Fix Assert failure in new plancache code.

The regression tests were failing with CLOBBER_CACHE_ALWAYS enabled,
as reported by buildfarm member jaguar.  There was an Assert in
BuildCachedPlan that asserted that the CachedPlanSource hadn't been
invalidated since we called RevalidateCachedQuery, which in theory can't
happen because we are holding locks on all the relevant database objects.
However, CLOBBER_CACHE_ALWAYS generates a false positive by making an
invalidation happen anyway; and on reflection, that could also occur as a
result of a badly-timed sinval reset due to queue overflow.  We could just
remove the Assert and forge ahead with the not-really-stale querytree, but
it seems safer to do another RevalidateCachedQuery call just to make real
sure everything's OK.

src/backend/utils/cache/plancache.c

@@ -718,8 +718,21 @@ BuildCachedPlan(CachedPlanSource *plansource, List *qlist,
 	MemoryContext plan_context;
 	MemoryContext oldcxt;
 
-	/* Assert that caller checked the querytree */
-	Assert(plansource->is_valid);
+	/*
+	 * Normally the querytree should be valid already, but if it's not,
+	 * rebuild it.
+	 *
+	 * NOTE: GetCachedPlan should have called RevalidateCachedQuery first, so
+	 * we ought to be holding sufficient locks to prevent any invalidation.
+	 * However, if we're building a custom plan after having built and
+	 * rejected a generic plan, it's possible to reach here with is_valid
+	 * false due to an invalidation while making the generic plan.  In theory
+	 * the invalidation must be a false positive, perhaps a consequence of an
+	 * sinval reset event or the CLOBBER_CACHE_ALWAYS debug code.  But for
+	 * safety, let's treat it as real and redo the RevalidateCachedQuery call.
+	 */
+	if (!plansource->is_valid)
+		qlist = RevalidateCachedQuery(plansource);
 
 	/*
 	 * If we don't already have a copy of the querytree list that can be