retrybackend: Fail operations when context is already canceled

Depending on the used backend, operations started with a canceled
context may fail or not. For example the local backend still works in
large parts when called with a canceled context. Backends transfering
data via http don't work. It is also not possible to retry failed
operations in that state as the RetryBackend will abort with a 'context
canceled' error.

Ensure uniform behavior of all backends by checking for a canceled
context by checking for a canceled context as a first step in the
RetryBackend. This ensures uniform behavior across all backends, as
backends are always wrapped in a RetryBackend.

internal/backend/backend_retry.go

@@ -33,6 +33,16 @@ func NewRetryBackend(be restic.Backend, maxTries int, report func(string, error,
 }
 
 func (be *RetryBackend) retry(ctx context.Context, msg string, f func() error) error {
+	// Don't do anything when called with an already cancelled context. There would be
+	// no retries in that case either, so be consistent and abort always.
+	// This enforces a strict contract for backend methods: Using a cancelled context
+	// will prevent any backup repository modifications. This simplifies ensuring that
+	// a backup repository is not modified any further after a context was cancelled.
+	// The 'local' backend for example does not provide this guarantee on its own.
+	if ctx.Err() != nil {
+		return ctx.Err()
+	}
+
 	err := backoff.RetryNotify(f,
 		backoff.WithContext(backoff.WithMaxRetries(backoff.NewExponentialBackOff(), uint64(be.MaxTries)), ctx),
 		func(err error, d time.Duration) {