Add a citus_is_coordinator function

src/backend/distributed/metadata/node_metadata.c

@@ -150,6 +150,7 @@ PG_FUNCTION_INFO_V1(get_shard_id_for_distribution_column);
 PG_FUNCTION_INFO_V1(citus_nodename_for_nodeid);
 PG_FUNCTION_INFO_V1(citus_nodeport_for_nodeid);
 PG_FUNCTION_INFO_V1(citus_coordinator_nodeid);
+PG_FUNCTION_INFO_V1(citus_is_coordinator);
 
 
 /*
@@ -1558,6 +1559,29 @@ citus_coordinator_nodeid(PG_FUNCTION_ARGS)
 }
 
 
+/*
+ * citus_is_coordinator returns whether the current node is a coordinator.
+ * We consider the node a coordinator if its group ID is 0 and it has
+ * pg_dist_node entries (only group ID 0 could indicate a worker without
+ * metadata).
+ */
+Datum
+citus_is_coordinator(PG_FUNCTION_ARGS)
+{
+	CheckCitusVersion(ERROR);
+
+	bool isCoordinator = false;
+
+	if (GetLocalGroupId() == COORDINATOR_GROUP_ID &&
+		ActivePrimaryNodeCount() > 0)
+	{
+		isCoordinator = true;
+	}
+
+	PG_RETURN_BOOL(isCoordinator);
+}
+
+
 /*
  * FindWorkerNode searches over the worker nodes and returns the workerNode
  * if it already exists. Else, the function returns NULL.

src/backend/distributed/sql/citus--11.0-1--11.0-2.sql

@@ -1,2 +1,3 @@
 #include "udfs/citus_shards_on_worker/11.0-2.sql"
 #include "udfs/citus_shard_indexes_on_worker/11.0-2.sql"
+#include "udfs/citus_is_coordinator/11.0-2.sql"

src/backend/distributed/sql/udfs/citus_is_coordinator/11.0-2.sql

@@ -0,0 +1,7 @@
+CREATE FUNCTION pg_catalog.citus_is_coordinator()
+ RETURNS bool
+ LANGUAGE c
+ STRICT
+AS 'MODULE_PATHNAME', $$citus_is_coordinator$$;
+COMMENT ON FUNCTION pg_catalog.citus_is_coordinator()
+ IS 'returns whether the current node is a coordinator';

src/backend/distributed/sql/udfs/citus_is_coordinator/latest.sql

@@ -0,0 +1,7 @@
+CREATE FUNCTION pg_catalog.citus_is_coordinator()
+ RETURNS bool
+ LANGUAGE c
+ STRICT
+AS 'MODULE_PATHNAME', $$citus_is_coordinator$$;
+COMMENT ON FUNCTION pg_catalog.citus_is_coordinator()
+ IS 'returns whether the current node is a coordinator';

src/test/regress/expected/multi_cluster_management.out

@@ -15,6 +15,21 @@ SELECT 1 FROM master_add_node('localhost', :worker_2_port);
         1
 (1 row)
 
+-- I am coordinator
+SELECT citus_is_coordinator();
+ citus_is_coordinator
+---------------------------------------------------------------------
+ t
+(1 row)
+
+-- workers are not coordinator
+SELECT result FROM run_command_on_workers('SELECT citus_is_coordinator()');
+ result
+---------------------------------------------------------------------
+ f
+ f
+(2 rows)
+
 -- get the active nodes
 SELECT master_get_active_worker_nodes();
  master_get_active_worker_nodes

src/test/regress/expected/multi_extension.out

@@ -1034,9 +1034,10 @@ SELECT * FROM multi_extension.print_extension_changes();
 -- Snapshot of state at 11.0-2
 ALTER EXTENSION citus UPDATE TO '11.0-2';
 SELECT * FROM multi_extension.print_extension_changes();
- previous_object | current_object
+ previous_object |             current_object
 ---------------------------------------------------------------------
-(0 rows)
+                 | function citus_is_coordinator() boolean
+(1 row)
 
 DROP TABLE multi_extension.prev_objects, multi_extension.extension_diff;
 -- show running version

src/test/regress/expected/single_node.out

@@ -10,6 +10,13 @@ ALTER SYSTEM SET citus.max_cached_conns_per_worker TO 0;
 -- adding the coordinator as inactive is disallowed
 SELECT 1 FROM master_add_inactive_node('localhost', :master_port, groupid => 0);
 ERROR:  coordinator node cannot be added as inactive node
+-- before adding a node we are not officially a coordinator
+SELECT citus_is_coordinator();
+ citus_is_coordinator
+---------------------------------------------------------------------
+ f
+(1 row)
+
 -- idempotently add node to allow this test to run without add_coordinator
 SET client_min_messages TO WARNING;
 SELECT 1 FROM citus_set_coordinator_host('localhost', :master_port);
@@ -18,6 +25,13 @@ SELECT 1 FROM citus_set_coordinator_host('localhost', :master_port);
         1
 (1 row)
 
+-- after adding a node we are officially a coordinator
+SELECT citus_is_coordinator();
+ citus_is_coordinator
+---------------------------------------------------------------------
+ t
+(1 row)
+
 -- coordinator cannot be disabled
 SELECT 1 FROM citus_disable_node('localhost', :master_port);
 ERROR:  cannot change "isactive" field of the coordinator node

src/test/regress/expected/upgrade_list_citus_objects.out

@@ -83,6 +83,7 @@ ORDER BY 1;
  function citus_internal_local_blocked_processes()
  function citus_internal_update_placement_metadata(bigint,integer,integer)
  function citus_internal_update_relation_colocation(oid,integer)
+ function citus_is_coordinator()
  function citus_isolation_test_session_is_blocked(integer,integer[])
  function citus_json_concatenate(json,json)
  function citus_json_concatenate_final(json)
@@ -283,5 +284,5 @@ ORDER BY 1;
  view citus_stat_statements
  view pg_dist_shard_placement
  view time_partitions
-(267 rows)
+(268 rows)
 

src/test/regress/sql/multi_cluster_management.sql

@@ -8,6 +8,11 @@ ALTER SEQUENCE pg_catalog.pg_dist_groupid_seq RESTART 1;
 SELECT 1 FROM master_add_node('localhost', :worker_1_port);
 SELECT 1 FROM master_add_node('localhost', :worker_2_port);
 
+-- I am coordinator
+SELECT citus_is_coordinator();
+-- workers are not coordinator
+SELECT result FROM run_command_on_workers('SELECT citus_is_coordinator()');
+
 -- get the active nodes
 SELECT master_get_active_worker_nodes();
 

src/test/regress/sql/single_node.sql

@@ -13,10 +13,16 @@ ALTER SYSTEM SET citus.max_cached_conns_per_worker TO 0;
 -- adding the coordinator as inactive is disallowed
 SELECT 1 FROM master_add_inactive_node('localhost', :master_port, groupid => 0);
 
+-- before adding a node we are not officially a coordinator
+SELECT citus_is_coordinator();
+
 -- idempotently add node to allow this test to run without add_coordinator
 SET client_min_messages TO WARNING;
 SELECT 1 FROM citus_set_coordinator_host('localhost', :master_port);
 
+-- after adding a node we are officially a coordinator
+SELECT citus_is_coordinator();
+
 -- coordinator cannot be disabled
 SELECT 1 FROM citus_disable_node('localhost', :master_port);