tsweb: log cancelled requests as 499

Fixes #12860

Signed-off-by: Paul Scott <paul@tailscale.com>

tsweb/tsweb.go

@@ -472,15 +472,9 @@ func (h logHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	// Let errorHandler tell us what error it wrote to the client.
 	r = r.WithContext(errCallback.WithValue(ctx, func(e string) {
-		if ctx.Err() == context.Canceled {
-			msg.Code = 499 // nginx convention: Client Closed Request
-			msg.Err = context.Canceled.Error()
-			return
-		}
-		if msg.Err != "" {
-			return
+		if msg.Err == "" {
+			msg.Err = e // Keep the first error.
 		}
-		msg.Err = e
 	}))
 
 	lw := newLogResponseWriter(h.opts.Logf, w, r)
@@ -513,9 +507,14 @@ func (h logHandler) logRequest(r *http.Request, lw *loggingResponseWriter, msg A
 		// switched protocols away from HTTP.
 		msg.Code = http.StatusSwitchingProtocols
 	case lw.code == 0:
-		// If the handler didn't write and didn't send a header, that still means 200.
-		// (See https://play.golang.org/p/4P7nx_Tap7p)
-		msg.Code = 200
+		if r.Context().Err() != nil {
+			// We didn't write a response before the client disconnected.
+			msg.Code = 499
+		} else {
+			// If the handler didn't write and didn't send a header, that still means 200.
+			// (See https://play.golang.org/p/4P7nx_Tap7p)
+			msg.Code = 200
+		}
 	default:
 		msg.Code = lw.code
 	}
@@ -669,7 +668,9 @@ func (h errorHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	err = h.rh.ServeHTTPReturn(lw, r)
 }
 
-func (h errorHandler) handleError(w http.ResponseWriter, r *http.Request, lw *loggingResponseWriter, err error) (logged bool) {
+func (h errorHandler) handleError(w http.ResponseWriter, r *http.Request, lw *loggingResponseWriter, err error) bool {
+	var logged bool
+
 	// Extract a presentable, loggable error.
 	var hOK bool
 	var hErr HTTPError
@@ -681,6 +682,8 @@ func (h errorHandler) handleError(w http.ResponseWriter, r *http.Request, lw *lo
 		}
 	} else if v, ok := vizerror.As(err); ok {
 		hErr = Error(http.StatusInternalServerError, v.Error(), nil)
+	} else if errors.Is(err, context.Canceled) || errors.Is(err, http.ErrAbortHandler) {
+		hErr = Error(499, "", err) // Nginx convention
 	} else {
 		// Omit the friendly message so HTTP logs show the bare error that was
 		// returned and we know it's not a HTTPError.
@@ -699,21 +702,31 @@ func (h errorHandler) handleError(w http.ResponseWriter, r *http.Request, lw *lo
 		logged = true
 	}
 
+	if r.Context().Err() != nil {
+		return logged
+	}
+
 	if lw.code != 0 {
-		if hOK {
-			lw.logf("[unexpected] handler returned HTTPError %v, but already sent a response with code %d", hErr, lw.code)
+		if hOK && hErr.Code != lw.code {
+			lw.logf("[unexpected] handler returned HTTPError %v, but already sent response with code %d", hErr, lw.code)
 		}
-		return
+		return logged
 	}
 
 	// Set a default error message from the status code. Do this after we pass
 	// the error back to the logger so that `return errors.New("oh")` logs as
 	// `"err": "oh"`, not `"err": "Internal Server Error: oh"`.
 	if hErr.Msg == "" {
-		hErr.Msg = http.StatusText(hErr.Code)
+		switch hErr.Code {
+		case 499:
+			hErr.Msg = "Client Closed Request"
+		default:
+			hErr.Msg = http.StatusText(hErr.Code)
+		}
 	}
 
 	// If OnError panics before a response is written, write a bare 500 back.
+	// OnError panics are thrown further up the stack.
 	defer func() {
 		if lw.code == 0 {
 			if rec := recover(); rec != nil {

tsweb/tsweb_test.go

@@ -22,6 +22,7 @@ import (
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"tailscale.com/metrics"
 	"tailscale.com/tstest"
+	"tailscale.com/util/httpm"
 	"tailscale.com/util/must"
 	"tailscale.com/util/vizerror"
 )
@@ -693,6 +694,81 @@ func TestStdHandler_Panic(t *testing.T) {
 	res.Body.Close()
 }
 
+func TestStdHandler_Canceled(t *testing.T) {
+	now := time.Now()
+
+	r := make(chan AccessLogRecord)
+	var e *HTTPError
+	handlerOpen := make(chan struct{})
+	h := StdHandler(
+		ReturnHandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+			close(handlerOpen)
+			ctx := r.Context()
+			<-ctx.Done()
+			return ctx.Err()
+		}),
+		HandlerOptions{
+			Logf: t.Logf,
+			Now:  func() time.Time { return now },
+			OnError: func(w http.ResponseWriter, r *http.Request, h HTTPError) {
+				e = &h
+			},
+			OnCompletion: func(_ *http.Request, alr AccessLogRecord) {
+				r <- alr
+			},
+		},
+	)
+
+	// Create a context which gets canceled after the handler starts processing
+	// the request.
+	ctx, cancelReq := context.WithCancel(context.Background())
+	go func() {
+		<-handlerOpen
+		cancelReq()
+	}()
+
+	s := httptest.NewServer(h)
+	t.Cleanup(s.Close)
+
+	// Send a request to our server.
+	req, err := http.NewRequestWithContext(ctx, httpm.GET, s.URL, nil)
+	if err != nil {
+		t.Fatalf("making request: %s", err)
+	}
+	res, err := http.DefaultClient.Do(req)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("got error %v, want context.Canceled", err)
+	}
+	if res != nil {
+		t.Errorf("got response %#v, want nil", res)
+	}
+
+	// Check that we got the expected log record.
+	got := <-r
+	got.Seconds = 0
+	got.RemoteAddr = ""
+	got.Host = ""
+	got.UserAgent = ""
+	want := AccessLogRecord{
+		Time:       now,
+		Code:       499,
+		Method:     "GET",
+		Err:        "context canceled",
+		Proto:      "HTTP/1.1",
+		RequestURI: "/",
+	}
+	if d := cmp.Diff(want, got); d != "" {
+		t.Errorf("AccessLogRecord wrong (-want +got)\n%s", d)
+	}
+
+	// Check that we rendered no response to the client after
+	// logHandler.OnCompletion has been called.
+	if e != nil {
+		t.Errorf("got OnError callback with %#v, want no callback", e)
+	}
+
+}
+
 func TestStdHandler_OnErrorPanic(t *testing.T) {
 	var r AccessLogRecord
 	h := StdHandler(