 +3011,6 @@ StatementCancelHandler(SIGNAL_ARGS)
 
 	/* If we're still here, waken anything waiting on the process latch */
 	SetLatch(MyLatch);
-
-	errno = save_errno;
 }
 
 /* signal handler for floating point exception */

src/backend/utils/misc/timeout.c

@@ -363,8 +363,6 @@ schedule_alarm(TimestampTz now)
 static void
 handle_sig_alarm(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
-
 	/*
 	 * Bump the holdoff counter, to make sure nothing we call will process
 	 * interrupts directly. No timeout handler should do that, but these
@@ -452,8 +450,6 @@ handle_sig_alarm(SIGNAL_ARGS)
 	}
 
 	RESUME_INTERRUPTS();
-
-	errno = save_errno;
 }
 
 

src/fe_utils/cancel.c

@@ -152,7 +152,6 @@ ResetCancelConn(void)
 static void
 handle_sigint(SIGNAL_ARGS)
 {
-	int			save_errno = errno;
 	char		errbuf[256];
 
 	CancelRequested = true;
@@ -173,8 +172,6 @@ handle_sigint(SIGNAL_ARGS)
 			write_stderr(errbuf);
 		}
 	}
-
-	errno = save_errno;			/* just in case the write changed it */
 }
 
 /*

src/port/pqsignal.c

@@ -80,10 +80,14 @@ static volatile pqsigfunc pqsignal_handlers[PG_NSIG];
  * processes do not modify shared memory, which is often detrimental.  If the
  * check succeeds, the function originally provided to pqsignal() is called.
  * Otherwise, the default signal handler is installed and then called.
+ *
+ * This wrapper also handles restoring the value of errno.
  */
 static void
 wrapper_handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 #ifndef FRONTEND
 
 	/*
@@ -102,6 +106,8 @@ wrapper_handler(SIGNAL_ARGS)
 #endif
 
 	(*pqsignal_handlers[postgres_signal_arg]) (postgres_signal_arg);
+
+	errno = save_errno;
 }
 
 /*