Code improvements for ALTER SYSTEM .. SET.

Move FreeConfigVariables() later to make sure ErrorConfFile is valid
when we use it, and get rid of an unnecessary string copy operation.

Amit Kapila, kibitzed by me.

Author: robertmhaas

src/backend/utils/misc/guc-file.l

@@ -120,7 +120,6 @@ ProcessConfigFile(GucContext context)
 				   *head,
 				   *tail;
 	int			i;
-	char		ConfigAutoFileName[MAXPGPATH];
 	char		*ErrorConfFile;
 	char		*CallingFileName;
 
@@ -155,17 +154,16 @@ ProcessConfigFile(GucContext context)
 	 * data directory, however when called during initdb data directory is not
 	 * set till this point, so use ConfigFile path which will be same.
 	 */
-	snprintf(ConfigAutoFileName,sizeof(ConfigAutoFileName),"%s", PG_AUTOCONF_FILENAME);
 	if (data_directory)
 		CallingFileName = NULL;
 	else
 		CallingFileName = ConfigFileName;
 
-	if (!ParseConfigFile(ConfigAutoFileName, CallingFileName, false, 0, elevel, &head, &tail))
+	if (!ParseConfigFile(PG_AUTOCONF_FILENAME, CallingFileName, false, 0, elevel, &head, &tail))
 	{
 		/* Syntax error(s) detected in the file, so bail out */
 		error = true;
-		ErrorConfFile = ConfigAutoFileName;
+		ErrorConfFile = PG_AUTOCONF_FILENAME;
 		goto cleanup_list;
 	}
 
@@ -368,8 +366,6 @@ ProcessConfigFile(GucContext context)
 	PgReloadTime = GetCurrentTimestamp();
 
  cleanup_list:
-	FreeConfigVariables(head);
-
 	if (error)
 	{
 		/* During postmaster startup, any error is fatal */
@@ -389,6 +385,13 @@ ProcessConfigFile(GucContext context)
 					 errmsg("configuration file \"%s\" contains errors; no changes were applied",
 							ErrorConfFile)));
 	}
+
+	/*
+	 * Calling FreeConfigVariables() any earlier than this can cause problems,
+	 * because ErrorConfFile could be pointing to a string that will be freed
+	 * here.
+	 */
+	FreeConfigVariables(head);
 }
 
 /*