Adds a method to determine if current node is primary (#7720)

DESCRIPTION: Adds citus_is_primary_node() UDF to determine if the
current node is a primary node in the cluster.

---------

Co-authored-by: German Eichberger <geeichbe@microsoft.com>
Co-authored-by: Onur Tirtir <onurcantirtir@gmail.com>

src/backend/distributed/metadata/node_metadata.c

@@ -167,6 +167,7 @@ PG_FUNCTION_INFO_V1(citus_nodeport_for_nodeid);
 PG_FUNCTION_INFO_V1(citus_coordinator_nodeid);
 PG_FUNCTION_INFO_V1(citus_is_coordinator);
 PG_FUNCTION_INFO_V1(citus_internal_mark_node_not_synced);
+PG_FUNCTION_INFO_V1(citus_is_primary_node);
 
 /*
  * DefaultNodeMetadata creates a NodeMetadata struct with the fields set to
@@ -1665,6 +1666,36 @@ citus_is_coordinator(PG_FUNCTION_ARGS)
 }
 
 
+/*
+ * citus_is_primary_node returns whether the current node is a primary for
+ * a given group_id. We consider the node a primary if it has
+ * pg_dist_node entries marked as primary
+ */
+Datum
+citus_is_primary_node(PG_FUNCTION_ARGS)
+{
+	CheckCitusVersion(ERROR);
+
+	int32 groupId = GetLocalGroupId();
+	WorkerNode *workerNode = PrimaryNodeForGroup(groupId, NULL);
+	if (workerNode == NULL)
+	{
+		ereport(WARNING, (errmsg("could not find the current node in pg_dist_node"),
+						  errdetail("If this is the coordinator node, consider adding it "
+									"into the metadata by using citus_set_coordinator_host() "
+									"UDF. Otherwise, if you're going to use this node as a "
+									"worker node for a new cluster, make sure to add this "
+									"node into the metadata from the coordinator by using "
+									"citus_add_node() UDF.")));
+		PG_RETURN_NULL();
+	}
+
+	bool isPrimary = workerNode->nodeId == GetLocalNodeId();
+
+	PG_RETURN_BOOL(isPrimary);
+}
+
+
 /*
  * EnsureParentSessionHasExclusiveLockOnPgDistNode ensures given session id
  * holds Exclusive lock on pg_dist_node.

src/backend/distributed/sql/citus--13.0-1--13.1-1.sql

@@ -47,3 +47,4 @@ DROP VIEW IF EXISTS pg_catalog.citus_lock_waits;
 #include "udfs/citus_internal_update_relation_colocation/13.1-1.sql"
 #include "udfs/repl_origin_helper/13.1-1.sql"
 #include "udfs/citus_finish_pg_upgrade/13.1-1.sql"
+#include "udfs/citus_is_primary_node/13.1-1.sql"

src/backend/distributed/sql/downgrades/citus--13.1-1--13.0-1.sql

@@ -11,6 +11,7 @@ DROP FUNCTION pg_catalog.citus_unmark_object_distributed(oid,oid,int,boolean);
 ALTER TABLE pg_catalog.pg_dist_transaction DROP COLUMN outer_xid;
 REVOKE USAGE ON SCHEMA citus_internal FROM PUBLIC;
 
+DROP FUNCTION pg_catalog.citus_is_primary_node();
 DROP FUNCTION citus_internal.add_colocation_metadata(int, int, int, regtype, oid);
 DROP FUNCTION citus_internal.add_object_metadata(text, text[], text[], integer, integer, boolean);
 DROP FUNCTION citus_internal.add_partition_metadata(regclass, "char", text, integer, "char");

src/backend/distributed/sql/udfs/citus_is_primary_node/13.1-1.sql

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

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

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

src/test/regress/expected/multi_cluster_management.out

@@ -25,6 +25,15 @@ SELECT citus_is_coordinator();
  t
 (1 row)
 
+-- I am primary node (fails beacuse not set in pg_dist)
+select citus_is_primary_node();
+WARNING:  could not find the current node in pg_dist_node
+DETAIL:  If this is the coordinator node, consider adding it into the metadata by using citus_set_coordinator_host() UDF. Otherwise, if you're going to use this node as a worker node for a new cluster, make sure to add this node into the metadata from the coordinator by using citus_add_node() UDF.
+ citus_is_primary_node
+---------------------------------------------------------------------
+
+(1 row)
+
 -- make sure coordinator is always in metadata.
 SELECT citus_set_coordinator_host('localhost');
  citus_set_coordinator_host
@@ -32,6 +41,13 @@ SELECT citus_set_coordinator_host('localhost');
 
 (1 row)
 
+-- I am primary node
+select citus_is_primary_node();
+ citus_is_primary_node
+---------------------------------------------------------------------
+ t
+(1 row)
+
 -- workers are not coordinator
 SELECT result FROM run_command_on_workers('SELECT citus_is_coordinator()');
  result
@@ -40,6 +56,14 @@ SELECT result FROM run_command_on_workers('SELECT citus_is_coordinator()');
  f
 (2 rows)
 
+-- primary workers are primary node
+SELECT result FROM run_command_on_workers('SELECT citus_is_primary_node()');
+ result
+---------------------------------------------------------------------
+ t
+ t
+(2 rows)
+
 -- get the active nodes
 SELECT master_get_active_worker_nodes();
  master_get_active_worker_nodes

src/test/regress/expected/multi_extension.out

@@ -1479,8 +1479,9 @@ SELECT * FROM multi_extension.print_extension_changes();
                                                                 | function citus_internal.update_none_dist_table_metadata(oid,"char",bigint,boolean) void
                                                                 | function citus_internal.update_placement_metadata(bigint,integer,integer) void
                                                                 | function citus_internal.update_relation_colocation(oid,integer) void
+                                                                | function citus_is_primary_node() boolean
                                                                 | function citus_unmark_object_distributed(oid,oid,integer,boolean) void
-(26 rows)
+(27 rows)
 
 DROP TABLE multi_extension.prev_objects, multi_extension.extension_diff;
 -- show running version

src/test/regress/expected/upgrade_list_citus_objects.out

@@ -133,6 +133,7 @@ ORDER BY 1;
  function citus_internal_update_relation_colocation(oid,integer)
  function citus_is_clock_after(cluster_clock,cluster_clock)
  function citus_is_coordinator()
+ function citus_is_primary_node()
  function citus_isolation_test_session_is_blocked(integer,integer[])
  function citus_job_cancel(bigint)
  function citus_job_list()
@@ -388,6 +389,6 @@ ORDER BY 1;
  view citus_stat_tenants_local
  view pg_dist_shard_placement
  view time_partitions
-(357 rows)
+(358 rows)
 
 DROP TABLE extension_basic_types;

src/test/regress/sql/multi_cluster_management.sql

@@ -14,12 +14,21 @@ RESET citus.metadata_sync_mode;
 -- I am coordinator
 SELECT citus_is_coordinator();
 
+-- I am primary node (fails beacuse not set in pg_dist)
+select citus_is_primary_node();
+
 -- make sure coordinator is always in metadata.
 SELECT citus_set_coordinator_host('localhost');
 
+-- I am primary node
+select citus_is_primary_node();
+
 -- workers are not coordinator
 SELECT result FROM run_command_on_workers('SELECT citus_is_coordinator()');
 
+-- primary workers are primary node
+SELECT result FROM run_command_on_workers('SELECT citus_is_primary_node()');
+
 -- get the active nodes
 SELECT master_get_active_worker_nodes();