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.

Author: robertmhaas

doc/src/sgml/config.sgml

@@ -4280,6 +4280,11 @@ SELECT * FROM parent WHERE key = 2400;
       <primary>where to log</primary>
      </indexterm>
 
+     <indexterm>
+       <primary>current_logfiles</primary>
+       <secondary>and the log_destination configuration parameter</secondary>
+     </indexterm>
+
      <variablelist>
 
      <varlistentry id="guc-log-destination" xreflabel="log_destination">
@@ -4310,6 +4315,27 @@ SELECT * FROM parent WHERE key = 2400;
         <xref linkend="guc-logging-collector"> must be enabled to generate
         CSV-format log output.
        </para>
+       <para>
+        When either <systemitem>stderr</systemitem> or
+        <systemitem>csvlog</systemitem> are included, the file
+        <filename>current_logfiles</> is created to record the location
+        of the log file(s) currently in use by the logging collector and the
+        associated logging destination. This provides a convenient way to
+        find the logs currently in use by the instance. Here is an example of
+        this file's content:
+<programlisting>
+stderr pg_log/postgresql.log
+csvlog pg_log/postgresql.csv
+</programlisting>
+
+        <filename>current_logfiles</filename> is recreated when a new log file
+        is created as an effect of rotation, and
+        when <varname>log_destination</> is reloaded.  It is removed when
+        neither <systemitem>stderr</systemitem>
+        nor <systemitem>csvlog</systemitem> are included
+        in <varname>log_destination</>, and when the logging collector is
+        disabled.
+       </para>
 
        <note>
         <para>

doc/src/sgml/func.sgml

@@ -15478,6 +15478,13 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
        <entry>configuration load time</entry>
       </row>
 
+      <row>
+       <entry><literal><function>pg_current_logfile(<optional><type>text</></optional>)</function></literal></entry>
+       <entry><type>text</type></entry>
+       <entry>Primary log file name, or log in the requested format,
+       currently in use by the logging collector</entry>
+      </row>
+
       <row>
        <entry><literal><function>pg_my_temp_schema()</function></literal></entry>
        <entry><type>oid</type></entry>
@@ -15696,6 +15703,45 @@ SET search_path TO <replaceable>schema</> <optional>, <replaceable>schema</>, ..
     the time when the postmaster process re-read the configuration files.)
    </para>
 
+   <indexterm>
+    <primary>pg_current_logfile</primary>
+   </indexterm>
+
+   <indexterm>
+    <primary>Logging</primary>
+    <secondary>pg_current_logfile function</secondary>
+   </indexterm>
+
+   <indexterm>
+     <primary>current_logfiles</primary>
+     <secondary>and the pg_current_logfile function</secondary>
+   </indexterm>
+
+   <indexterm>
+    <primary>Logging</primary>
+    <secondary>current_logfiles file and the pg_current_logfile
+    function</secondary>
+   </indexterm>
+
+   <para>
+    <function>pg_current_logfile</function> returns, as <type>text</type>,
+    the path of the log file(s) currently in use by the logging collector.
+    The path includes the <xref linkend="guc-log-directory"> directory
+    and the log file name.  Log collection must be enabled or the return value
+    is <literal>NULL</literal>.  When multiple log files exist, each in a
+    different format, <function>pg_current_logfile</function> called
+    without arguments returns the path of the file having the first format
+    found in the ordered list: <systemitem>stderr</>, <systemitem>csvlog</>.
+    <literal>NULL</literal> is returned when no log file has any of these
+    formats.  To request a specific file format supply, as <type>text</type>,
+    either <systemitem>csvlog</> or <systemitem>stderr</> as the value of the
+    optional parameter. The return value is <literal>NULL</literal> when the
+    log format requested is not a configured
+    <xref linkend="guc-log-destination">.  The
+    <function>pg_current_logfiles</function> reflects the contents of the
+    <filename>current_logfiles</> file.
+   </para>
+
    <indexterm>
     <primary>pg_my_temp_schema</primary>
    </indexterm>

doc/src/sgml/storage.sgml

@@ -60,6 +60,12 @@ Item
  <entry>Subdirectory containing per-database subdirectories</entry>
 </row>
 
+<row>
+ <entry><filename>current_logfiles</></entry>
+ <entry>File recording the log file(s) currently written to by the logging
+  collector</entry>
+</row>
+
 <row>
  <entry><filename>global</></entry>
  <entry>Subdirectory containing cluster-wide tables, such as

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;

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
 	 */

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
  * --------------------------------

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

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);
+}