backend: improve error on http request timeout

Now yields a "request timeout" error instead of "context canceled".

internal/backend/watchdog_roundtriper.go

@@ -2,11 +2,16 @@ package backend
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"io"
 	"net/http"
+	"sync/atomic"
 	"time"
 )
 
+var errRequestTimeout = fmt.Errorf("request timeout")
+
 // watchdogRoundtripper cancels an http request if an upload or download did not make progress
 // within timeout. The time between fully sending the request and receiving an response is also
 // limited by this timeout. This ensures that stuck requests are cancelled after some time.
@@ -32,12 +37,14 @@ func newWatchdogRoundtripper(rt http.RoundTripper, timeout time.Duration, chunkS
 func (w *watchdogRoundtripper) RoundTrip(req *http.Request) (*http.Response, error) {
 	timer := time.NewTimer(w.timeout)
 	ctx, cancel := context.WithCancel(req.Context())
+	timedOut := &atomic.Bool{}
 
 	// cancel context if timer expires
 	go func() {
 		defer timer.Stop()
 		select {
 		case <-timer.C:
+			timedOut.Store(true)
 			cancel()
 		case <-ctx.Done():
 		}
@@ -46,11 +53,14 @@ func (w *watchdogRoundtripper) RoundTrip(req *http.Request) (*http.Response, err
 	kick := func() {
 		timer.Reset(w.timeout)
 	}
+	isTimeout := func(err error) bool {
+		return timedOut.Load() && errors.Is(err, context.Canceled)
+	}
 
 	req = req.Clone(ctx)
 	if req.Body != nil {
 		// kick watchdog timer as long as uploading makes progress
-		req.Body = newWatchdogReadCloser(req.Body, w.chunkSize, kick, nil)
+		req.Body = newWatchdogReadCloser(req.Body, w.chunkSize, kick, nil, isTimeout)
 	}
 
 	resp, err := w.rt.RoundTrip(req)
@@ -60,16 +70,17 @@ func (w *watchdogRoundtripper) RoundTrip(req *http.Request) (*http.Response, err
 
 	// kick watchdog timer as long as downloading makes progress
 	// cancel context to stop goroutine once response body is closed
-	resp.Body = newWatchdogReadCloser(resp.Body, w.chunkSize, kick, cancel)
+	resp.Body = newWatchdogReadCloser(resp.Body, w.chunkSize, kick, cancel, isTimeout)
 	return resp, nil
 }
 
-func newWatchdogReadCloser(rc io.ReadCloser, chunkSize int, kick func(), close func()) *watchdogReadCloser {
+func newWatchdogReadCloser(rc io.ReadCloser, chunkSize int, kick func(), close func(), isTimeout func(err error) bool) *watchdogReadCloser {
 	return &watchdogReadCloser{
 		rc:        rc,
 		chunkSize: chunkSize,
 		kick:      kick,
 		close:     close,
+		isTimeout: isTimeout,
 	}
 }
 
@@ -78,6 +89,7 @@ type watchdogReadCloser struct {
 	chunkSize int
 	kick      func()
 	close     func()
+	isTimeout func(err error) bool
 }
 
 var _ io.ReadCloser = &watchdogReadCloser{}
@@ -93,6 +105,9 @@ func (w *watchdogReadCloser) Read(p []byte) (n int, err error) {
 	n, err = w.rc.Read(p)
 	w.kick()
 
+	if err != nil && w.isTimeout(err) {
+		err = errRequestTimeout
+	}
 	return n, err
 }
 

internal/backend/watchdog_roundtriper_test.go

@@ -23,8 +23,11 @@ func TestRead(t *testing.T) {
 	onClose := func() {
 		closed = true
 	}
+	isTimeout := func(err error) bool {
+		return false
+	}
 
-	wd := newWatchdogReadCloser(io.NopCloser(bytes.NewReader(data)), 1, kick, onClose)
+	wd := newWatchdogReadCloser(io.NopCloser(bytes.NewReader(data)), 1, kick, onClose, isTimeout)
 
 	out, err := io.ReadAll(wd)
 	rtest.OK(t, err)
@@ -196,6 +199,6 @@ func TestDownloadTimeout(t *testing.T) {
 	rtest.Equals(t, 200, resp.StatusCode, "unexpected status code")
 
 	_, err = io.ReadAll(resp.Body)
-	rtest.Equals(t, context.Canceled, err, "response download not canceled")
+	rtest.Equals(t, errRequestTimeout, err, "response download not canceled")
 	rtest.OK(t, resp.Body.Close())
 }