Improve "invalid reference" error message

Use the same error message as is used in
docker/reference/reference.go to provide
slightly more information about the error.

This change tests if the reference passes
after lowercasing its characters, to determine
if the reference was invalid due to
it containing uppercase characters.

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

reference/reference.go

@@ -24,6 +24,7 @@ package reference
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/docker/distribution/digest"
 )
@@ -43,6 +44,9 @@ var (
 	// ErrDigestInvalidFormat represents an error while trying to parse a string as a tag.
 	ErrDigestInvalidFormat = errors.New("invalid digest format")
 
+	// ErrNameContainsUppercase is returned for invalid repository names that contain uppercase characters.
+	ErrNameContainsUppercase = errors.New("repository name must be lowercase")
+
 	// ErrNameEmpty is returned for empty, invalid repository names.
 	ErrNameEmpty = errors.New("repository name must have at least one component")
 
@@ -149,7 +153,9 @@ func Parse(s string) (Reference, error) {
 		if s == "" {
 			return nil, ErrNameEmpty
 		}
-		// TODO(dmcgowan): Provide more specific and helpful error
+		if ReferenceRegexp.FindStringSubmatch(strings.ToLower(s)) != nil {
+			return nil, ErrNameContainsUppercase
+		}
 		return nil, ErrReferenceInvalidFormat
 	}
 

reference/reference_test.go

@@ -95,6 +95,25 @@ func TestReferenceParse(t *testing.T) {
 			input: "validname@invaliddigest:ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
 			err:   digest.ErrDigestUnsupported,
 		},
+		{
+			input: "Uppercase:tag",
+			err:   ErrNameContainsUppercase,
+		},
+		// FIXME "Uppercase" is incorrectly handled as a domain-name here, therefore passes.
+		// See https://github.com/docker/distribution/pull/1778, and https://github.com/docker/docker/pull/20175
+		//{
+		//	input: "Uppercase/lowercase:tag",
+		//	err:   ErrNameContainsUppercase,
+		//},
+		{
+			input: "test:5000/Uppercase/lowercase:tag",
+			err:   ErrNameContainsUppercase,
+		},
+		{
+			input:      "lowercase:Uppercase",
+			repository: "lowercase",
+			tag:        "Uppercase",
+		},
 		{
 			input: strings.Repeat("a/", 128) + "a:tag",
 			err:   ErrNameTooLong,