Add test for master_create_empty_shard on coordinator

src/backend/distributed/operations/stage_protocol.c

@@ -415,7 +415,7 @@ CreateAppendDistributedShardPlacements(Oid relationId, int64 shardId,
 		if (NodeIsCoordinator(workerNode))
 		{
 			ereport(NOTICE, (errmsg(
-								 "Creating placements for the append partitioned tables on the coordinator is not allowed.")));
+								 "Creating placements for the append partitioned tables on the coordinator is not supported, skipping coordinator ...")));
 			continue;
 		}
 

src/test/regress/expected/coordinator_shouldhaveshards.out

@@ -510,6 +510,36 @@ NOTICE:  executing the command locally: SELECT count(*) AS count FROM (coordinat
 (1 row)
 
 ROLLBACK;
+-- issue #4237: preventing empty placement creation on coordinator
+CREATE TABLE test_append_table(a int);
+SELECT create_distributed_table('test_append_table', 'a', 'append');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- this will fail since it will try to create an empty placement in the
+-- coordinator as well
+SET citus.shard_replication_factor TO 3;
+SELECT master_create_empty_shard('test_append_table');
+NOTICE:  Creating placements for the append partitioned tables on the coordinator is not supported, skipping coordinator ...
+ERROR:  could only create 2 of 3 of required shard replicas
+-- this will create an empty shard with replicas in the two worker nodes
+SET citus.shard_replication_factor TO 2;
+SELECT 1 FROM master_create_empty_shard('test_append_table');
+ ?column?
+---------------------------------------------------------------------
+        1
+(1 row)
+
+-- verify groupid is not 0 for each placement
+SELECT COUNT(*) FROM pg_dist_placement p, pg_dist_shard s WHERE p.shardid = s.shardid AND s.logicalrelid = 'test_append_table'::regclass AND p.groupid > 0;
+ count
+---------------------------------------------------------------------
+     2
+(1 row)
+
+SET citus.shard_replication_factor TO 1;
 \set VERBOSITY terse
 DROP TABLE ref_table;
 NOTICE:  executing the command locally: DROP TABLE IF EXISTS coordinator_shouldhaveshards.ref_table_xxxxx CASCADE
@@ -518,6 +548,7 @@ DROP TABLE test;
 DROP TABLE dist_table;
 DROP TABLE ref;
 NOTICE:  executing the command locally: DROP TABLE IF EXISTS coordinator_shouldhaveshards.ref_xxxxx CASCADE
+DROP TABLE test_append_table;
 DROP SCHEMA coordinator_shouldhaveshards CASCADE;
 NOTICE:  drop cascades to table local
 SELECT 1 FROM master_set_node_property('localhost', :master_port, 'shouldhaveshards', false);

src/test/regress/sql/coordinator_shouldhaveshards.sql

@@ -214,6 +214,20 @@ INSERT INTO ref_table SELECT *, * FROM generate_series(1, 100);
 SELECT COUNT(*) FROM test JOIN ref_table USING(x);
 ROLLBACK;
 
+-- issue #4237: preventing empty placement creation on coordinator
+CREATE TABLE test_append_table(a int);
+SELECT create_distributed_table('test_append_table', 'a', 'append');
+-- this will fail since it will try to create an empty placement in the
+-- coordinator as well
+SET citus.shard_replication_factor TO 3;
+SELECT master_create_empty_shard('test_append_table');
+-- this will create an empty shard with replicas in the two worker nodes
+SET citus.shard_replication_factor TO 2;
+SELECT 1 FROM master_create_empty_shard('test_append_table');
+-- verify groupid is not 0 for each placement
+SELECT COUNT(*) FROM pg_dist_placement p, pg_dist_shard s WHERE p.shardid = s.shardid AND s.logicalrelid = 'test_append_table'::regclass AND p.groupid > 0;
+SET citus.shard_replication_factor TO 1;
+
 \set VERBOSITY terse
 DROP TABLE ref_table;
 
@@ -221,6 +235,7 @@ DELETE FROM test;
 DROP TABLE test;
 DROP TABLE dist_table;
 DROP TABLE ref;
+DROP TABLE test_append_table;
 
 DROP SCHEMA coordinator_shouldhaveshards CASCADE;