Restrict 'ON MASTER' feature to custom protocols only

ON MASTER feature is not fully supported by built-in protocols other
than s3 yet, disable it with those protocols for now.

src/backend/optimizer/plan/createplan.c

@@ -1195,6 +1195,7 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 	int			total_primaries = 0;
 	int			i;
 	Oid			fmtErrTblOid = InvalidOid;
+	char       *on_clause = NULL;
 
 	/* various processing flags */
 	bool		using_execute = false;	/* true if EXECUTE is used */
@@ -1291,6 +1292,14 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 		uri = ParseExternalTableUri(first_uri_str);
 	}
 
+	/* get the ON clause information, and restrict 'ON MASTER' to custom
+	 * protocols only */
+	on_clause = (char *) strVal(lfirst(list_head(rel->execlocationlist)));
+	if ((strcmp(on_clause, "MASTER_ONLY") == 0)
+		&& using_location && (uri->protocol != URI_CUSTOM)) {
+		ereport(ERROR, (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
+				errmsg("\'ON MASTER\' is not supported by this protocol yet.")));
+	}
 
 	/*
 	 * Now we do the actual assignment of work to the segment databases (where
@@ -1434,14 +1443,11 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 
 	}
 	/* (2) */
-	else if(using_location && (uri->protocol == URI_GPFDIST || 
-							   uri->protocol == URI_GPFDISTS || 
+	else if (using_location && (uri->protocol == URI_GPFDIST ||
+							   uri->protocol == URI_GPFDISTS ||
 							   uri->protocol == URI_CUSTOM))
 	{
-		char       *on_clause = NULL;
-		on_clause = (char *) strVal(lfirst(list_head(rel->execlocationlist)));
-
-		if (strcmp(on_clause, "MASTER_ONLY") == 0 && uri->protocol == URI_CUSTOM){
+		if ((strcmp(on_clause, "MASTER_ONLY") == 0) && (uri->protocol == URI_CUSTOM)) {
 			const char *uri_str = (char *) strVal(lfirst(list_head(rel->urilocationlist)));
 			segdb_file_map[0] = pstrdup(uri_str);
 			ismasteronly = true;
@@ -1621,7 +1627,6 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 								" when trying to assign segments for gpfdist(s)")));
 			}		
 		}
-
 	}
 	}
 	/* (3) */
@@ -1630,7 +1635,6 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 		const char *command = rel->execcommand;
 		const char *prefix = "execute:";
 		char	   *prefixed_command = NULL;
-		char	   *on_clause = NULL;
 		bool		match_found = false;
 
 		/* build the command string for the executor - 'execute:command' */
@@ -1643,9 +1647,6 @@ create_externalscan_plan(PlannerInfo *root, Path *best_path,
 		pfree(buf);
 		buf = NULL;
 
-		/* get the ON clause (execute location) information */
-		on_clause = (char *) strVal(lfirst(list_head(rel->execlocationlist)));
-
 		/*
 		 * Now we handle each one of the ON locations separately:
 		 *

src/test/regress/input/external_table.source

@@ -1336,7 +1336,7 @@ DROP EXTERNAL TABLE IF EXISTS exttab_with_on_master;
 CREATE EXTERNAL TABLE exttab_with_on_master( i int, j text )
 LOCATION ('file://@hostname@@abs_srcdir@/data/exttab_few_errors.data') ON MASTER FORMAT 'TEXT' (DELIMITER '|');
 
-\d exttab_with_on_master
+SELECT * FROM exttab_with_on_master;
 
 DROP EXTERNAL TABLE IF EXISTS exttab_with_on_master;
 

src/test/regress/output/external_table.source

@@ -2721,20 +2721,8 @@ NOTICE:  table "exttab_with_on_master" does not exist, skipping
 -- Create external table with on clause
 CREATE EXTERNAL TABLE exttab_with_on_master( i int, j text )
 LOCATION ('file://@hostname@@abs_srcdir@/data/exttab_few_errors.data') ON MASTER FORMAT 'TEXT' (DELIMITER '|');
-\d exttab_with_on_master
-External table "public.exttab_with_on_master"
- Column |  Type   | Modifiers 
---------+---------+-----------
- i      | integer | 
- j      | text    | 
-Type: readable
-Encoding: UTF8
-Format type: text
-Format options: delimiter '|' null '\N' escape '\'
-External options: {}
-External location: file://@hostname@@abs_srcdir@/data/exttab_few_errors.data
-Execute on: master segment
-
+SELECT * FROM exttab_with_on_master;
+ERROR:  'ON MASTER' is not supported by this protocol yet.
 DROP EXTERNAL TABLE IF EXISTS exttab_with_on_master;
 -- start_ignore
 DROP EXTERNAL TABLE IF EXISTS exttab_with_option_empty;