tsweb: implement interceptor for error page presentation

Updates https://github.com/tailscale/corp/issues/5605

Signed-off-by: Tom DNetto <tom@tailscale.com>

tsweb/tsweb.go

@@ -197,8 +197,16 @@ type HandlerOptions struct {
 	// codes for handled responses.
 	// The keys are HTTP numeric response codes e.g. 200, 404, ...
 	StatusCodeCountersFull *expvar.Map
+
+	// OnError is called if the handler returned a HTTPError. This
+	// is intended to be used to present pretty error pages if
+	// the user agent is determined to be a browser.
+	OnError ErrorHandlerFunc
 }
 
+// ErrorHandlerFunc is called to present a error response.
+type ErrorHandlerFunc func(http.ResponseWriter, *http.Request, HTTPError)
+
 // ReturnHandlerFunc is an adapter to allow the use of ordinary
 // functions as ReturnHandlers. If f is a function with the
 // appropriate signature, ReturnHandlerFunc(f) is a ReturnHandler that
@@ -287,7 +295,11 @@ func (h retHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			h.opts.Logf("[unexpected] HTTPError %v did not contain an HTTP status code, sending internal server error", hErr)
 			msg.Code = http.StatusInternalServerError
 		}
-		http.Error(lw, hErr.Msg, msg.Code)
+		if h.opts.OnError != nil {
+			h.opts.OnError(lw, r, hErr)
+		} else {
+			http.Error(lw, hErr.Msg, msg.Code)
+		}
 	case err != nil:
 		// Handler returned a generic error. Serve an internal server
 		// error, if necessary.

tsweb/tsweb_test.go

@@ -76,11 +76,12 @@ func TestStdHandler(t *testing.T) {
 	// cancel()
 
 	tests := []struct {
-		name     string
-		rh       ReturnHandler
-		r        *http.Request
-		wantCode int
-		wantLog  AccessLogRecord
+		name       string
+		rh         ReturnHandler
+		r          *http.Request
+		errHandler ErrorHandlerFunc
+		wantCode   int
+		wantLog    AccessLogRecord
 	}{
 		{
 			name:     "handler returns 200",
@@ -238,6 +239,26 @@ func TestStdHandler(t *testing.T) {
 				Code:       101,
 			},
 		},
+		{
+			name:     "error handler gets run",
+			rh:       handlerErr(0, Error(404, "not found", nil)), // status code changed in errHandler
+			r:        req(bgCtx, "http://example.com/"),
+			wantCode: 200,
+			errHandler: func(w http.ResponseWriter, r *http.Request, e HTTPError) {
+				http.Error(w, e.Msg, 200)
+			},
+			wantLog: AccessLogRecord{
+				When:       clock.Start,
+				Seconds:    1.0,
+				Proto:      "HTTP/1.1",
+				TLS:        false,
+				Host:       "example.com",
+				Method:     "GET",
+				Code:       404,
+				Err:        "not found",
+				RequestURI: "/",
+			},
+		},
 	}
 
 	for _, test := range tests {
@@ -253,7 +274,7 @@ func TestStdHandler(t *testing.T) {
 			clock.Reset()
 
 			rec := noopHijacker{httptest.NewRecorder(), false}
-			h := StdHandler(test.rh, HandlerOptions{Logf: logf, Now: clock.Now})
+			h := StdHandler(test.rh, HandlerOptions{Logf: logf, Now: clock.Now, OnError: test.errHandler})
 			h.ServeHTTP(&rec, test.r)
 			res := rec.Result()
 			if res.StatusCode != test.wantCode {