fix: Add latency-check for DERP over HTTP(s) (#3788)

* fix: Add latency-check for DERP over HTTP(s)

This fixes scenarios where latency wasn't being reported if
a connection had UDP entirely blocked.

* Add inactivity ping

* Improve coordinator error reporting consistency

coderd/coderd.go

@@ -165,7 +165,13 @@ func New(options *Options) *API {
 	// other applications might not as well.
 	r.Route("/%40{user}/{workspace_and_agent}/apps/{workspaceapp}", apps)
 	r.Route("/@{user}/{workspace_and_agent}/apps/{workspaceapp}", apps)
-	r.Get("/derp", derphttp.Handler(api.derpServer).ServeHTTP)
+	r.Route("/derp", func(r chi.Router) {
+		r.Get("/", derphttp.Handler(api.derpServer).ServeHTTP)
+		// This is used when UDP is blocked, and latency must be checked via HTTP(s).
+		r.Get("/latency-check", func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})
+	})
 
 	r.Route("/api/v2", func(r chi.Router) {
 		r.NotFound(func(rw http.ResponseWriter, r *http.Request) {

coderd/coderd_test.go

@@ -2,6 +2,7 @@ package coderd_test
 
 import (
 	"context"
+	"net/http"
 	"net/netip"
 	"strconv"
 	"testing"
@@ -114,3 +115,12 @@ func TestDERP(t *testing.T) {
 	w1.Close()
 	w2.Close()
 }
+
+func TestDERPLatencyCheck(t *testing.T) {
+	t.Parallel()
+	client := coderdtest.New(t, nil)
+	res, err := client.Request(context.Background(), http.MethodGet, "/derp/latency-check", nil)
+	require.NoError(t, err)
+	defer res.Body.Close()
+	require.Equal(t, http.StatusOK, res.StatusCode)
+}

coderd/coderdtest/authtest.go

@@ -168,6 +168,7 @@ func AGPLRoutes(a *AuthTester) (map[string]string, map[string]RouteCheck) {
 	skipRoutes := map[string]string{
 		"POST:/api/v2/users/logout": "Logging out deletes the API Key for other routes",
 		"GET:/derp":                 "This requires a WebSocket upgrade!",
+		"GET:/derp/latency-check":   "This always returns a 200!",
 	}
 
 	assertRoute := map[string]RouteCheck{