Return warnings from service create and service update when digest pinning fails

Modify the service update and create APIs to return optional warning
messages as part of the response. Populate these messages with an
informative reason when digest resolution fails.

This is a small API change, but significantly improves the UX. The user
can now get immediate feedback when they've specified a nonexistent
image or unreachable registry.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

interface.go

@@ -124,7 +124,7 @@ type ServiceAPIClient interface {
 	ServiceInspectWithRaw(ctx context.Context, serviceID string) (swarm.Service, []byte, error)
 	ServiceList(ctx context.Context, options types.ServiceListOptions) ([]swarm.Service, error)
 	ServiceRemove(ctx context.Context, serviceID string) error
-	ServiceUpdate(ctx context.Context, serviceID string, version swarm.Version, service swarm.ServiceSpec, options types.ServiceUpdateOptions) error
+	ServiceUpdate(ctx context.Context, serviceID string, version swarm.Version, service swarm.ServiceSpec, options types.ServiceUpdateOptions) (types.ServiceUpdateResponse, error)
 	ServiceLogs(ctx context.Context, serviceID string, options types.ContainerLogsOptions) (io.ReadCloser, error)
 	TaskInspectWithRaw(ctx context.Context, taskID string) (swarm.Task, []byte, error)
 	TaskList(ctx context.Context, options types.TaskListOptions) ([]swarm.Task, error)

service_update.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"encoding/json"
 	"net/url"
 	"strconv"
 
@@ -10,7 +11,7 @@ import (
 )
 
 // ServiceUpdate updates a Service.
-func (cli *Client) ServiceUpdate(ctx context.Context, serviceID string, version swarm.Version, service swarm.ServiceSpec, options types.ServiceUpdateOptions) error {
+func (cli *Client) ServiceUpdate(ctx context.Context, serviceID string, version swarm.Version, service swarm.ServiceSpec, options types.ServiceUpdateOptions) (types.ServiceUpdateResponse, error) {
 	var (
 		headers map[string][]string
 		query   = url.Values{}
@@ -28,7 +29,13 @@ func (cli *Client) ServiceUpdate(ctx context.Context, serviceID string, version
 
 	query.Set("version", strconv.FormatUint(version.Index, 10))
 
+	var response types.ServiceUpdateResponse
 	resp, err := cli.post(ctx, "/services/"+serviceID+"/update", query, service, headers)
-	ensureReaderClosed(resp)
+	if err != nil {
-	return err
+		return response, err
+	}
+
+	err = json.NewDecoder(resp.body).Decode(&response)
+	ensureReaderClosed(resp)
+	return response, err
 }

service_update_test.go

@@ -19,7 +19,7 @@ func TestServiceUpdateError(t *testing.T) {
 		client: newMockClient(errorMock(http.StatusInternalServerError, "Server error")),
 	}
 
-	err := client.ServiceUpdate(context.Background(), "service_id", swarm.Version{}, swarm.ServiceSpec{}, types.ServiceUpdateOptions{})
+	_, err := client.ServiceUpdate(context.Background(), "service_id", swarm.Version{}, swarm.ServiceSpec{}, types.ServiceUpdateOptions{})
 	if err == nil || err.Error() != "Error response from daemon: Server error" {
 		t.Fatalf("expected a Server Error, got %v", err)
 	}
@@ -64,12 +64,12 @@ func TestServiceUpdate(t *testing.T) {
 				}
 				return &http.Response{
 					StatusCode: http.StatusOK,
-					Body:       ioutil.NopCloser(bytes.NewReader([]byte("body"))),
+					Body:       ioutil.NopCloser(bytes.NewReader([]byte("{}"))),
 				}, nil
 			}),
 		}
 
-		err := client.ServiceUpdate(context.Background(), "service_id", updateCase.swarmVersion, swarm.ServiceSpec{}, types.ServiceUpdateOptions{})
+		_, err := client.ServiceUpdate(context.Background(), "service_id", updateCase.swarmVersion, swarm.ServiceSpec{}, types.ServiceUpdateOptions{})
 		if err != nil {
 			t.Fatal(err)
 		}