Make sure no modification done instead seq one

src/backend/distributed/metadata/node_metadata.c

@@ -787,7 +787,7 @@ SyncDistributedObjectsToNode(WorkerNode *workerNode)
 		return;
 	}
 
-	EnsureSequentialModeMetadataOperations();
+	EnsureNoModificationsHaveBeenDone();
 	Assert(ShouldPropagate());
 
 	List *commandList = SyncDistributedObjectsCommandList(workerNode);

src/test/regress/expected/multi_replicate_reference_table.out

@@ -464,11 +464,7 @@ SELECT create_reference_table('replicate_reference_table_insert');
 BEGIN;
 INSERT INTO replicate_reference_table_insert VALUES(1);
 SELECT 1 FROM master_add_node('localhost', :worker_2_port);
- ?column?
+ERROR:  cannot open new connections after the first modification command within a transaction
----------------------------------------------------------------------
-        1
-(1 row)
-
 ROLLBACK;
 DROP TABLE replicate_reference_table_insert;
 -- test COPY then adding a new node in a transaction
@@ -483,11 +479,7 @@ SET citus.enable_local_execution = 'off';
 BEGIN;
 COPY replicate_reference_table_copy FROM STDIN;
 SELECT 1 FROM master_add_node('localhost', :worker_2_port);
- ?column?
+ERROR:  cannot open new connections after the first modification command within a transaction
----------------------------------------------------------------------
-        1
-(1 row)
-
 ROLLBACK;
 RESET citus.enable_local_execution;
 DROP TABLE replicate_reference_table_copy;
@@ -502,11 +494,7 @@ SELECT create_reference_table('replicate_reference_table_ddl');
 BEGIN;
 ALTER TABLE replicate_reference_table_ddl ADD column2 int;
 SELECT 1 FROM master_add_node('localhost', :worker_2_port);
- ?column?
+ERROR:  cannot open new connections after the first modification command within a transaction
----------------------------------------------------------------------
-        1
-(1 row)
-
 ROLLBACK;
 DROP TABLE replicate_reference_table_ddl;
 -- test DROP table after adding new node in a transaction

src/test/regress/expected/start_stop_metadata_sync.out

@@ -468,9 +468,7 @@ BEGIN;
 (1 row)
 
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
-ERROR:  cannot execute metadata syncing operation because there was a parallel operation on a distributed table in the transaction
+ERROR:  cannot open new connections after the first modification command within a transaction
-DETAIL:  When modifying metadata, Citus needs to perform all operations over a single connection per node to ensure consistency.
-HINT:  Try re-running the transaction with "SET LOCAL citus.multi_shard_modify_mode TO 'sequential';"
 ROLLBACK;
 -- this is safe because start_metadata_sync_to_node already switches to
 -- sequential execution
@@ -507,9 +505,7 @@ BEGIN;
 (1 row)
 
 ROLLBACK;
--- multi-shard commands are allowed with start_metadata_sync
+-- multi-shard commands are not allowed with start_metadata_sync
--- as long as the start_metadata_sync_to_node executed
--- when it is OK to switch to sequential execution
 BEGIN;
 	-- sync at the start of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
@@ -543,15 +539,9 @@ BEGIN;
 
 	-- sync at the end of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
- start_metadata_sync_to_node
+ERROR:  cannot open new connections after the first modification command within a transaction
----------------------------------------------------------------------
-
-(1 row)
-
 ROLLBACK;
--- multi-shard commands are allowed with start_metadata_sync
+-- multi-shard commands are not allowed with start_metadata_sync
--- as long as the start_metadata_sync_to_node executed
--- when it is OK to switch to sequential execution
 BEGIN;
 	-- sync at the start of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
@@ -586,11 +576,7 @@ BEGIN;
 
 	-- sync at the end of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
- start_metadata_sync_to_node
+ERROR:  cannot open new connections after the first modification command within a transaction
----------------------------------------------------------------------
-
-(1 row)
-
 ROLLBACK;
 -- cleanup
 \c - - - :master_port

src/test/regress/sql/start_stop_metadata_sync.sql

@@ -244,9 +244,7 @@ BEGIN;
 	SELECT create_distributed_table('test_table_rep', 'a');
 ROLLBACK;
 
--- multi-shard commands are allowed with start_metadata_sync
+-- multi-shard commands are not allowed with start_metadata_sync
--- as long as the start_metadata_sync_to_node executed
--- when it is OK to switch to sequential execution
 BEGIN;
 	-- sync at the start of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
@@ -262,9 +260,7 @@ BEGIN;
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);
 ROLLBACK;
 
--- multi-shard commands are allowed with start_metadata_sync
+-- multi-shard commands are not allowed with start_metadata_sync
--- as long as the start_metadata_sync_to_node executed
--- when it is OK to switch to sequential execution
 BEGIN;
 	-- sync at the start of the tx
 	SELECT start_metadata_sync_to_node('localhost', :worker_1_port);