Allow hostname components in component names.

Fixes https://github.com/docker/distribution/issues/1062

This relaxes the naming restrictions places on Docker images to permit
valid hostnames according to [RFC-2396](https://www.ietf.org/rfc/rfc2396.txt).

It deviates from the RFC in the following ways:
 1) Allow underscores where we allow hyphens (hostnames don't allow
 underscores, which we must for backwards compatibility).
 2) Leave "top-level" name segments unrestricted (domains require an
 alpha character to begin a top-level domain, e.g. "com").
 3) DO NOT allow a trailing dot, as permitted by FQDNs.

Signed-off-by: Matt Moore <mattmoor@google.com>

docs/api/v2/names.go

@@ -15,10 +15,23 @@ const (
 	RepositoryNameTotalLengthMax = 255
 )
 
+// domainLabelRegexp represents the following RFC-2396 BNF construct:
+//   domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
+var domainLabelRegexp = regexp.MustCompile(`[a-z0-9](?:-*[a-z0-9])*`)
+
 // RepositoryNameComponentRegexp restricts registry path component names to
-// start with at least one letter or number, with following parts able to
-// be separated by one period, dash or underscore.
-var RepositoryNameComponentRegexp = regexp.MustCompile(`[a-z0-9]+(?:[._-][a-z0-9]+)*`)
+// the allow valid hostnames according to: https://www.ietf.org/rfc/rfc2396.txt
+// with the following differences:
+//  1) It DOES NOT allow for fully-qualified domain names, which include a
+//    trailing '.', e.g. "google.com."
+//  2) It DOES NOT restrict 'top-level' domain labels to start with just alpha
+//    characters.
+//  3) It DOES allow for underscores to appear in the same situations as dots.
+//
+// RFC-2396 uses the BNF construct:
+//   hostname      = *( domainlabel "." ) toplabel [ "." ]
+var RepositoryNameComponentRegexp = regexp.MustCompile(
+	domainLabelRegexp.String() + `(?:[._]` + domainLabelRegexp.String() + `)*`)
 
 // RepositoryNameComponentAnchoredRegexp is the version of
 // RepositoryNameComponentRegexp which must completely match the content

docs/api/v2/names_test.go

@@ -165,21 +165,46 @@ var (
 			invalid: true,
 		},
 		{
-			input: "b.gcr.io/test.example.com/my-app", // embedded domain component
-		},
-		// TODO(stevvooe): The following is a punycode domain name that we may
-		// want to allow in the future. Currently, this is not allowed but we
-		// may want to change this in the future. Adding this here as invalid
-		// for the time being.
-		{
-			input:   "xn--n3h.com/myimage", // http://☃.com in punycode
+			input:   "do__cker/docker",
 			err:     ErrRepositoryNameComponentInvalid,
 			invalid: true,
 		},
+		{
+			input:   "docker./docker",
+			err:     ErrRepositoryNameComponentInvalid,
+			invalid: true,
+		},
+		{
+			input:   ".docker/docker",
+			err:     ErrRepositoryNameComponentInvalid,
+			invalid: true,
+		},
+		{
+			input:   "do..cker/docker",
+			err:     ErrRepositoryNameComponentInvalid,
+			invalid: true,
+		},
+		{
+			input:   "docker-/docker",
+			err:     ErrRepositoryNameComponentInvalid,
+			invalid: true,
+		},
+		{
+			input:   "-docker/docker",
+			err:     ErrRepositoryNameComponentInvalid,
+			invalid: true,
+		},
+		{
+			input: "b.gcr.io/test.example.com/my-app", // embedded domain component
+		},
+		{
+			input: "xn--n3h.com/myimage", // http://☃.com in punycode
+		},
 		{
 			input: "xn--7o8h.com/myimage", // http://🐳.com in punycode
-			err:     ErrRepositoryNameComponentInvalid,
-			invalid: true,
+		},
+		{
+			input: "registry.io/foo/project--id.module--name.ver---sion--name", // image with hostname
 		},
 	}
 )