Make syslogger more robust against failures in opening CSV log files.

The previous coding figured it'd be good enough to postpone opening
the first CSV log file until we got a message we needed to write there.
This is unsafe, though, because if the open fails we end up in infinite
recursion trying to report the failure.  Instead make the CSV log file
management code look as nearly as possible like the longstanding logic
for the stderr log file.  In particular, open it immediately at postmaster
startup (if enabled), or when we get a SIGHUP in which we find that
log_destination has been changed to enable CSV logging.

It seems OK to fail if a postmaster-start-time open attempt fails, as
we've long done for the stderr log file.  But we can't die if we fail
to open a CSV log file during SIGHUP, so we're still left with a problem.
In that case, write any output meant for the CSV log file to the stderr
log file.  (This will also cover race-condition cases in which backends
send CSV log data before or after we have the CSV log file open.)

This patch also fixes an ancient oversight that, if CSV logging was
turned off during a SIGHUP, we never actually closed the last CSV
log file.

In passing, remember to reset whereToSendOutput = DestNone during syslogger
start, since (unlike all other postmaster children) it's forked before the
postmaster has done that.  This made for a platform-dependent difference
in error reporting behavior between the syslogger and other children:
except on Windows, it'd report problems to the original postmaster stderr
as well as the normal error log file(s).  It's barely possible that that
was intentional at some point; but it doesn't seem likely to be desirable
in production, and the platform dependency definitely isn't desirable.

Per report from Alexander Kukushkin.  It's been like this for a long time,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/CAFh8B==iLUD_gqC-dAENS0V+kVrCeGiKujtKqSQ7++S-caaChw@mail.gmail.com

src/backend/postmaster/syslogger.c

@@ -45,6 +45,7 @@
 #include "storage/ipc.h"
 #include "storage/latch.h"
 #include "storage/pg_shmem.h"
+#include "tcop/tcopprot.h"
 #include "utils/guc.h"
 #include "utils/ps_status.h"
 #include "utils/timestamp.h"
@@ -135,7 +136,6 @@ static void syslogger_parseArgs(int argc, char *argv[]);
 NON_EXEC_STATIC void SysLoggerMain(int argc, char *argv[]) pg_attribute_noreturn();
 static void process_pipe_input(char *logbuffer, int *bytes_in_logbuffer);
 static void flush_pipe_input(char *logbuffer, int *bytes_in_logbuffer);
-static void open_csvlogfile(void);
 static FILE *logfile_open(const char *filename, const char *mode,
 			 bool allow_errors);
 
@@ -272,11 +272,13 @@ SysLoggerMain(int argc, char *argv[])
 #endif							/* WIN32 */
 
 	/*
-	 * Remember active logfile's name.  We recompute this from the reference
+	 * Remember active logfiles' name(s).  We recompute 'em from the reference
 	 * time because passing down just the pg_time_t is a lot cheaper than
 	 * passing a whole file path in the EXEC_BACKEND case.
 	 */
 	last_file_name = logfile_getname(first_syslogger_file_time, NULL);
+	if (csvlogFile != NULL)
+		last_csv_file_name = logfile_getname(first_syslogger_file_time, ".csv");
 
 	/* remember active logfile parameters */
 	currentLogDir = pstrdup(Log_directory);
@@ -286,6 +288,13 @@ SysLoggerMain(int argc, char *argv[])
 	set_next_rotation_time();
 	update_metainfo_datafile();
 
+	/*
+	 * Reset whereToSendOutput, as the postmaster will do (but hasn't yet, at
+	 * the point where we forked).  This prevents duplicate output of messages
+	 * from syslogger itself.
+	 */
+	whereToSendOutput = DestNone;
+
 	/* main worker loop */
 	for (;;)
 	{
@@ -332,6 +341,14 @@ SysLoggerMain(int argc, char *argv[])
 				rotation_requested = true;
 			}
 
+			/*
+			 * Force a rotation if CSVLOG output was just turned on or off and
+			 * we need to open or close csvlogFile accordingly.
+			 */
+			if (((Log_destination & LOG_DESTINATION_CSVLOG) != 0) !=
+				(csvlogFile != NULL))
+				rotation_requested = true;
+
 			/*
 			 * If rotation time parameter changed, reset next rotation time,
 			 * but don't immediately force a rotation.
@@ -580,12 +597,27 @@ SysLogger_Start(void)
 	 * a time-based rotation.
 	 */
 	first_syslogger_file_time = time(NULL);
+
 	filename = logfile_getname(first_syslogger_file_time, NULL);
 
 	syslogFile = logfile_open(filename, "a", false);
 
 	pfree(filename);
 
