Mop up some undue familiarity with the innards of Bitmapsets.

nodeAppend.c used non-nullness of appendstate->as_valid_subplans as
a state flag to indicate whether it'd done ExecFindMatchingSubPlans
(or some sufficient approximation to that).  This was pretty
questionable even in the beginning, since it wouldn't really work
right if there are no valid subplans.  It got more questionable
after commit 27e1f1456 added logic that could reduce as_valid_subplans
to an empty set: at that point we were depending on unspecified
behavior of bms_del_members, namely that it'd not return an empty
set as NULL.  It's about to start doing that, which breaks this
logic entirely.  Hence, add a separate boolean flag to signal
whether as_valid_subplans has been computed.

Also fix a previously-cosmetic bug in nodeAgg.c, wherein it ignored
the return value of bms_del_member instead of updating its pointer.

Patch by me; thanks to Nathan Bossart and Richard Guo for review.

Discussion: https://postgr.es/m/1159933.1677621588@sss.pgh.pa.us

src/backend/executor/nodeAgg.c

@@ -1642,7 +1642,7 @@ find_hash_columns(AggState *aggstate)
 			perhash->hashGrpColIdxHash[i] = i + 1;
 			perhash->numhashGrpCols++;
 			/* delete already mapped columns */
-			bms_del_member(colnos, grpColIdx[i]);
+			colnos = bms_del_member(colnos, grpColIdx[i]);
 		}
 
 		/* and add the remaining columns */

src/backend/executor/nodeAppend.c

@@ -157,7 +157,10 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 		 * later calls to ExecFindMatchingSubPlans.
 		 */
 		if (!prunestate->do_exec_prune && nplans > 0)
+		{
 			appendstate->as_valid_subplans = bms_add_range(NULL, 0, nplans - 1);
+			appendstate->as_valid_subplans_identified = true;
+		}
 	}
 	else
 	{
@@ -170,6 +173,7 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 		Assert(nplans > 0);
 		appendstate->as_valid_subplans = validsubplans =
 			bms_add_range(NULL, 0, nplans - 1);
+		appendstate->as_valid_subplans_identified = true;
 		appendstate->as_prune_state = NULL;
 	}
 
@@ -259,7 +263,7 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 		appendstate->as_asyncresults = (TupleTableSlot **)
 			palloc0(nasyncplans * sizeof(TupleTableSlot *));
 
-		if (appendstate->as_valid_subplans != NULL)
+		if (appendstate->as_valid_subplans_identified)
 			classify_matching_subplans(appendstate);
 	}
 
@@ -414,14 +418,12 @@ ExecReScanAppend(AppendState *node)
 		bms_overlap(node->ps.chgParam,
 					node->as_prune_state->execparamids))
 	{
+		node->as_valid_subplans_identified = false;
 		bms_free(node->as_valid_subplans);
 		node->as_valid_subplans = NULL;
-		if (nasyncplans > 0)
-		{
 		bms_free(node->as_valid_asyncplans);
 		node->as_valid_asyncplans = NULL;
 	}
-	}
 
 	for (i = 0; i < node->as_nplans; i++)
 	{
@@ -574,11 +576,14 @@ choose_next_subplan_locally(AppendState *node)
 		if (node->as_nasyncplans > 0)
 		{
 			/* We'd have filled as_valid_subplans already */
-			Assert(node->as_valid_subplans);
+			Assert(node->as_valid_subplans_identified);
 		}
-		else if (node->as_valid_subplans == NULL)
+		else if (!node->as_valid_subplans_identified)
+		{
 			node->as_valid_subplans =
 				ExecFindMatchingSubPlans(node->as_prune_state, false);
+			node->as_valid_subplans_identified = true;
+		}
 
 		whichplan = -1;
 	}
@@ -640,10 +645,11 @@ choose_next_subplan_for_leader(AppendState *node)
 		 * run-time pruning is disabled then the valid subplans will always be
 		 * set to all subplans.
 		 */
-		if (node->as_valid_subplans == NULL)
+		if (!node->as_valid_subplans_identified)
 		{
 			node->as_valid_subplans =
 				ExecFindMatchingSubPlans(node->as_prune_state, false);
+			node->as_valid_subplans_identified = true;
 
 			/*
 			 * Mark each invalid plan as finished to allow the loop below to
@@ -715,10 +721,12 @@ choose_next_subplan_for_worker(AppendState *node)
 	 * run-time pruning is disabled then the valid subplans will always be set
 	 * to all subplans.
 	 */
-	else if (node->as_valid_subplans == NULL)
+	else if (!node->as_valid_subplans_identified)
 	{
 		node->as_valid_subplans =
 			ExecFindMatchingSubPlans(node->as_prune_state, false);
+		node->as_valid_subplans_identified = true;
+
 		mark_invalid_subplans_as_finished(node);
 	}
 
@@ -866,10 +874,11 @@ ExecAppendAsyncBegin(AppendState *node)
 	Assert(node->as_nasyncplans > 0);
 
 	/* If we've yet to determine the valid subplans then do so now. */
-	if (node->as_valid_subplans == NULL)
+	if (!node->as_valid_subplans_identified)
 	{
 		node->as_valid_subplans =
 			ExecFindMatchingSubPlans(node->as_prune_state, false);
+		node->as_valid_subplans_identified = true;
 
 		classify_matching_subplans(node);
 	}
@@ -1143,6 +1152,7 @@ classify_matching_subplans(AppendState *node)
 {
 	Bitmapset  *valid_asyncplans;
 
+	Assert(node->as_valid_subplans_identified);
 	Assert(node->as_valid_asyncplans == NULL);
 
 	/* Nothing to do if there are no valid subplans. */
@@ -1161,8 +1171,7 @@ classify_matching_subplans(AppendState *node)
 	}
 
 	/* Get valid async subplans. */
-	valid_asyncplans = bms_copy(node->as_asyncplans);
+	valid_asyncplans = bms_intersect(node->as_asyncplans,
-	valid_asyncplans = bms_int_members(valid_asyncplans,
 									 node->as_valid_subplans);
 
 	/* Adjust the valid subplans to contain sync subplans only. */

src/include/nodes/execnodes.h

@@ -1350,6 +1350,7 @@ struct AppendState
 	ParallelAppendState *as_pstate; /* parallel coordination info */
 	Size		pstate_len;		/* size of parallel coordination info */
 	struct PartitionPruneState *as_prune_state;
+	bool		as_valid_subplans_identified;	/* is as_valid_subplans valid? */
 	Bitmapset  *as_valid_subplans;
 	Bitmapset  *as_valid_asyncplans;	/* valid asynchronous plans indexes */
 	bool		(*choose_next_subplan) (AppendState *);