Revert reading images from the docker daemon

See cnabio/duffle#828 (comment)

README.md

@@ -67,6 +67,12 @@ The Go packages provided by this repository include:
 
 For details, please refer to the [package documentation](https://godoc.org/github.com/pivotal/image-relocation).
 
+### Docker daemon
+
+This repository reads images directly from their repositories and does not attempt to read images
+from the Docker daemon. This is primarily because the daemon doesn't guarantee to provide the 
+same digest of an image as when the image has been pushed to a repository.
+
 ## Command line interface
 
 A CLI, `irel`, is provided for manual use and experimentation. Issue `make irel` to build it.

pkg/registry/remote.go

@@ -18,19 +18,16 @@ package registry
 
 import (
 	"errors"
-	"fmt"
 	"net/http"
 
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
-	"github.com/google/go-containerregistry/pkg/v1/daemon"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/pivotal/image-relocation/pkg/image"
 )
 
 var (
-	daemonImageFunc = daemon.Image
 	repoImageFunc   = remote.Image
 	resolveFunc     = authn.DefaultKeychain.Resolve
 	repoWriteFunc   = remote.Write
@@ -54,16 +51,7 @@ func readRemoteImage(n image.Name) (v1.Image, error) {
 		return nil, err
 	}
 
-	img, err := daemonImageFunc(ref)
-	if err != nil {
-		var remoteErr error
-		img, remoteErr = repoImageFunc(ref, remote.WithAuth(auth))
-		if remoteErr != nil {
-			return nil, fmt.Errorf("reading remote image %s failed: %v; attempting to read from daemon also failed: %v", n.String(), remoteErr, err)
-		}
-	}
-
-	return img, nil
+	return remote.Image(ref, remote.WithAuth(auth))
 }
 
 func writeRemoteImage(i v1.Image, n image.Name) error {

pkg/registry/remote_test.go

@@ -6,7 +6,6 @@ import (
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
-	"github.com/google/go-containerregistry/pkg/v1/daemon"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
@@ -36,107 +35,21 @@ var _ = Describe("remote utilities", func() {
 	})
 
 	Describe("readRemoteImage", func() {
-		var (
-			mockImage2 *imagefakes.FakeImage
-
-			img v1.Image
-		)
-
 		JustBeforeEach(func() {
-			img, err = readRemoteImage(imageName)
+			_, err = readRemoteImage(imageName)
 		})
 
 		BeforeEach(func() {
 			var err error
 			imageName, err = image.NewName("imagename")
 			Expect(err).NotTo(HaveOccurred())
 
-			mockImage2 = &imagefakes.FakeImage{}
-			h2, err := v1.NewHash("sha256:1111111111111111111111111111111111111111111111111111111111111111")
-			Expect(err).NotTo(HaveOccurred())
-			mockImage2.DigestReturns(h2, nil)
-
-			Expect(mockImage).NotTo(Equal(mockImage2)) // crucial for the correctness of certain tests
-
 			// In most tests, keychain resolution succeeds
 			resolveFunc = func(registry name.Registry) (authn.Authenticator, error) {
 				return nil, nil
 			}
 		})
 
-		Context("when the daemon returns an image", func() {
-			BeforeEach(func() {
-				daemonImageFunc = func(ref name.Reference, options ...daemon.ImageOption) (v1.Image, error) {
-					return mockImage, nil
-				}
-			})
-
-			It("should return the image", func() {
-				Expect(err).NotTo(HaveOccurred())
-				Expect(img).To(Equal(mockImage))
-			})
-
-			Context("when the remote repository returns an image", func() {
-				BeforeEach(func() {
-					repoImageFunc = func(ref name.Reference, options ...remote.Option) (v1.Image, error) {
-						return mockImage2, nil
-					}
-				})
-
-				It("should return the image from the daemon", func() {
-					Expect(err).NotTo(HaveOccurred())
-					Expect(img).To(Equal(mockImage))
-				})
-			})
-
-			Context("when the remote repository returns an error", func() {
-				BeforeEach(func() {
-					repoImageFunc = func(ref name.Reference, options ...remote.Option) (v1.Image, error) {
-						return nil, testError
-					}
-				})
-
-				It("should return the image from the daemon", func() {
-					Expect(err).NotTo(HaveOccurred())
-					Expect(img).To(Equal(mockImage))
-				})
-			})
-
-		})
-
-		Context("when the daemon returns an error", func() {
-			BeforeEach(func() {
-				daemonImageFunc = func(ref name.Reference, options ...daemon.ImageOption) (v1.Image, error) {
-					return nil, testError
-				}
-			})
-
-			Context("when the remote repository returns an image", func() {
-				BeforeEach(func() {
-					repoImageFunc = func(ref name.Reference, options ...remote.Option) (v1.Image, error) {
-						return mockImage, nil
-					}
-				})
-
-				It("should return the image", func() {
-					Expect(err).NotTo(HaveOccurred())
-					Expect(img).To(Equal(mockImage))
-				})
-			})
-
-			Context("when the remote repository returns an error", func() {
-				BeforeEach(func() {
-					repoImageFunc = func(ref name.Reference, options ...remote.Option) (v1.Image, error) {
-						return nil, errors.New("repo error")
-					}
-				})
-
-				It("should return a combined error", func() {
-					Expect(err).To(MatchError("reading remote image docker.io/library/imagename:latest failed: repo error; attempting to read from daemon also failed: hard cheese"))
-				})
-			})
-		})
-
 		Context("when keychain resolution fails", func() {
 			BeforeEach(func() {
 				resolveFunc = func(registry name.Registry) (authn.Authenticator, error) {