Add pg_current_logfile() function.

The syslogger will write out the current stderr and csvlog names, if
it's running and there are any, to a new file in the data directory
called "current_logfiles".  We take care to remove this file when it
might no longer be valid (but not at shutdown).  The function
pg_current_logfile() can be used to read the entries in the file.

Gilles Darold, reviewed and modified by Karl O.  Pinc, Michael
Paquier, and me.  Further review by Álvaro Herrera and Christoph Berg.

1 files changed, 79 insertions, 0 deletions

diff --git a/src/backend/postmaster/syslogger.c b/src/backend/postmaster/syslogger.c
index 13a03014ebb..aaefdaebad9 100644
--- a/src/backend/postmaster/syslogger.c
+++ b/src/backend/postmaster/syslogger.c
@@ -146,6 +146,7 @@ static char *logfile_getname(pg_time_t timestamp, const char *suffix);
 static void set_next_rotation_time(void);
 static void sigHupHandler(SIGNAL_ARGS);
 static void sigUsr1Handler(SIGNAL_ARGS);
+static void update_metainfo_datafile(void);
 
 
 /*
@@ -282,6 +283,7 @@ SysLoggerMain(int argc, char *argv[])
 	currentLogRotationAge = Log_RotationAge;
 	/* set next planned rotation time */
 	set_next_rotation_time();
+	update_metainfo_datafile();
 
 	/* main worker loop */
 	for (;;)
@@ -348,6 +350,13 @@ SysLoggerMain(int argc, char *argv[])
 				rotation_disabled = false;
 				rotation_requested = true;
 			}
+
+			/*
+			 * Force rewriting last log filename when reloading configuration.
+			 * Even if rotation_requested is false, log_destination may have
+			 * been changed and we don't want to wait the next file rotation.
+			 */
+			update_metainfo_datafile();
 		}
 
 		if (Log_RotationAge > 0 && !rotation_disabled)
@@ -1098,6 +1107,8 @@ open_csvlogfile(void)
 		pfree(last_csv_file_name);
 
 	last_csv_file_name = filename;
+
+	update_metainfo_datafile();
 }
 
 /*
@@ -1268,6 +1279,8 @@ logfile_rotate(bool time_based_rotation, int size_rotation_for)
 	if (csvfilename)
 		pfree(csvfilename);
 
+	update_metainfo_datafile();
+
 	set_next_rotation_time();
 }
 
@@ -1337,6 +1350,72 @@ set_next_rotation_time(void)
 	next_rotation_time = now;
 }
 
+/*
+ * Store the name of the file(s) where the log collector, when enabled, writes
+ * log messages.  Useful for finding the name(s) of the current log file(s)
+ * when there is time-based logfile rotation.  Filenames are stored in a
+ * temporary file and which is renamed into the final destination for
+ * atomicity.
+ */
+static void
+update_metainfo_datafile(void)
+{
+	FILE    *fh;
+
+	if (!(Log_destination & LOG_DESTINATION_STDERR) &&
+		!(Log_destination & LOG_DESTINATION_CSVLOG))
+	{
+		if (unlink(LOG_METAINFO_DATAFILE) < 0 && errno != ENOENT)
+			ereport(LOG,
+					(errcode_for_file_access(),
+					 errmsg("could not remove file \"%s\": %m",
+						LOG_METAINFO_DATAFILE)));
+		return;
+	}
+
+	if ((fh = logfile_open(LOG_METAINFO_DATAFILE_TMP, "w", true)) == NULL)
+	{
+		ereport(LOG,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m",
+					LOG_METAINFO_DATAFILE_TMP)));
+		return;
+	}
+
+	if (last_file_name && (Log_destination & LOG_DESTINATION_STDERR))
+	{
+		if (fprintf(fh, "stderr %s\n", last_file_name) < 0)
+		{
+			ereport(LOG,
+					(errcode_for_file_access(),
+					errmsg("could not write file \"%s\": %m",
+							LOG_METAINFO_DATAFILE_TMP)));
+			fclose(fh);
+			return;
+		}
+	}
+
+	if (last_csv_file_name && (Log_destination & LOG_DESTINATION_CSVLOG))
+	{
+		if (fprintf(fh, "csvlog %s\n", last_csv_file_name) < 0)
+		{
+			ereport(LOG,
+					(errcode_for_file_access(),
+					errmsg("could not write file \"%s\": %m",
+							LOG_METAINFO_DATAFILE_TMP)));
+			fclose(fh);
+			return;
+		}
+	}
+	fclose(fh);
+
+	if (rename(LOG_METAINFO_DATAFILE_TMP, LOG_METAINFO_DATAFILE) != 0)
+		ereport(LOG,
+				(errcode_for_file_access(),
+				errmsg("could not rename file \"%s\" to \"%s\": %m",
+					   LOG_METAINFO_DATAFILE_TMP, LOG_METAINFO_DATAFILE)));
+}
+
 /* --------------------------------
  *		signal handler routines
  * --------------------------------