Default cableBackend to authBackend

Docs already mention this but we are actually defaulting this to
localhost:8080

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65068

workhorse/config_test.go

@@ -101,6 +101,38 @@ func TestConfigDefaults(t *testing.T) {
 	require.Equal(t, expectedCfg, cfg)
 }
 
+func TestCableConfigDefault(t *testing.T) {
+	backendURL, err := url.Parse("http://localhost:1234")
+	require.NoError(t, err)
+
+	args := []string{
+		"-authBackend", backendURL.String(),
+	}
+	boot, cfg, err := buildConfig("test", args)
+	require.NoError(t, err, "build config")
+
+	expectedBoot := &bootConfig{
+		secretPath:    "./.gitlab_workhorse_secret",
+		listenAddr:    "localhost:8181",
+		listenNetwork: "tcp",
+		logFormat:     "text",
+	}
+
+	require.Equal(t, expectedBoot, boot)
+
+	expectedCfg := &config.Config{
+		Backend:                  backendURL,
+		CableBackend:             backendURL,
+		Version:                  "(unknown version)",
+		DocumentRoot:             "public",
+		ProxyHeadersTimeout:      5 * time.Minute,
+		APIQueueTimeout:          queueing.DefaultTimeout,
+		APICILongPollingDuration: 50 * time.Nanosecond,
+		ImageResizerConfig:       config.DefaultImageResizerConfig,
+	}
+	require.Equal(t, expectedCfg, cfg)
+}
+
 func TestConfigFlagParsing(t *testing.T) {
 	backendURL, err := url.Parse("http://localhost:1234")
 	require.NoError(t, err)

workhorse/main.go

@@ -95,7 +95,7 @@ func buildConfig(arg0 string, args []string) (*bootConfig, *config.Config, error
 	fset.StringVar(&cfg.Socket, "authSocket", "", "Optional: Unix domain socket to dial authBackend at")
 
 	// actioncable backend
-	cableBackend := fset.String("cableBackend", upstream.DefaultBackend.String(), "ActionCable backend")
+	cableBackend := fset.String("cableBackend", "", "ActionCable backend")
 	fset.StringVar(&cfg.CableSocket, "cableSocket", "", "Optional: Unix domain socket to dial cableBackend at")
 
 	fset.StringVar(&cfg.DocumentRoot, "documentRoot", "public", "Path to static files content")
@@ -123,9 +123,14 @@ func buildConfig(arg0 string, args []string) (*bootConfig, *config.Config, error
 		return nil, nil, fmt.Errorf("authBackend: %v", err)
 	}
 
-	cfg.CableBackend, err = parseAuthBackend(*cableBackend)
-	if err != nil {
-		return nil, nil, fmt.Errorf("cableBackend: %v", err)
+	if *cableBackend != "" {
+		// A custom -cableBackend has been specified
+		cfg.CableBackend, err = parseAuthBackend(*cableBackend)
+		if err != nil {
+			return nil, nil, fmt.Errorf("cableBackend: %v", err)
+		}
+	} else {
+		cfg.CableBackend = cfg.Backend
 	}
 
 	tomlData := ""