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.

5 files changed, 195 insertions, 0 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 38be9cf1a0c..ada542c530b 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1091,6 +1091,8 @@ REVOKE EXECUTE ON FUNCTION pg_wal_replay_pause() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_wal_replay_resume() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_rotate_logfile() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_reload_conf() FROM public;
+REVOKE EXECUTE ON FUNCTION pg_current_logfile() FROM public;
+REVOKE EXECUTE ON FUNCTION pg_current_logfile(text) FROM public;
 
 REVOKE EXECUTE ON FUNCTION pg_stat_reset() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_stat_reset_shared(text) FROM public;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 2cf17ac42e0..68313424ee2 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1232,6 +1232,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	RemovePromoteSignalFiles();
 
+	/* Remove any outdated file holding the current log filenames. */
+	if (unlink(LOG_METAINFO_DATAFILE) < 0 && errno != ENOENT)
+		ereport(LOG,
+				(errcode_for_file_access(),
+				 errmsg("could not remove file \"%s\": %m",
+					LOG_METAINFO_DATAFILE)));
+
 	/*
 	 * If enabled, start up syslogger collection subprocess
 	 */
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
  * --------------------------------
diff --git a/src/backend/replication/basebackup.c b/src/backend/replication/basebackup.c
index 7414048f4eb..e3a7ad5e9ab 100644
--- a/src/backend/replication/basebackup.c
+++ b/src/backend/replication/basebackup.c
@@ -26,6 +26,7 @@
 #include "nodes/pg_list.h"
 #include "pgtar.h"
 #include "pgstat.h"
+#include "postmaster/syslogger.h"
 #include "replication/basebackup.h"
 #include "replication/walsender.h"
 #include "replication/walsender_private.h"
@@ -147,6 +148,9 @@ static const char *excludeFiles[] =
 	/* Skip auto conf temporary file. */
 	PG_AUTOCONF_FILENAME ".tmp",
 
+	/* Skip current log file temporary file */
+	LOG_METAINFO_DATAFILE_TMP,
+
 	/*
 	 * If there's a backup_label or tablespace_map file, it belongs to a
 	 * backup started by the user with pg_start_backup().  It is *not* correct
diff --git a/src/backend/utils/adt/misc.c b/src/backend/utils/adt/misc.c
index 8f7c1f81fd4..ff6a25d2b60 100644
--- a/src/backend/utils/adt/misc.c
+++ b/src/backend/utils/adt/misc.c
@@ -885,3 +885,106 @@ parse_ident(PG_FUNCTION_ARGS)
 
 	PG_RETURN_DATUM(makeArrayResult(astate, CurrentMemoryContext));
 }
+
+/*
+ * pg_current_logfile
+ *
+ * Report current log file used by log collector by scanning current_logfiles.
+ */
+Datum
+pg_current_logfile(PG_FUNCTION_ARGS)
+{
+	FILE	   *fd;
+	char		lbuffer[MAXPGPATH];
+	char	   *logfmt;
+	char	   *log_filepath;
+	char	   *log_format = lbuffer;
+	char	   *nlpos;
+
+	/* The log format parameter is optional */
+	if (PG_NARGS() == 0 || PG_ARGISNULL(0))
+		logfmt = NULL;
+	else
+	{
+		logfmt = text_to_cstring(PG_GETARG_TEXT_PP(0));
+
+		if (strcmp(logfmt, "stderr") != 0 && strcmp(logfmt, "csvlog") != 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("log format \"%s\" is not supported", logfmt),
+					 errhint("The supported log formats are \"stderr\" and \"csvlog\".")));
+	}
+
+	fd = AllocateFile(LOG_METAINFO_DATAFILE, "r");
+	if (fd == NULL)
+	{
+		if (errno != ENOENT)
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not read file \"%s\": %m",
+							LOG_METAINFO_DATAFILE)));
+		PG_RETURN_NULL();
+	}
+
+	/*
+	 * Read the file to gather current log filename(s) registered by the
+	 * syslogger.
+	 */
+	while (fgets(lbuffer, sizeof(lbuffer), fd) != NULL)
+	{
+		/*
+		 * Extract log format and log file path from the line; lbuffer ==
+		 * log_format, they share storage.
+		 */
+		log_filepath = strchr(lbuffer, ' ');
+		if (log_filepath == NULL)
+		{
+			/*
+			 * No space found, file content is corrupted.  Return NULL to the
+			 * caller and inform him on the situation.
+			 */
+			elog(ERROR,
+				 "missing space character in \"%s\"", LOG_METAINFO_DATAFILE);
+			break;
+		}
+
+		*log_filepath = '\0';
+		log_filepath++;
+		nlpos = strchr(log_filepath, '\n');
+		if (nlpos == NULL)
+		{
+			/*
+			 * No newlinei found, file content is corrupted.  Return NULL to
+			 * the caller and inform him on the situation.
+			 */
+			elog(ERROR,
+			   "missing newline character in \"%s\"", LOG_METAINFO_DATAFILE);
+			break;
+		}
+		*nlpos = '\0';
+
+		if (logfmt == NULL || strcmp(logfmt, log_format) == 0)
+		{
+			FreeFile(fd);
+			PG_RETURN_TEXT_P(cstring_to_text(log_filepath));
+		}
+	}
+
+	/* Close the current log filename file. */
+	FreeFile(fd);
+
+	PG_RETURN_NULL();
+}
+
+/*
+ * Report current log file used by log collector (1 argument version)
+ *
+ * note: this wrapper is necessary to pass the sanity check in opr_sanity,
+ * which checks that all built-in functions that share the implementing C
+ * function take the same number of arguments
+ */
+Datum
+pg_current_logfile_1arg(PG_FUNCTION_ARGS)
+{
+	return pg_current_logfile(fcinfo);
+}