swarm/controller: allow cancellation to propagate

Ensure that cancellation of a pull propagates rather than continuing to
container creation. This ensures that the `Prepare` method is properly
re-entrant.

Signed-off-by: Stephen J Day <stephen.day@docker.com>
(cherry picked from commit d99c6b837f)
Signed-off-by: Tibor Vass <tibor@docker.com>

daemon/cluster/executor/container/controller.go

@@ -85,6 +85,11 @@ func (r *controller) Prepare(ctx context.Context) error {
 	}
 
 	if err := r.adapter.pullImage(ctx); err != nil {
+		cause := errors.Cause(err)
+		if cause == context.Canceled || cause == context.DeadlineExceeded {
+			return err
+		}
+
 		// NOTE(stevvooe): We always try to pull the image to make sure we have
 		// the most up to date version. This will return an error, but we only
 		// log it. If the image truly doesn't exist, the create below will

distribution/errors.go

@@ -40,7 +40,11 @@ type fallbackError struct {
 
 // Error renders the FallbackError as a string.
 func (f fallbackError) Error() string {
-	return f.err.Error()
+	return f.Cause().Error()
+}
+
+func (f fallbackError) Cause() error {
+	return f.err
 }
 
 // shouldV2Fallback returns true if this error is a reason to fall back to v1.