feat: Workspace Proxy picker show latency to each proxy (#7486)

* chore: Add cors to workspace proxies to allow for latency checks
* Add latency check to wsproxy

Use performance API timings.
- Fix cors and timing headers
- Accept custom headers

coderd/coderd.go

@@ -806,6 +806,17 @@ func New(options *Options) *API {
 		return []string{}
 	})
 	r.NotFound(cspMW(compressHandler(http.HandlerFunc(api.siteHandler.ServeHTTP))).ServeHTTP)
+
+	// This must be before all middleware to improve the response time.
+	// So make a new router, and mount the old one as the root.
+	rootRouter := chi.NewRouter()
+	// This is the only route we add before all the middleware.
+	// We want to time the latency of the request, so any middleware will
+	// interfere with that timing.
+	rootRouter.Get("/latency-check", LatencyCheck(api.AccessURL))
+	rootRouter.Mount("/", r)
+	api.RootHandler = rootRouter
+
 	return api
 }
 

coderd/coderd_test.go

@@ -124,6 +124,15 @@ func TestDERPLatencyCheck(t *testing.T) {
 	require.Equal(t, http.StatusOK, res.StatusCode)
 }
 
+func TestFastLatencyCheck(t *testing.T) {
+	t.Parallel()
+	client := coderdtest.New(t, nil)
+	res, err := client.Request(context.Background(), http.MethodGet, "/latency-check", nil)
+	require.NoError(t, err)
+	defer res.Body.Close()
+	require.Equal(t, http.StatusOK, res.StatusCode)
+}
+
 func TestHealthz(t *testing.T) {
 	t.Parallel()
 	client := coderdtest.New(t, nil)

coderd/latencycheck.go

@@ -0,0 +1,24 @@
+package coderd
+
+import (
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+func LatencyCheck(allowedOrigins ...*url.URL) http.HandlerFunc {
+	allowed := make([]string, 0, len(allowedOrigins))
+	for _, origin := range allowedOrigins {
+		// Allow the origin without a path
+		tmp := *origin
+		tmp.Path = ""
+		allowed = append(allowed, strings.TrimSuffix(origin.String(), "/"))
+	}
+	origins := strings.Join(allowed, ",")
+	return func(rw http.ResponseWriter, r *http.Request) {
+		// Allowing timing information to be shared. This allows the browser
+		// to exclude TLS handshake timing.
+		rw.Header().Set("Timing-Allow-Origin", origins)
+		rw.WriteHeader(http.StatusOK)
+	}
+}