Address the issues/comments from the original PR# 6315

1) Regular users fail to use clock UDF with permission issue.
2) Clock functions were declared as STABLE, whereas by definition they are VOLATILE. By design, any clock/time
   functions will return different results for each call even within a single SQL statement.

Note: UDF citus_get_transaction_clock() is a misnomer as it internally calls the clock tick which always returns
      different results for every invocation in the same transaction.

src/backend/distributed/clock/causal_clock.c

@@ -44,10 +44,15 @@
 
 #define SAVE_AND_PERSIST(c) \
 	do { \
+		Oid savedUserId = InvalidOid; \
+		int savedSecurityContext = 0; \
 		LogicalClockShmem->clusterClockValue = *(c); \
+		GetUserIdAndSecContext(&savedUserId, &savedSecurityContext); \
+		SetUserIdAndSecContext(CitusExtensionOwner(), SECURITY_LOCAL_USERID_CHANGE); \
 		DirectFunctionCall2(setval_oid, \
 							ObjectIdGetDatum(DistClockLogicalSequenceId()), \
 							Int64GetDatum((c)->logical)); \
+		SetUserIdAndSecContext(savedUserId, savedSecurityContext); \
 	} while (0)
 
 PG_FUNCTION_INFO_V1(citus_get_node_clock);
@@ -61,7 +66,7 @@ PG_FUNCTION_INFO_V1(citus_get_transaction_clock);
 typedef enum ClockState
 {
 	CLOCKSTATE_INITIALIZED,
-	CLOCKSTATE_UNINITIALIZED,
+	CLOCKSTATE_UNINITIALIZED
 } ClockState;
 
 /*
@@ -504,9 +509,17 @@ InitClockAtFirstUse(void)
 	 * a higher rate than once every 32 seconds.
 	 *
 	 */
+	Oid saveUserId = InvalidOid;
+	int savedSecurityCtx = 0;
+
+	GetUserIdAndSecContext(&saveUserId, &savedSecurityCtx);
+	SetUserIdAndSecContext(CitusExtensionOwner(), SECURITY_LOCAL_USERID_CHANGE);
+
 	persistedMaxClock.logical =
 		DirectFunctionCall1(nextval_oid, ObjectIdGetDatum(DistClockLogicalSequenceId()));
 
+	SetUserIdAndSecContext(saveUserId, savedSecurityCtx);
+
 	/*
 	 * Sequence 1 indicates no prior clock timestamps on this server, retain
 	 * the wall clock i.e. no adjustment needed.

src/backend/distributed/sql/udfs/citus_get_node_clock/11.2-1.sql

@@ -1,6 +1,6 @@
 CREATE OR REPLACE FUNCTION pg_catalog.citus_get_node_clock()
     RETURNS pg_catalog.cluster_clock
-    LANGUAGE C STABLE PARALLEL SAFE STRICT
+    LANGUAGE C VOLATILE PARALLEL UNSAFE STRICT
     AS 'MODULE_PATHNAME',$$citus_get_node_clock$$;
 COMMENT ON FUNCTION pg_catalog.citus_get_node_clock()
     IS 'Returns monotonically increasing timestamp with logical clock value as close to epoch value (in milli seconds) as possible, and a counter for ticks(maximum of 4 million) within the logical clock';

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

@@ -1,6 +1,6 @@
 CREATE OR REPLACE FUNCTION pg_catalog.citus_get_node_clock()
     RETURNS pg_catalog.cluster_clock
-    LANGUAGE C STABLE PARALLEL SAFE STRICT
+    LANGUAGE C VOLATILE PARALLEL UNSAFE STRICT
     AS 'MODULE_PATHNAME',$$citus_get_node_clock$$;
 COMMENT ON FUNCTION pg_catalog.citus_get_node_clock()
     IS 'Returns monotonically increasing timestamp with logical clock value as close to epoch value (in milli seconds) as possible, and a counter for ticks(maximum of 4 million) within the logical clock';

src/backend/distributed/sql/udfs/citus_get_transaction_clock/11.2-1.sql

@@ -1,6 +1,6 @@
 CREATE OR REPLACE FUNCTION pg_catalog.citus_get_transaction_clock()
     RETURNS pg_catalog.cluster_clock
-    LANGUAGE C STABLE PARALLEL SAFE STRICT
+    LANGUAGE C VOLATILE PARALLEL UNSAFE STRICT
     AS 'MODULE_PATHNAME',$$citus_get_transaction_clock$$;
 COMMENT ON FUNCTION pg_catalog.citus_get_transaction_clock()
     IS 'Returns a transaction timestamp logical clock';

