Don't enter parallel mode when holding interrupts.

Doing so caused the leader to hang in wait_event=ParallelFinish, which
required an immediate shutdown to resolve.  Back-patch to v12 (all
supported versions).

Francesco Degrassi

Discussion: https://postgr.es/m/CAC-SaSzHUKT=vZJ8MPxYdC_URPfax+yoA1hKTcF4ROz_Q6z0_Q@mail.gmail.com

src/backend/optimizer/plan/planner.c

@@ -327,6 +327,11 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	 * if we want to allow parallel inserts in general; updates and deletes
 	 * have additional problems especially around combo CIDs.)
 	 *
+	 * We don't try to use parallel mode unless interruptible.  The leader
+	 * expects ProcessInterrupts() calls to reach HandleParallelMessages().
+	 * Even if we called HandleParallelMessages() another way, starting a
+	 * parallel worker is too delay-prone to be prudent when uncancellable.
+	 *
 	 * For now, we don't try to use parallel mode if we're running inside a
 	 * parallel worker.  We might eventually be able to relax this
 	 * restriction, but for now it seems best not to have parallel workers
@@ -337,6 +342,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 		parse->commandType == CMD_SELECT &&
 		!parse->hasModifyingCTE &&
 		max_parallel_workers_per_gather > 0 &&
+		INTERRUPTS_CAN_BE_PROCESSED() &&
 		!IsParallelWorker())
 	{
 		/* all the cheap tests pass, so scan the query tree */

src/test/regress/expected/select_parallel.out

@@ -1252,3 +1252,27 @@ reset force_parallel_mode;
 drop function set_and_report_role();
 drop function set_role_and_error(int);
 drop role regress_parallel_worker;
+-- don't freeze in ParallelFinish while holding an LWLock
+BEGIN;
+CREATE FUNCTION my_cmp (int4, int4)
+RETURNS int LANGUAGE sql AS
+$$
+	SELECT
+		CASE WHEN $1 < $2 THEN -1
+				WHEN $1 > $2 THEN  1
+				ELSE 0
+		END;
+$$;
+CREATE TABLE parallel_hang (i int4);
+INSERT INTO parallel_hang
+	(SELECT * FROM generate_series(1, 400) gs);
+CREATE OPERATOR CLASS int4_custom_ops FOR TYPE int4 USING btree AS
+	OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+	OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+	OPERATOR 5 > (int4, int4), FUNCTION 1 my_cmp(int4, int4);
+CREATE UNIQUE INDEX parallel_hang_idx
+					ON parallel_hang
+					USING btree (i int4_custom_ops);
+SET force_parallel_mode = on;
+DELETE FROM parallel_hang WHERE 380 <= i AND i <= 420;
+ROLLBACK;

src/test/regress/sql/select_parallel.sql

@@ -485,3 +485,34 @@ reset force_parallel_mode;
 drop function set_and_report_role();
 drop function set_role_and_error(int);
 drop role regress_parallel_worker;
+
+-- don't freeze in ParallelFinish while holding an LWLock
+BEGIN;
+
+CREATE FUNCTION my_cmp (int4, int4)
+RETURNS int LANGUAGE sql AS
+$$
+	SELECT
+		CASE WHEN $1 < $2 THEN -1
+				WHEN $1 > $2 THEN  1
+				ELSE 0
+		END;
+$$;
+
+CREATE TABLE parallel_hang (i int4);
+INSERT INTO parallel_hang
+	(SELECT * FROM generate_series(1, 400) gs);
+
+CREATE OPERATOR CLASS int4_custom_ops FOR TYPE int4 USING btree AS
+	OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+	OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+	OPERATOR 5 > (int4, int4), FUNCTION 1 my_cmp(int4, int4);
+
+CREATE UNIQUE INDEX parallel_hang_idx
+					ON parallel_hang
+					USING btree (i int4_custom_ops);
+
+SET force_parallel_mode = on;
+DELETE FROM parallel_hang WHERE 380 <= i AND i <= 420;
+
+ROLLBACK;