Added a test to check for alternative host on timeout

ably/rest_client_integration_test.go

@@ -337,8 +337,7 @@ func TestRest_hostfallback(t *testing.T) {
 		client, err := ably.NewREST(app.Options(append(options, ably.WithHTTPClient(newHTTPClientMock(server)))...)...)
 		assert.NoError(t, err)
 		err = client.Channels.Get("test").Publish(context.Background(), "ping", "pong")
-		assert.Error(t, err,
-			"expected an error")
+		assert.Error(t, err, "expected an error")
 		return retryCount, hosts
 	}
 	t.Run("RSC15d RSC15a must use alternative host", func(t *testing.T) {
@@ -368,6 +367,56 @@ func TestRest_hostfallback(t *testing.T) {
 			uniq[h] = true
 		}
 	})
+
+	runTestServerWithRequestTimeout := func(t *testing.T, options []ably.ClientOption) (int, []string) {
+		var retryCount int
+		var hosts []string
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			hosts = append(hosts, r.Host)
+			retryCount++
+			time.Sleep(2 * time.Second)
+			w.WriteHeader(http.StatusInternalServerError)
+		}))
+		defer server.Close()
+		httpClientMock := &http.Client{
+			Timeout: 1 * time.Second,
+			Transport: &http.Transport{
+				Proxy: func(*http.Request) (*url.URL, error) { return url.Parse(server.URL) },
+			},
+		}
+		client, err := ably.NewREST(app.Options(append(options, ably.WithHTTPClient(httpClientMock))...)...)
+		assert.NoError(t, err)
+		err = client.Channels.Get("test").Publish(context.Background(), "ping", "pong")
+		assert.Contains(t, err.Error(), "context deadline exceeded (Client.Timeout exceeded while awaiting headers)")
+		return retryCount, hosts
+	}
+
+	t.Run("RSC15l2 must use alternative host on timeout", func(t *testing.T) {
+
+		options := []ably.ClientOption{
+			ably.WithFallbackHosts(ably.DefaultFallbackHosts()),
+			ably.WithTLS(false),
+			ably.WithUseTokenAuth(true),
+		}
+		retryCount, hosts := runTestServerWithRequestTimeout(t, options)
+		assert.Equal(t, 4, retryCount, "expected 4 http calls got %d", retryCount)
+		// make sure the host header is set. Since we are using defaults from the spec
+		// the hosts should be in [a..e].ably-realtime.com
+		expect := strings.Join(ably.DefaultFallbackHosts(), ", ")
+		for _, host := range hosts[1:] {
+			assert.Contains(t, expect, host, "expected %s got be in %s", host, expect)
+		}
+
+		// ensure all picked fallbacks are unique
+		uniq := make(map[string]bool)
+		for _, h := range hosts {
+			_, ok := uniq[h]
+			assert.False(t, ok,
+				"duplicate fallback %s", h)
+			uniq[h] = true
+		}
+	})
+
 	t.Run("rsc15b", func(t *testing.T) {
 		t.Run("must not occur when default rest.ably.io is overridden", func(t *testing.T) {