client: RouteCache should account for empty Route

This can happen if a journal has no current broker assignments.

broker/client/route_cache.go

@@ -30,10 +30,8 @@ func NewRouteCache(size int, ttl time.Duration) *RouteCache {
 
 // UpdateRoute caches the provided Route for |item|, or invalidates it if |route| is nil.
 func (rc *RouteCache) UpdateRoute(item string, route *pb.Route) {
-	if route == nil {
+	if route == nil || len(route.Members) == 0 {
 		rc.cache.Remove(item)
-	} else if len(route.Members) == 0 {
-		panic("expected members")
 	} else {
 		var cr = cachedRoute{
 			route: route.Copy(),

broker/client/route_cache_test.go

@@ -30,8 +30,9 @@ func (s *RouteCacheSuite) TestCachingCases(c *gc.C) {
 	// Case: Routes which have fallen out of cache are not.
 	c.Check(rc.Route(ctx, "A"), gc.DeepEquals, pb.Route{Primary: -1}) // Miss.
 
-	// Case: Invalidations remove routes from the cache.
+	// Case: Nil or empty routes invalidate the cache.
 	rc.UpdateRoute("C", nil)
+	rc.UpdateRoute("C", new(pb.Route))
 	c.Check(rc.Route(ctx, "C"), gc.DeepEquals, pb.Route{Primary: -1}) // Miss.
 
 	// Case: TTLs are enforced.