Add standing unit test for #4 (#5)

Just as a precautionary measure, this updates an existing unit test to
specifically check whether the fix introduced in #4 works as expected.
This has been tested by reverting #4 and checking that it does indeed
trigger the panic, and with #4 the panic is fixed.

cache_test.go

@@ -338,30 +338,36 @@ func TestNotifyMetadata(t *testing.T) {
 // loop. The loop should abort and not call the remaining subscribers. It should instead restart and run through each
 // subscriber with the updated value.
 func TestWatchableValueUpdateCancel(t *testing.T) {
+	prefix := ads.XDSTPScheme + "/" + diderot.TypeOf[*Timestamp]().TrimmedURL() + "/foo/"
+
 	c := newCache()
 
-	r1 := newResource(name1, "0")
+	r1 := newResource(prefix+name1, "0")
 
 	var r1Wg, r2Wg sync.WaitGroup
 	r1Wg.Add(1)
-	r2Wg.Add(2)
+	r2Wg.Add(3)
 	notify := func(name string, r *ads.Resource[*Timestamp], _ ads.SubscriptionMetadata) {
 		// r is nil during the initial invocation of notify since the resource does not yet exist.
 		if r == nil {
 			return
 		}
 
 		if r == r1 {
-			c.Set(name1, "1", Now(), noTime)
+			c.Set(prefix+name1, "1", Now(), noTime)
 			// if notify is invoked with r1 more than once, this will panic
 			r1Wg.Done()
 		} else {
 			r2Wg.Done()
 		}
 	}
 
-	c.Subscribe(name1, testutils.NewSubscriptionHandler(notify))
-	c.Subscribe(name1, testutils.NewSubscriptionHandler(notify))
+	c.Subscribe(prefix+name1, testutils.NewSubscriptionHandler(notify))
+	// Use a glob and wildcard subscriptions, this flexes the code path in watchableValue that iterates
+	// through the various subscriber sets, and had a bug where breaking before exhausting the iterator
+	// would panic.
+	c.Subscribe(prefix+ads.WildcardSubscription, testutils.NewSubscriptionHandler(notify))
+	c.Subscribe(ads.WildcardSubscription, testutils.NewSubscriptionHandler(notify))
 
 	c.SetResource(r1, noTime)