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.

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);
+}