reference: ParseDockerRef: slight refactor, and update docs

- improve documentation
- remove redundant error-check
- simplify interface matching, which slightly improves readability
- touch-up some docs

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

reference/helpers.go

@@ -32,7 +32,7 @@ func FamiliarString(ref Reference) string {
 }
 
 // FamiliarMatch reports whether ref matches the specified pattern.
-// See https://godoc.org/path#Match for supported patterns.
+// See [path.Match] for supported patterns.
 func FamiliarMatch(pattern string, ref Reference) (bool, error) {
 	matched, err := path.Match(pattern, FamiliarString(ref))
 	if namedRef, isNamed := ref.(Named); isNamed && !matched {

reference/normalize.go

@@ -54,31 +54,43 @@ func ParseNormalizedNamed(s string) (Named, error) {
 	return named, nil
 }
 
-// ParseDockerRef normalizes the image reference following the docker convention. This is added
-// mainly for backward compatibility.
-// The reference returned can only be either tagged or digested. For reference contains both tag
-// and digest, the function returns digested reference, e.g. docker.io/library/busybox:latest@
-// sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa will be returned as
-// docker.io/library/busybox@sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa.
+// namedTaggedDigested is a reference that has both a tag and a digest.
+type namedTaggedDigested interface {
+	NamedTagged
+	Digested
+}
+
+// ParseDockerRef normalizes the image reference following the docker convention,
+// which allows for references to contain both a tag and a digest. It returns a
+// reference that is either tagged or digested. For references containing both
+// a tag and a digest, it returns a digested reference. For example, the following
+// reference:
+//
+//	docker.io/library/busybox:latest@sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa
+//
+// Is returned as a digested reference (with the ":latest" tag removed):
+//
+//	docker.io/library/busybox@sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa
+//
+// References that are already "tagged" or "digested" are returned unmodified:
+//
+//	// Already a digested reference
+//	docker.io/library/busybox@sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa
+//
+//	// Already a named reference
+//	docker.io/library/busybox:latest
 func ParseDockerRef(ref string) (Named, error) {
 	named, err := ParseNormalizedNamed(ref)
 	if err != nil {
 		return nil, err
 	}
-	if _, ok := named.(NamedTagged); ok {
-		if canonical, ok := named.(Canonical); ok {
-			// The reference is both tagged and digested, only
-			// return digested.
+	if canonical, ok := named.(namedTaggedDigested); ok {
+		// The reference is both tagged and digested; only return digested.
 		newNamed, err := WithName(canonical.Name())
 		if err != nil {
 			return nil, err
 		}
-			newCanonical, err := WithDigest(newNamed, canonical.Digest())
-			if err != nil {
-				return nil, err
-			}
-			return newCanonical, nil
-		}
+		return WithDigest(newNamed, canonical.Digest())
 	}
 	return TagNameOnly(named), nil
 }

reference/reference.go

@@ -146,7 +146,7 @@ type namedRepository interface {
 	Path() string
 }
 
-// Domain returns the domain part of the Named reference
+// Domain returns the domain part of the [Named] reference.
 func Domain(named Named) string {
 	if r, ok := named.(namedRepository); ok {
 		return r.Domain()
@@ -155,7 +155,7 @@ func Domain(named Named) string {
 	return domain
 }
 
-// Path returns the name without the domain part of the Named reference
+// Path returns the name without the domain part of the [Named] reference.
 func Path(named Named) (name string) {
 	if r, ok := named.(namedRepository); ok {
 		return r.Path()

reference/sort.go

@@ -20,13 +20,15 @@ import (
 	"sort"
 )
 
-// Sort sorts string references preferring higher information references
+// Sort sorts string references preferring higher information references.
+//
 // The precedence is as follows:
-// 1. Name + Tag + Digest
-// 2. Name + Tag
-// 3. Name + Digest
-// 4. Name
-// 5. Digest
+//
+//  1. [Named] + [Tagged] + [Digested] (e.g., "docker.io/library/busybox:latest@sha256:<digest>")
+//  2. [Named] + [Tagged]              (e.g., "docker.io/library/busybox:latest")
+//  3. [Named] + [Digested]            (e.g., "docker.io/library/busybo@sha256:<digest>")
+//  4. [Named]                         (e.g., "docker.io/library/busybox")
+//  5. [Digested]                      (e.g., "docker.io@sha256:<digest>")
 //  6. Parse error
 func Sort(references []string) []string {
 	var prefs []Reference