added test for host unresolvable or unreachable

ably/rest_client_integration_test.go

@@ -400,6 +400,27 @@ func TestRest_RSC15_HostFallback(t *testing.T) {
 		assertUnique(t, hosts)                                  // ensure all picked fallbacks are unique
 	})
 
+	t.Run("RSC15l1 must use alternative host on host unresolvable or unreachable", func(t *testing.T) {
+		options := []ably.ClientOption{
+			ably.WithFallbackHosts(ably.DefaultFallbackHosts()),
+			ably.WithRESTHost("foobar.ably.com"),
+			ably.WithFallbackHosts([]string{
+				"spam.ably.com",
+				"tatto.ably.com",
+				"rest.ably.io"}),
+			ably.WithTLS(false),
+			ably.WithUseTokenAuth(true),
+		}
+		client, err := ably.NewREST(app.Options(options...)...)
+		assert.NoError(t, err)
+		tm, err := client.Time(context.Background())
+		assert.Nil(t, err)
+		assert.NotNil(t, tm)
+		time.Sleep(2 * time.Second)
+		cachedFallbackHost := client.GetCachedFallbackHost()
+		assert.Equal(t, "rest.ably.io", cachedFallbackHost)
+	})
+
 	t.Run("rsc15b", func(t *testing.T) {
 		t.Run("must not occur when default rest.ably.io is overridden", func(t *testing.T) {