Fix / -> /graph redirect when using a path prefix.

util/route/route.go

@@ -85,6 +85,13 @@ func (r *Router) Post(path string, h http.HandlerFunc) {
 	r.rtr.POST(r.prefix+path, handle(h))
 }
 
+// Redirect takes an absolute path and sends an internal HTTP redirect for it,
+// prefixed by the router's path prefix. Note that this method does not include
+// functionality for handling relative paths or full URL redirects.
+func (r *Router) Redirect(w http.ResponseWriter, req *http.Request, path string, code int) {
+	http.Redirect(w, req, r.prefix+path, code)
+}
+
 // ServeHTTP implements http.Handler.
 func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	r.rtr.ServeHTTP(w, req)

util/route/route_test.go

@@ -0,0 +1,27 @@
+package route
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestRedirect(t *testing.T) {
+	router := New().WithPrefix("/test/prefix")
+	w := httptest.NewRecorder()
+	r, err := http.NewRequest("GET", "http://localhost:9090/foo", nil)
+	if err != nil {
+		t.Fatalf("Error building test request: %s", err)
+	}
+
+	router.Redirect(w, r, "/some/endpoint", http.StatusFound)
+	if w.Code != http.StatusFound {
+		t.Fatalf("Unexpected redirect status code: got %d, want %d", w.Code, http.StatusFound)
+	}
+
+	want := "/test/prefix/some/endpoint"
+	got := w.Header()["Location"][0]
+	if want != got {
+		t.Fatalf("Unexpected redirect location: got %s, want %s", got, want)
+	}
+}

web/web.go

@@ -160,7 +160,7 @@ func New(st local.Storage, qe *promql.Engine, rm *rules.Manager, status *Prometh
 	instrf := prometheus.InstrumentHandlerFunc
 
 	router.Get("/", func(w http.ResponseWriter, r *http.Request) {
-		http.Redirect(w, r, "/graph", http.StatusFound)
+		router.Redirect(w, r, "/graph", http.StatusFound)
 	})
 	router.Get("/graph", instrf("graph", h.graph))