Initial implementation of Manifest HTTP API

Push, pull and delete of manifest files in the registry have been implemented
on top of the storage services. Basic workflows, including reporting of missing
manifests are tested, including various proposed response codes. Common testing
functionality has been collected into shared methods. A test suite may be
emerging but it might better to capture more edge cases (such as resumable
upload, range requests, etc.) before we commit to a full approach.

To support clearer test cases and simpler handler methods, an application aware
urlBuilder has been added. We may want to export the functionality for use in
the client, which could allow us to abstract away from gorilla/mux.

A few error codes have been added to fill in error conditions missing from the
proposal. Some use cases have identified some problems with the approach to
error reporting that requires more work to reconcile. To resolve this, the
mapping of Go errors into error types needs to pulled out of the handlers and
into the application. We also need to move to type-based errors, with rich
information, rather than value-based errors. ErrorHandlers will probably
replace the http.Handlers to make this work correctly.

Unrelated to the above, the "length" parameter has been migrated to "size" for
completing layer uploads. This change should have gone out before but these
diffs ending up being coupled with the parameter name change due to updates to
the layer unit tests.

errors.go

@@ -34,6 +34,14 @@ const (
 	// match the provided tag.
 	ErrorCodeInvalidTag
 
+	// ErrorCodeUnknownManifest returned when image manifest name and tag is
+	// unknown, accompanied by a 404 status.
+	ErrorCodeUnknownManifest
+
+	// ErrorCodeInvalidManifest returned when an image manifest is invalid,
+	// typically during a PUT operation.
+	ErrorCodeInvalidManifest
+
 	// ErrorCodeUnverifiedManifest is returned when the manifest fails signature
 	// validation.
 	ErrorCodeUnverifiedManifest
@@ -56,6 +64,8 @@ var errorCodeStrings = map[ErrorCode]string{
 	ErrorCodeInvalidLength:      "INVALID_LENGTH",
 	ErrorCodeInvalidName:        "INVALID_NAME",
 	ErrorCodeInvalidTag:         "INVALID_TAG",
+	ErrorCodeUnknownManifest:    "UNKNOWN_MANIFEST",
+	ErrorCodeInvalidManifest:    "INVALID_MANIFEST",
 	ErrorCodeUnverifiedManifest: "UNVERIFIED_MANIFEST",
 	ErrorCodeUnknownLayer:       "UNKNOWN_LAYER",
 	ErrorCodeUnknownLayerUpload: "UNKNOWN_LAYER_UPLOAD",
@@ -66,12 +76,14 @@ var errorCodesMessages = map[ErrorCode]string{
 	ErrorCodeUnknown:            "unknown error",
 	ErrorCodeInvalidDigest:      "provided digest did not match uploaded content",
 	ErrorCodeInvalidLength:      "provided length did not match content length",
-	ErrorCodeInvalidName:        "Manifest name did not match URI",
-	ErrorCodeInvalidTag:         "Manifest tag did not match URI",
-	ErrorCodeUnverifiedManifest: "Manifest failed signature validation",
-	ErrorCodeUnknownLayer:       "Referenced layer not available",
+	ErrorCodeInvalidName:        "manifest name did not match URI",
+	ErrorCodeInvalidTag:         "manifest tag did not match URI",
+	ErrorCodeUnknownManifest:    "manifest not known",
+	ErrorCodeInvalidManifest:    "manifest is invalid",
+	ErrorCodeUnverifiedManifest: "manifest failed signature validation",
+	ErrorCodeUnknownLayer:       "referenced layer not available",
 	ErrorCodeUnknownLayerUpload: "cannot resume unknown layer upload",
-	ErrorCodeUntrustedSignature: "Manifest signed by untrusted source",
+	ErrorCodeUntrustedSignature: "manifest signed by untrusted source",
 }
 
 var stringToErrorCode map[string]ErrorCode
@@ -178,7 +190,12 @@ func (errs *Errors) Push(code ErrorCode, details ...interface{}) {
 
 // PushErr pushes an error interface onto the error stack.
 func (errs *Errors) PushErr(err error) {
-	errs.Errors = append(errs.Errors, err)
+	switch err.(type) {
+	case Error:
+		errs.Errors = append(errs.Errors, err)
+	default:
+		errs.Errors = append(errs.Errors, Error{Message: err.Error()})
+	}
 }
 
 func (errs *Errors) Error() string {