+	/*
+	 * Likewise for the initial CSV log file, if that's enabled.  (Note that
+	 * we open syslogFile even when only CSV output is nominally enabled,
+	 * since some code paths will write to syslogFile anyway.)
+	 */
+	if (Log_destination & LOG_DESTINATION_CSVLOG)
+	{
+		filename = logfile_getname(first_syslogger_file_time, ".csv");
+
+		csvlogFile = logfile_open(filename, "a", false);
+
+		pfree(filename);
+	}
+
 #ifdef EXEC_BACKEND
 	switch ((sysloggerPid = syslogger_forkexec()))
 #else
@@ -675,9 +707,14 @@ SysLogger_Start(void)
 				redirection_done = true;
 			}
 
-			/* postmaster will never write the file; close it */
+			/* postmaster will never write the file(s); close 'em */
 			fclose(syslogFile);
 			syslogFile = NULL;
+			if (csvlogFile != NULL)
+			{
+				fclose(csvlogFile);
+				csvlogFile = NULL;
+			}
 			return (int) sysloggerPid;
 	}
 
@@ -699,6 +736,7 @@ syslogger_forkexec(void)
 	char	   *av[10];
 	int			ac = 0;
 	char		filenobuf[32];
+	char		csvfilenobuf[32];
 
 	av[ac++] = "postgres";
 	av[ac++] = "--forklog";
@@ -720,6 +758,21 @@ syslogger_forkexec(void)
 #endif							/* WIN32 */
 	av[ac++] = filenobuf;
 
+#ifndef WIN32
+	if (csvlogFile != NULL)
+		snprintf(csvfilenobuf, sizeof(csvfilenobuf), "%d",
+				 fileno(csvlogFile));
+	else
+		strcpy(csvfilenobuf, "-1");
+#else							/* WIN32 */
+	if (csvlogFile != NULL)
+		snprintf(csvfilenobuf, sizeof(csvfilenobuf), "%ld",
+				 (long) _get_osfhandle(_fileno(csvlogFile)));
+	else
+		strcpy(csvfilenobuf, "0");
+#endif							/* WIN32 */
+	av[ac++] = csvfilenobuf;
+
 	av[ac] = NULL;
 	Assert(ac < lengthof(av));
 
@@ -736,9 +789,16 @@ syslogger_parseArgs(int argc, char *argv[])
 {
 	int			fd;
 
-	Assert(argc == 4);
+	Assert(argc == 5);
 	argv += 3;
 
+	/*
+	 * Re-open the error output files that were opened by SysLogger_Start().
+	 *
+	 * We expect this will always succeed, which is too optimistic, but if it
+	 * fails there's not a lot we can do to report the problem anyway.  As
+	 * coded, we'll just crash on a null pointer dereference after failure...
+	 */
 #ifndef WIN32
 	fd = atoi(*argv++);
 	if (fd != -1)
@@ -746,6 +806,12 @@ syslogger_parseArgs(int argc, char *argv[])
 		syslogFile = fdopen(fd, "a");
 		setvbuf(syslogFile, NULL, PG_IOLBF, 0);
 	}
+	fd = atoi(*argv++);
+	if (fd != -1)
+	{
+		csvlogFile = fdopen(fd, "a");
+		setvbuf(csvlogFile, NULL, PG_IOLBF, 0);
+	}
 #else							/* WIN32 */
 	fd = atoi(*argv++);
 	if (fd != 0)
@@ -757,6 +823,16 @@ syslogger_parseArgs(int argc, char *argv[])
 			setvbuf(syslogFile, NULL, PG_IOLBF, 0);
 		}
 	}
+	fd = atoi(*argv++);
+	if (fd != 0)
+	{
+		fd = _open_osfhandle(fd, _O_APPEND | _O_TEXT);
+		if (fd > 0)
+		{
+			csvlogFile = fdopen(fd, "a");
+			setvbuf(csvlogFile, NULL, PG_IOLBF, 0);
+		}
+	}
 #endif							/* WIN32 */
 }
 #endif							/* EXEC_BACKEND */
@@ -998,13 +1074,29 @@ write_syslogger_file(const char *buffer, int count, int destination)
 	int			rc;
 	FILE	   *logfile;
 
