Keep returning image for default arch to old clients fetching lists

This puts back the original flow where old clients are fetching manifest
lists schema1 images where we want to try returning some image for the
default architecture. This was incorrectly removed by one of the
previous commits.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/handlers/manifests.go

@@ -23,6 +23,8 @@ import (
 )
 
 const (
+	defaultArch         = "amd64"
+	defaultOS           = "linux"
 	maxManifestBodySize = 4 << 20
 	imageClass          = "image"
 )
@@ -166,6 +168,43 @@ func (imh *manifestHandler) GetManifest(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	if imh.Tag != "" && manifestType == manifestlistSchema && !supports[manifestlistSchema] {
+		// Rewrite manifest in schema1 format
+		dcontext.GetLogger(imh).Infof("rewriting manifest list %s in schema1 format to support old client", imh.Digest.String())
+
+		// Find the image manifest corresponding to the default
+		// platform
+		var manifestDigest digest.Digest
+		for _, manifestDescriptor := range manifestList.Manifests {
+			if manifestDescriptor.Platform.Architecture == defaultArch && manifestDescriptor.Platform.OS == defaultOS {
+				manifestDigest = manifestDescriptor.Digest
+				break
+			}
+		}
+
+		if manifestDigest == "" {
+			imh.Errors = append(imh.Errors, v2.ErrorCodeManifestUnknown)
+			return
+		}
+
+		manifest, err = manifests.Get(imh, manifestDigest)
+		if err != nil {
+			if _, ok := err.(distribution.ErrManifestUnknownRevision); ok {
+				imh.Errors = append(imh.Errors, v2.ErrorCodeManifestUnknown.WithDetail(err))
+			} else {
+				imh.Errors = append(imh.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+			}
+			return
+		}
+
+		if _, isSchema2 := manifest.(*schema2.DeserializedManifest); isSchema2 && !supports[manifestSchema2] {
+			imh.Errors = append(imh.Errors, v2.ErrorCodeManifestInvalid.WithMessage("Schema 2 manifest not supported by client"))
+			return
+		} else {
+			imh.Digest = manifestDigest
+		}
+	}
+
 	ct, p, err := manifest.Payload()
 	if err != nil {
 		return