Suppress log spam from multiple reports of SIGQUIT shutdown.

When the postmaster sends SIGQUIT to its children, there's no real
need for all the children to log that fact; the postmaster already
made a log entry about it, so adding perhaps dozens or hundreds of
child-process log entries adds nothing of value.  So, let's introduce
a new ereport level to specify "WARNING, but never send to log" and
use that for these messages.

Such a change wouldn't have been desirable before commit 7e784d1d,
because if someone manually SIGQUIT's a backend, we *do* want to log
that.  But now we can tell the difference between a signal that was
issued by the postmaster and one that was not with reasonable
certainty.

While we're here, also clear error_context_stack before ereport'ing,
to prevent error callbacks from being invoked in the signal-handler
context.  This should reduce the odds of getting hung up while trying
to notify the client.

Per a suggestion from Andres Freund.

Discussion: https://postgr.es/m/20201225230331.hru3u6obyy6j53tk@alap3.anarazel.de

src/backend/tcop/postgres.c

@@ -2789,6 +2789,18 @@ quickdie(SIGNAL_ARGS)
 	 * wrong, so there's not much to lose.  Assuming the postmaster is still
 	 * running, it will SIGKILL us soon if we get stuck for some reason.
 	 *
+	 * One thing we can do to make this a tad safer is to clear the error
+	 * context stack, so that context callbacks are not called.  That's a lot
+	 * less code that could be reached here, and the context info is unlikely
+	 * to be very relevant to a SIGQUIT report anyway.
+	 */
+	error_context_stack = NULL;
+
+	/*
+	 * When responding to a postmaster-issued signal, we send the message only
+	 * to the client; sending to the server log just creates log spam, plus
+	 * it's more code that we need to hope will work in a signal handler.
+	 *
 	 * Ideally these should be ereport(FATAL), but then we'd not get control
 	 * back to force the correct type of process exit.
 	 */
@@ -2802,7 +2814,7 @@ quickdie(SIGNAL_ARGS)
 			break;
 		case PMQUIT_FOR_CRASH:
 			/* A crash-and-restart cycle is in progress */
-			ereport(WARNING,
+			ereport(WARNING_CLIENT_ONLY,
 					(errcode(ERRCODE_CRASH_SHUTDOWN),
 					 errmsg("terminating connection because of crash of another server process"),
 					 errdetail("The postmaster has commanded this server process to roll back"
@@ -2814,7 +2826,7 @@ quickdie(SIGNAL_ARGS)
 			break;
 		case PMQUIT_FOR_STOP:
 			/* Immediate-mode stop */
-			ereport(WARNING,
+			ereport(WARNING_CLIENT_ONLY,
 					(errcode(ERRCODE_ADMIN_SHUTDOWN),
 					 errmsg("terminating connection due to immediate shutdown command")));
 			break;

src/backend/utils/error/elog.c

@@ -202,6 +202,11 @@ is_log_level_output(int elevel, int log_min_level)
 		if (log_min_level == LOG || log_min_level <= ERROR)
 			return true;
 	}
+	else if (elevel == WARNING_CLIENT_ONLY)
+	{
+		/* never sent to log, regardless of log_min_level */
+		return false;
+	}
 	else if (log_min_level == LOG)
 	{
 		/* elevel != LOG */
@@ -453,7 +458,7 @@ errstart(int elevel, const char *domain)
 	/* Select default errcode based on elevel */
 	if (elevel >= ERROR)
 		edata->sqlerrcode = ERRCODE_INTERNAL_ERROR;
-	else if (elevel == WARNING)
+	else if (elevel >= WARNING)
 		edata->sqlerrcode = ERRCODE_WARNING;
 	else
 		edata->sqlerrcode = ERRCODE_SUCCESSFUL_COMPLETION;
@@ -2152,6 +2157,7 @@ write_eventlog(int level, const char *line, int len)
 			eventlevel = EVENTLOG_INFORMATION_TYPE;
 			break;
 		case WARNING:
+		case WARNING_CLIENT_ONLY:
 			eventlevel = EVENTLOG_WARNING_TYPE;
 			break;
 		case ERROR:
@@ -3109,6 +3115,7 @@ send_message_to_server_log(ErrorData *edata)
 				break;
 			case NOTICE:
 			case WARNING:
+			case WARNING_CLIENT_ONLY:
 				syslog_level = LOG_NOTICE;
 				break;
 			case ERROR:
@@ -3484,6 +3491,7 @@ error_severity(int elevel)
 			prefix = gettext_noop("NOTICE");
 			break;
 		case WARNING:
+		case WARNING_CLIENT_ONLY:
 			prefix = gettext_noop("WARNING");
 			break;
 		case ERROR:

src/include/utils/elog.h

@@ -40,19 +40,19 @@
 #define WARNING		19			/* Warnings.  NOTICE is for expected messages
 								 * like implicit sequence creation by SERIAL.
 								 * WARNING is for unexpected messages. */
-#define ERROR		20			/* user error - abort transaction; return to
+#define WARNING_CLIENT_ONLY	20	/* Warnings to be sent to client as usual, but
+								 * never to the server log. */
+#define ERROR		21			/* user error - abort transaction; return to
 								 * known state */
 /* Save ERROR value in PGERROR so it can be restored when Win32 includes
  * modify it.  We have to use a constant rather than ERROR because macros
  * are expanded only when referenced outside macros.
  */
 #ifdef WIN32
-#define PGERROR		20
+#define PGERROR		21
 #endif
-#define FATAL		21			/* fatal error - abort process */
-#define PANIC		22			/* take down the other backends with me */
-
- /* #define DEBUG DEBUG1 */	/* Backward compatibility with pre-7.3 */
+#define FATAL		22			/* fatal error - abort process */
+#define PANIC		23			/* take down the other backends with me */
 
 
 /* macros for representing SQLSTATE strings compactly */