Assert that ExecOpenIndices and ExecCloseIndices are not repeated.

These functions should be called at most once per ResultRelInfo;
it's wasteful to do otherwise, and certainly the pattern of
opening twice and then closing twice is a bad idea.  Moreover,
aminsertcleanup functions might not be prepared to be called twice,
as the just-hardened code in BRIN demonstrates.

This amounts to an API change, since such coding patterns were
safe even if wasteful before v17.  Hence, apply to HEAD only.
(Extension code violating this new rule faces some risk in v17,
but we just fixed brininsertcleanup and there are probably few
other aminsertcleanup functions as yet.  So the odds of breaking
usable code seem higher than the odds of doing something useful
with a back-patch.)

Bug: #18815
Reported-by: Sergey Belyashov <sergey.belyashov@gmail.com>
Discussion: https://postgr.es/m/18815-2a0407cc7f40b327@postgresql.org

src/backend/executor/execIndexing.c

@@ -181,6 +181,9 @@ ExecOpenIndices(ResultRelInfo *resultRelInfo, bool speculative)
 	if (len == 0)
 		return;
 
+	/* This Assert will fail if ExecOpenIndices is called twice */
+	Assert(resultRelInfo->ri_IndexRelationDescs == NULL);
+
 	/*
 	 * allocate space for result arrays
 	 */
@@ -246,19 +249,23 @@ ExecCloseIndices(ResultRelInfo *resultRelInfo)
 
 	for (i = 0; i < numIndices; i++)
 	{
-		if (indexDescs[i] == NULL)
-			continue;			/* shouldn't happen? */
+		/* This Assert will fail if ExecCloseIndices is called twice */
+		Assert(indexDescs[i] != NULL);
 
 		/* Give the index a chance to do some post-insert cleanup */
 		index_insert_cleanup(indexDescs[i], indexInfos[i]);
 
 		/* Drop lock acquired by ExecOpenIndices */
 		index_close(indexDescs[i], RowExclusiveLock);
+
+		/* Mark the index as closed */
+		indexDescs[i] = NULL;
 	}
 
 	/*
-	 * XXX should free indexInfo array here too?  Currently we assume that
-	 * such stuff will be cleaned up automatically in FreeExecutorState.
+	 * We don't attempt to free the IndexInfo data structures or the arrays,
+	 * instead assuming that such stuff will be cleaned up automatically in
+	 * FreeExecutorState.
 	 */
 }