backend/retry: don't trip circuit breaker if context is canceled

When the context used for a load operation is canceled, then the result
is always an error independent of whether the file could be retrieved
from the backend. Do not false positively trip the circuit breaker in
this case.

The old behavior was problematic when trying to lock a repository. When
`Lock.checkForOtherLocks` listed multiple lock files in parallel and one
of them fails to load, then all other loads were canceled. This
cancelation was remembered by the circuit breaker, such that locking
retries would fail.

internal/backend/retry/backend_retry_test.go

@@ -357,6 +357,30 @@ func TestBackendLoadCircuitBreaker(t *testing.T) {
 	test.Equals(t, notFound, err, "expected circuit breaker to reset, got %v")
 }
 
+func TestBackendLoadCircuitBreakerCancel(t *testing.T) {
+	cctx, cancel := context.WithCancel(context.Background())
+	be := mock.NewBackend()
+	be.OpenReaderFn = func(ctx context.Context, h backend.Handle, length int, offset int64) (io.ReadCloser, error) {
+		cancel()
+		return nil, errors.New("something")
+	}
+	nilRd := func(rd io.Reader) (err error) {
+		return nil
+	}
+
+	TestFastRetries(t)
+	retryBackend := New(be, 2, nil, nil)
+	// canceling the context should not trip the circuit breaker
+	err := retryBackend.Load(cctx, backend.Handle{Name: "other"}, 0, 0, nilRd)
+	test.Equals(t, context.Canceled, err, "unexpected error")
+
+	// reset context and check that the cirucit breaker does not return an error
+	cctx, cancel = context.WithCancel(context.Background())
+	defer cancel()
+	err = retryBackend.Load(cctx, backend.Handle{Name: "other"}, 0, 0, nilRd)
+	test.Equals(t, context.Canceled, err, "unexpected error")
+}
+
 func TestBackendStatNotExists(t *testing.T) {
 	// stat should not retry if the error matches IsNotExist
 	notFound := errors.New("not found")