-	if (destination == LOG_DESTINATION_CSVLOG && csvlogFile == NULL)
-		open_csvlogfile();
+	/*
+	 * If we're told to write to csvlogFile, but it's not open, dump the data
+	 * to syslogFile (which is always open) instead.  This can happen if CSV
+	 * output is enabled after postmaster start and we've been unable to open
+	 * csvlogFile.  There are also race conditions during a parameter change
+	 * whereby backends might send us CSV output before we open csvlogFile or
+	 * after we close it.  Writing CSV-formatted output to the regular log
+	 * file isn't great, but it beats dropping log output on the floor.
+	 *
+	 * Think not to improve this by trying to open csvlogFile on-the-fly.  Any
+	 * failure in that would lead to recursion.
+	 */
+	logfile = (destination == LOG_DESTINATION_CSVLOG &&
+			   csvlogFile != NULL) ? csvlogFile : syslogFile;
 
-	logfile = destination == LOG_DESTINATION_CSVLOG ? csvlogFile : syslogFile;
 	rc = fwrite(buffer, 1, count, logfile);
 
-	/* can't use ereport here because of possible recursion */
+	/*
+	 * Try to report any failure.  We mustn't use ereport because it would
+	 * just recurse right back here, but write_stderr is OK: it will write
+	 * either to the postmaster's original stderr, or to /dev/null, but never
+	 * to our input pipe which would result in a different sort of looping.
+	 */
 	if (rc != count)
 		write_stderr("could not write to log file: %s\n", strerror(errno));
 }
@@ -1087,31 +1179,6 @@ pipeThread(void *arg)
 }
 #endif							/* WIN32 */
 
-/*
- * Open the csv log file - we do this opportunistically, because
- * we don't know if CSV logging will be wanted.
- *
- * This is only used the first time we open the csv log in a given syslogger
- * process, not during rotations.  As with opening the main log file, we
- * always append in this situation.
- */
-static void
-open_csvlogfile(void)
-{
-	char	   *filename;
-
-	filename = logfile_getname(time(NULL), ".csv");
-
-	csvlogFile = logfile_open(filename, "a", false);
-
-	if (last_csv_file_name != NULL) /* probably shouldn't happen */
-		pfree(last_csv_file_name);
-
-	last_csv_file_name = filename;
-
-	update_metainfo_datafile();
-}
-
 /*
  * Open a new logfile with proper permissions and buffering options.
  *
@@ -1179,7 +1246,7 @@ logfile_rotate(bool time_based_rotation, int size_rotation_for)
 	else
 		fntime = time(NULL);
 	filename = logfile_getname(fntime, NULL);
-	if (csvlogFile != NULL)
+	if (Log_destination & LOG_DESTINATION_CSVLOG)
 		csvfilename = logfile_getname(fntime, ".csv");
 
 	/*
@@ -1231,10 +1298,16 @@ logfile_rotate(bool time_based_rotation, int size_rotation_for)
 		filename = NULL;
 	}
 
-	/* Same as above, but for csv file. */
-
-	if (csvlogFile != NULL &&
-		(time_based_rotation || (size_rotation_for & LOG_DESTINATION_CSVLOG)))
+	/*
+	 * Same as above, but for csv file.  Note that if LOG_DESTINATION_CSVLOG
+	 * was just turned on, we might have to open csvlogFile here though it was
+	 * not open before.  In such a case we'll append not overwrite (since
+	 * last_csv_file_name will be NULL); that is consistent with the normal
+	 * rules since it's not a time-based rotation.
+	 */
+	if ((Log_destination & LOG_DESTINATION_CSVLOG) &&
+		(csvlogFile == NULL ||
+		 time_based_rotation || (size_rotation_for & LOG_DESTINATION_CSVLOG)))
 	{
 		if (Log_truncate_on_rotation && time_based_rotation &&
 			last_csv_file_name != NULL &&
@@ -1265,6 +1338,7 @@ logfile_rotate(bool time_based_rotation, int size_rotation_for)
 			return;
 		}
 
+		if (csvlogFile != NULL)
 			fclose(csvlogFile);
 		csvlogFile = fh;
 
@@ -1274,6 +1348,16 @@ logfile_rotate(bool time_based_rotation, int size_rotation_for)
 		last_csv_file_name = csvfilename;
 		csvfilename = NULL;
 	}
+	else if (!(Log_destination & LOG_DESTINATION_CSVLOG) &&
+			 csvlogFile != NULL)
+	{
+		/* CSVLOG was just turned off, so close the old file */
+		fclose(csvlogFile);
+		csvlogFile = NULL;
+		if (last_csv_file_name != NULL)
+			pfree(last_csv_file_name);
+		last_csv_file_name = NULL;
+	}
 
 	if (filename)
 		pfree(filename);