Fix assertion failure with incorrect number of workers

In 18~ (581df2148737), the previous code causes an assertion failure in
the planner regarding the fact that a gather node should always have at
least 1 worker to spawn, but we were initializing this number to 0 in
pg_hint_plan when recalculating the number of workers to use for an
AppendPath.  So pg_hint_plan is wrong, and upstream is right, because a
gather node should always ask for at least 1 parallel worker.

Some tests of in ut-W are impacted here because this causes an increase
in the number of workers to spawn, based on what
max_parallel_workers_per_gather is set to.

Note for v17 and older branches: this change has been originally applied
as of 3c2e9c5, and has been backpatched to all the stable branches
following issue #218, that reported that the same assertion failure can
be reached, messing with the state of Gather node paths in the backend
with an incorrect worker number compilation.

Author: William Zhang, Michael Paquier
Batchpatch-through: 13

Per issues #93 and #218.

Author: michaelpq

expected/ut-W.out

@@ -1224,7 +1224,7 @@ Parallel()
                    QUERY PLAN                   
 ------------------------------------------------
  Gather
-   Workers Planned: 1
+   Workers Planned: 2
    ->  Parallel Append
          ->  Parallel Seq Scan on p1_c2 p1_2
          ->  Parallel Seq Scan on p1_c4 p1_4

pg_hint_plan.c

@@ -4832,13 +4832,17 @@ pg_hint_plan_set_rel_pathlist(PlannerInfo * root, RelOptInfo *rel,
 
 		foreach(lc, rel->partial_pathlist)
 		{
-			ListCell *lcp;
-			AppendPath *apath = (AppendPath *) lfirst(lc);
-			int		parallel_workers = 0;
+			ListCell   *lcp;
+			Node	   *path = (Node *) lfirst(lc);
+			AppendPath *apath;
+			int			parallel_workers;
 
-			if (!IsA(apath, AppendPath))
+			if (!IsA(path, AppendPath))
 				continue;
 
+			apath = (AppendPath *) path;
+			parallel_workers = apath->path.parallel_workers;
+
 			foreach (lcp, apath->subpaths)
 			{
 				Path *spath = (Path *) lfirst(lcp);
@@ -4848,7 +4852,8 @@ pg_hint_plan_set_rel_pathlist(PlannerInfo * root, RelOptInfo *rel,
 					parallel_workers = spath->parallel_workers;
 			}
 
-			apath->path.parallel_workers = parallel_workers;
+			if (apath->path.parallel_workers < parallel_workers)
+				apath->path.parallel_workers = parallel_workers;
 			inhibit_nonparallel = true;
 		}