feat(digest) Parse images digest

In some occasions the same tag can be assigned to different images.
Like for example the official Python images tagged with `3.7` or other
projects which may not have a labeling strategy which only uses a label
like `latest` or `master`. Using digests instead of labels ensures that
you don't receive unexpected upgrades

More info about tags and digests can be found
[here](https://success.docker.com/article/images-tagging-vs-digests)

Fixes fluxcd#885

image/image.go

@@ -108,10 +108,12 @@ func (i Name) ToRef(tag string) Ref {
 //  * alpine:3.5
 //  * library/alpine:3.5
 //  * docker.io/weaveworks/flux:1.1.0
-//  * localhost:5000/arbitrary/path/to/repo:revision-sha1
+//  * docker.io/weaveworks/flux@sha256:e5ac3944b44556916df9b97d78c28cf8e6b6ef419614a70e87b39f84c17428dd
+//  * docker.io/python:3.7@sha256:284a4057aac706aaafe50c5c0090d6d311f5d8d72014f0cd69f880a54e9df6fe
 type Ref struct {
 	Name
-	Tag string
+	Tag    string
+	Digest string
 }
 
 // CanonicalRef is an image ref with none of the fields left to be
@@ -122,11 +124,14 @@ type CanonicalRef struct {
 
 // String returns the Ref as a string (i.e., unparsed) without canonicalising it.
 func (i Ref) String() string {
-	var tag string
+	var tag, digest string
 	if i.Tag != "" {
 		tag = ":" + i.Tag
 	}
-	return fmt.Sprintf("%s%s", i.Name.String(), tag)
+	if i.Digest != "" {
+		digest = "@" + i.Digest
+	}
+	return fmt.Sprintf("%s%s%s", i.Name.String(), tag, digest)
 }
 
 // ParseRef parses a string representation of an image id into an
@@ -142,7 +147,13 @@ func ParseRef(s string) (Ref, error) {
 		return id, errors.Wrapf(ErrMalformedImageID, "parsing %q", s)
 	}
 
-	elements := strings.Split(s, "/")
+	repoWithDigest := strings.Split(s, "@")
+
+	if len(repoWithDigest) == 2 {
+		id.Digest = repoWithDigest[1]
+	}
+
+	elements := strings.Split(repoWithDigest[0], "/")
 	switch len(elements) {
 	case 0: // NB strings.Split will never return []
 		return id, errors.Wrapf(ErrMalformedImageID, "parsing %q", s)
@@ -169,6 +180,7 @@ func ParseRef(s string) (Ref, error) {
 		if imageParts[0] == "" || imageParts[1] == "" {
 			return id, errors.Wrapf(ErrMalformedImageID, "parsing %q", s)
 		}
+
 		id.Image = imageParts[0]
 		id.Tag = imageParts[1]
 	default:
@@ -205,8 +217,9 @@ func (i Ref) CanonicalRef() CanonicalRef {
 	name := i.CanonicalName()
 	return CanonicalRef{
 		Ref: Ref{
-			Name: name.Name,
-			Tag:  i.Tag,
+			Name:   name.Name,
+			Tag:    i.Tag,
+			Digest: i.Digest,
 		},
 	}
 }
@@ -242,7 +255,7 @@ type Labels struct {
 	BuildDate time.Time `json:"org.label-schema.build-date,omitempty"`
 	// Created holds the Open Container Image spec 'created' label
 	// Ref: https://github.com/opencontainers/image-spec/blob/master/annotations.md#pre-defined-annotation-keys
-	Created   time.Time `json:"org.opencontainers.image.created,omitempty"`
+	Created time.Time `json:"org.opencontainers.image.created,omitempty"`
 }
 
 // MarshalJSON returns the Labels value in JSON (as bytes). It is

image/image_test.go

@@ -52,6 +52,9 @@ func TestParseRef(t *testing.T) {
 		{"quay.io/library/alpine:latest", "quay.io", "library/alpine", "quay.io/library/alpine:latest"},
 		{"quay.io/library/alpine:mytag", "quay.io", "library/alpine", "quay.io/library/alpine:mytag"},
 		{"localhost:5000/path/to/repo/alpine:mytag", "localhost:5000", "path/to/repo/alpine", "localhost:5000/path/to/repo/alpine:mytag"},
+		// Images with digest
+		{"docker.io/weaveworks/flux@sha256:e5ac3944b44556916df9b97d78c28cf8e6b6ef419614a70e87b39f84c17428dd", "index.docker.io", "weaveworks/flux", "index.docker.io/weaveworks/flux@sha256:e5ac3944b44556916df9b97d78c28cf8e6b6ef419614a70e87b39f84c17428dd"},
+		{"localhost:5005/python:3.7@sha256:284a4057aac706aaafe50c5c0090d6d311f5d8d72014f0cd69f880a54e9df6fe", "localhost:5005", "python", "localhost:5005/python:3.7@sha256:284a4057aac706aaafe50c5c0090d6d311f5d8d72014f0cd69f880a54e9df6fe"},
 	} {
 		i, err := ParseRef(x.test)
 		if err != nil {
@@ -238,9 +241,9 @@ func TestImage_OrderByCreationDate(t *testing.T) {
 	imA := mustMakeInfo("my/Image:2", testTime)
 	imB := mustMakeInfo("my/Image:0", time.Time{}).setLabels(Labels{Created: time0})
 	imC := mustMakeInfo("my/Image:3", time.Time{}).setLabels(Labels{BuildDate: time2})
-	imD := mustMakeInfo("my/Image:4", time.Time{}) // test nil
+	imD := mustMakeInfo("my/Image:4", time.Time{})                                      // test nil
 	imE := mustMakeInfo("my/Image:1", time.Time{}).setLabels(Labels{Created: testTime}) // test equal
-	imF := mustMakeInfo("my/Image:5", time.Time{}) // test nil equal
+	imF := mustMakeInfo("my/Image:5", time.Time{})                                      // test nil equal
 	imgs := []Info{imA, imB, imC, imD, imE, imF}
 	Sort(imgs, NewerByCreated)
 	checkSorted(t, imgs)