Prevent create_distributed_function() from the workers

As this could cause weird edge cases.

src/backend/distributed/commands/function.c

@@ -148,6 +148,7 @@ create_distributed_function(PG_FUNCTION_ARGS)
 		colocateWithTableName = text_to_cstring(colocateWithText);
 	}
 
+	EnsureCoordinator();
 	EnsureFunctionOwner(funcOid);
 
 	ObjectAddressSet(functionAddress, ProcedureRelationId, funcOid);

src/test/regress/expected/multi_mx_function_call_delegation.out

@@ -692,6 +692,14 @@ DEBUG:  pushing down the function call
            28
 (1 row)
 
+\c - - - :worker_1_port
+SET search_path TO multi_mx_function_call_delegation, public;
+-- create_distributed_function is disallowed from worker nodes
+select create_distributed_function('mx_call_func(int,int)');
+ERROR:  operation is not allowed on this node
+HINT:  Connect to the coordinator and run it again.
+\c - - - :master_port
+SET search_path TO multi_mx_function_call_delegation, public;
 RESET client_min_messages;
 \set VERBOSITY terse
 DROP SCHEMA multi_mx_function_call_delegation CASCADE;

src/test/regress/sql/multi_mx_function_call_delegation.sql

@@ -83,6 +83,7 @@ select create_distributed_function('mx_call_func_bigint(bigint,bigint)');
 select create_distributed_function('mx_call_func_custom_types(mx_call_enum,mx_call_enum)');
 select create_distributed_function('squares(int)');
 
+
 -- We still don't route them to the workers, because they aren't
 -- colocated with any distributed tables.
 SET client_min_messages TO DEBUG1;
@@ -259,6 +260,14 @@ EXECUTE call_plan(2, 0);
 EXECUTE call_plan(2, 0);
 EXECUTE call_plan(2, 0);
 
+\c - - - :worker_1_port
+SET search_path TO multi_mx_function_call_delegation, public;
+-- create_distributed_function is disallowed from worker nodes
+select create_distributed_function('mx_call_func(int,int)');
+
+\c - - - :master_port
+SET search_path TO multi_mx_function_call_delegation, public;
+
 RESET client_min_messages;
 \set VERBOSITY terse
 DROP SCHEMA multi_mx_function_call_delegation CASCADE;