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

@@ -1,6 +1,6 @@
 CREATE OR REPLACE FUNCTION pg_catalog.citus_get_transaction_clock()
     RETURNS pg_catalog.cluster_clock
-    LANGUAGE C STABLE PARALLEL SAFE STRICT
+    LANGUAGE C VOLATILE PARALLEL UNSAFE STRICT
     AS 'MODULE_PATHNAME',$$citus_get_transaction_clock$$;
 COMMENT ON FUNCTION pg_catalog.citus_get_transaction_clock()
     IS 'Returns a transaction timestamp logical clock';

src/test/regress/expected/clock.out

@@ -328,9 +328,51 @@ WARNING:  GUC enable_cluster_clock is off
 (1 row)
 
 END;
+SET citus.enable_cluster_clock to ON;
+-- Test if the clock UDFs are volatile, result should never be the same
+SELECT citus_get_node_clock() = citus_get_node_clock();
+ ?column?
+---------------------------------------------------------------------
+ f
+(1 row)
+
+select citus_get_transaction_clock() = citus_get_transaction_clock();
+DEBUG:  node xxxx transaction clock xxxxxx
+DEBUG:  final global transaction clock xxxxxx
+DEBUG:  node xxxx transaction clock xxxxxx
+DEBUG:  final global transaction clock xxxxxx
+ ?column?
+---------------------------------------------------------------------
+ f
+(1 row)
+
+-- Test if the clock UDFs are usable by non-superusers
+CREATE ROLE non_super_user_clock;
+SET ROLE non_super_user_clock;
+SELECT citus_get_node_clock();
+ citus_get_node_clock
+---------------------------------------------------------------------
+ (xxxxxxxxxxxxx,x)
+(1 row)
+
+BEGIN;
+SELECT citus_get_transaction_clock();
+DEBUG:  node xxxx transaction clock xxxxxx
+DEBUG:  final global transaction clock xxxxxx
+ citus_get_transaction_clock
+---------------------------------------------------------------------
+ (xxxxxxxxxxxxx,x)
+(1 row)
+
+COMMIT;
+-- Test setting the persisted clock value (it must fail)
+SELECT setval('pg_dist_clock_logical_seq', 100, true);
+ERROR:  permission denied for sequence pg_dist_clock_logical_seq
+\c
 RESET client_min_messages;
 RESET citus.enable_cluster_clock;
+DROP ROLE non_super_user_clock;
 DROP SCHEMA clock CASCADE;
 NOTICE:  drop cascades to 2 other objects
-DETAIL:  drop cascades to table clock_test
-drop cascades to table cluster_clock_type
+DETAIL:  drop cascades to table clock.clock_test
+drop cascades to table clock.cluster_clock_type

src/test/regress/sql/clock.sql

@@ -141,6 +141,25 @@ BEGIN;
 SELECT citus_get_transaction_clock();
 END;
 
+SET citus.enable_cluster_clock to ON;
+
+-- Test if the clock UDFs are volatile, result should never be the same
+SELECT citus_get_node_clock() = citus_get_node_clock();
+select citus_get_transaction_clock() = citus_get_transaction_clock();
+
+-- Test if the clock UDFs are usable by non-superusers
+CREATE ROLE non_super_user_clock;
+SET ROLE non_super_user_clock;
+SELECT citus_get_node_clock();
+BEGIN;
+SELECT citus_get_transaction_clock();
+COMMIT;
+
+-- Test setting the persisted clock value (it must fail)
+SELECT setval('pg_dist_clock_logical_seq', 100, true);
+
+\c
 RESET client_min_messages;
 RESET citus.enable_cluster_clock;
+DROP ROLE non_super_user_clock;
 DROP SCHEMA clock CASCADE;