Fix syslogger NULL-pointer-dereference in EXEC_BACKEND

Commit 0c8e082fba changed the time at which MyBackendType is assigned,
breaking a careful choreography in syslogger to decide when to write
messages to its own log files.  Fix by flipping a boolean at the
(approximate) location where previously MyBackendType was set, instead
of depending on MyBackendType directly.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/ahP-JT4ZRPyobnLb@paquier.xyz

src/backend/postmaster/syslogger.c

@@ -76,6 +76,12 @@ char	   *Log_filename = NULL;
 bool		Log_truncate_on_rotation = false;
 int			Log_file_mode = S_IRUSR | S_IWUSR;
 
+/*
+ * Indicates to be running in the syslogger process, and that the logging
+ * file descriptor(s) have been set up.
+ */
+bool		syslogger_setup_done = false;
+
 /*
  * Private state
  */
@@ -175,6 +181,17 @@ SysLoggerMain(const void *startup_data, size_t startup_data_len)
 	pg_time_t	now;
 	WaitEventSet *wes;
 
+#ifndef EXEC_BACKEND
+
+	/*
+	 * In !EXEC_BACKEND, syslogger is immediately ready to take over: the
+	 * output files were already opened by postmaster before forking.  For the
+	 * other case we must wait until the file descriptors have been restored,
+	 * below.
+	 */
+	syslogger_setup_done = true;
+#endif
+
 	/*
 	 * Re-open the error output files that were opened by SysLogger_Start().
 	 *
@@ -190,6 +207,8 @@ SysLoggerMain(const void *startup_data, size_t startup_data_len)
 		syslogFile = syslogger_fdopen(slsdata->syslogFile);
 		csvlogFile = syslogger_fdopen(slsdata->csvlogFile);
 		jsonlogFile = syslogger_fdopen(slsdata->jsonlogFile);
+
+		syslogger_setup_done = true;
 	}
 #else
 	Assert(startup_data_len == 0);

src/backend/utils/error/csvlog.c

@@ -253,7 +253,7 @@ write_csvlog(ErrorData *edata)
 	appendStringInfoChar(&buf, '\n');
 
 	/* If in the syslogger process, try to write messages direct to file */
-	if (MyBackendType == B_LOGGER)
+	if (syslogger_setup_done)
 		write_syslogger_file(buf.data, buf.len, LOG_DESTINATION_CSVLOG);
 	else
 		write_pipe_chunks(buf.data, buf.len, LOG_DESTINATION_CSVLOG);

src/backend/utils/error/elog.c

@@ -3831,7 +3831,7 @@ send_message_to_server_log(ErrorData *edata)
 		 * pipe).  If this is not possible, fallback to an entry written to
 		 * stderr.
 		 */
-		if (redirection_done || MyBackendType == B_LOGGER)
+		if (redirection_done || syslogger_setup_done)
 			write_csvlog(edata);
 		else
 			fallback_to_stderr = true;
@@ -3845,7 +3845,7 @@ send_message_to_server_log(ErrorData *edata)
 		 * pipe).  If this is not possible, fallback to an entry written to
 		 * stderr.
 		 */
-		if (redirection_done || MyBackendType == B_LOGGER)
+		if (redirection_done || syslogger_setup_done)
 		{
 			write_jsonlog(edata);
 		}
@@ -3885,7 +3885,7 @@ send_message_to_server_log(ErrorData *edata)
 	}
 
 	/* If in the syslogger process, try to write messages direct to file */
-	if (MyBackendType == B_LOGGER)
+	if (syslogger_setup_done)
 		write_syslogger_file(buf.data, buf.len, LOG_DESTINATION_STDERR);
 
 	/* No more need of the message formatted for stderr */

src/backend/utils/error/jsonlog.c

@@ -292,7 +292,7 @@ write_jsonlog(ErrorData *edata)
 	appendStringInfoChar(&buf, '\n');
 
 	/* If in the syslogger process, try to write messages direct to file */
-	if (MyBackendType == B_LOGGER)
+	if (syslogger_setup_done)
 		write_syslogger_file(buf.data, buf.len, LOG_DESTINATION_JSONLOG);
 	else
 		write_pipe_chunks(buf.data, buf.len, LOG_DESTINATION_JSONLOG);

src/include/postmaster/syslogger.h

@@ -85,6 +85,7 @@ extern PGDLLIMPORT int syslogPipe[2];
 extern PGDLLIMPORT HANDLE syslogPipe[2];
 #endif
 
+extern bool syslogger_setup_done;
 
 extern int	SysLogger_Start(int child_slot);