introduce TASK_QUERY_NULL task type (#3774)

When we call SetTaskQueryString we would set the task type to
TASK_QUERY_TEXT, and some parts of the codebase rely on the fact that if
TASK_QUERY_TEXT is set, the data can be read safely. However if
SetTaskQueryString is called with a NULL taskQueryString this can cause
crashes. In that case taskQueryType will simply be set to
TASK_QUERY_NULL.

src/backend/distributed/planner/deparse_shard_query.c

@@ -443,8 +443,15 @@ SetTaskQueryIfShouldLazyDeparse(Task *task, Query *query)
 void
 SetTaskQueryString(Task *task, char *queryString)
 {
-	task->taskQuery.queryType = TASK_QUERY_TEXT;
-	task->taskQuery.data.queryStringLazy = queryString;
+	if (queryString == NULL)
+	{
+		task->taskQuery.queryType = TASK_QUERY_NULL;
+	}
+	else
+	{
+		task->taskQuery.queryType = TASK_QUERY_TEXT;
+		task->taskQuery.data.queryStringLazy = queryString;
+	}
 }
 
 
@@ -466,6 +473,7 @@ SetTaskPerPlacementQueryStrings(Task *task, List *perPlacementQueryStringList)
 void
 SetTaskQueryStringList(Task *task, List *queryStringList)
 {
+	Assert(queryStringList != NIL);
 	task->taskQuery.queryType = TASK_QUERY_TEXT_LIST;
 	task->taskQuery.data.queryStringList = queryStringList;
 }
@@ -520,16 +528,32 @@ GetTaskQueryType(Task *task)
 char *
 TaskQueryStringForAllPlacements(Task *task)
 {
-	if (GetTaskQueryType(task) == TASK_QUERY_TEXT_LIST)
+	int taskQueryType = GetTaskQueryType(task);
+	if (taskQueryType == TASK_QUERY_NULL)
+	{
+		/* if task query type is TASK_QUERY_NULL then the data will be NULL,
+		 * this is unexpected state */
+		ereport(ERROR, (errcode(ERRCODE_INTERNAL_ERROR),
+						errmsg("unexpected task query state: task query type is null"),
+						errdetail("Please report this to the Citus core team.")));
+	}
+	else if (taskQueryType == TASK_QUERY_TEXT_LIST)
 	{
 		return StringJoin(task->taskQuery.data.queryStringList, ';');
 	}
-	if (GetTaskQueryType(task) == TASK_QUERY_TEXT)
+	else if (taskQueryType == TASK_QUERY_TEXT)
 	{
 		return task->taskQuery.data.queryStringLazy;
 	}
+
 	Query *jobQueryReferenceForLazyDeparsing =
 		task->taskQuery.data.jobQueryReferenceForLazyDeparsing;
+
+	/*
+	 *	At this point task query type should be TASK_QUERY_OBJECT.
+	 *  if someone calls this method inappropriately with TASK_QUERY_TEXT_PER_PLACEMENT case
+	 *  (instead of TaskQueryStringForPlacement), they will hit this assert.
+	 */
 	Assert(task->taskQuery.queryType == TASK_QUERY_OBJECT &&
 		   jobQueryReferenceForLazyDeparsing != NULL);
 

src/include/distributed/multi_physical_planner.h

@@ -205,6 +205,7 @@ typedef struct TaskExecution TaskExecution;
 
 typedef enum TaskQueryType
 {
+	TASK_QUERY_NULL,
 	TASK_QUERY_TEXT,
 	TASK_QUERY_OBJECT,
 	TASK_QUERY_TEXT_PER_PLACEMENT,

src/test/regress/expected/multi_mx_reference_table.out

@@ -909,6 +909,37 @@ LOG:  join order: [ "colocated_table_test" ][ reference join "reference_table_te
 
 SET client_min_messages TO NOTICE;
 SET citus.log_multi_join_order TO FALSE;
--- clean up tables
 \c - - - :master_port
-DROP TABLE reference_table_test, reference_table_test_second, reference_table_test_third;;
+-- issue 3766
+CREATE TABLE numbers(a int);
+SELECT create_reference_table('numbers');
+ create_reference_table
+---------------------------------------------------------------------
+
+(1 row)
+
+SET client_min_messages TO debug4;
+INSERT INTO numbers VALUES (1), (2), (3), (4);
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+DEBUG:  query before rebuilding: (null)
+DEBUG:  query after rebuilding:  INSERT INTO public.numbers_1250015 AS citus_table_alias (a) VALUES (1), (2), (3), (4)
+DEBUG:  assigned task to node localhost:xxxxx
+DEBUG:  opening 1 new connections to localhost:xxxxx
+DEBUG:  established connection to localhost:xxxxx for session 4
+DEBUG:  opening 1 new connections to localhost:xxxxx
+DEBUG:  established connection to localhost:xxxxx for session 5
+DEBUG:  Total number of commands sent over the session 4: 1
+DEBUG:  Total number of commands sent over the session 5: 1
+SELECT count(*) FROM numbers;
+DEBUG:  Distributed planning for a fast-path router query
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+ count
+---------------------------------------------------------------------
+     4
+(1 row)
+
+RESET client_min_messages;
+-- clean up tables
+DROP TABLE reference_table_test, reference_table_test_second, reference_table_test_third, numbers;

src/test/regress/sql/multi_mx_reference_table.sql

@@ -557,6 +557,15 @@ ORDER BY 1;
 SET client_min_messages TO NOTICE;
 SET citus.log_multi_join_order TO FALSE;
 
--- clean up tables
 \c - - - :master_port
-DROP TABLE reference_table_test, reference_table_test_second, reference_table_test_third;;
+
+-- issue 3766
+CREATE TABLE numbers(a int);
+SELECT create_reference_table('numbers');
+SET client_min_messages TO debug4;
+INSERT INTO numbers VALUES (1), (2), (3), (4);
+SELECT count(*) FROM numbers;
+RESET client_min_messages;
+
+-- clean up tables
+DROP TABLE reference_table_test, reference_table_test_second, reference_table_test_third, numbers;