Include headers when serving blob through proxy

In commit 1795292 we updated ServeBlob() to use an io.MultiWriter to
write simultaneously to the local store and the HTTP response.

However, copyContent was using a type assertion to only add headers if
the io.Writer was a http.ResponseWriter. Therefore, this change caused
us to stop sending the expected headers (i.e. Content-Length, Etag,
etc.) on the first request for a blob.

Resolve the issue by explicitly passing in http.Header and setting it
unconditionally.

Signed-off-by: Mikel Rychliski <mikel@mikelr.com>

registry/proxy/proxyblobstore.go

@@ -33,22 +33,20 @@ var inflight = make(map[digest.Digest]struct{})
 // mu protects inflight
 var mu sync.Mutex
 
-func setResponseHeaders(w http.ResponseWriter, length int64, mediaType string, digest digest.Digest) {
-	w.Header().Set("Content-Length", strconv.FormatInt(length, 10))
-	w.Header().Set("Content-Type", mediaType)
-	w.Header().Set("Docker-Content-Digest", digest.String())
-	w.Header().Set("Etag", digest.String())
+func setResponseHeaders(h http.Header, length int64, mediaType string, digest digest.Digest) {
+	h.Set("Content-Length", strconv.FormatInt(length, 10))
+	h.Set("Content-Type", mediaType)
+	h.Set("Docker-Content-Digest", digest.String())
+	h.Set("Etag", digest.String())
 }
 
-func (pbs *proxyBlobStore) copyContent(ctx context.Context, dgst digest.Digest, writer io.Writer) (distribution.Descriptor, error) {
+func (pbs *proxyBlobStore) copyContent(ctx context.Context, dgst digest.Digest, writer io.Writer, h http.Header) (distribution.Descriptor, error) {
 	desc, err := pbs.remoteStore.Stat(ctx, dgst)
 	if err != nil {
 		return distribution.Descriptor{}, err
 	}
 
-	if w, ok := writer.(http.ResponseWriter); ok {
-		setResponseHeaders(w, desc.Size, desc.MediaType, dgst)
-	}
+	setResponseHeaders(h, desc.Size, desc.MediaType, dgst)
 
 	remoteReader, err := pbs.remoteStore.Open(ctx, dgst)
 	if err != nil {
@@ -102,7 +100,7 @@ func (pbs *proxyBlobStore) ServeBlob(ctx context.Context, w http.ResponseWriter,
 		// Will return the blob from the remote store directly.
 		// TODO Maybe we could reuse the these blobs are serving remotely and caching locally.
 		mu.Unlock()
-		_, err := pbs.copyContent(ctx, dgst, w)
+		_, err := pbs.copyContent(ctx, dgst, w, w.Header())
 		return err
 	}
 	inflight[dgst] = struct{}{}
@@ -122,7 +120,7 @@ func (pbs *proxyBlobStore) ServeBlob(ctx context.Context, w http.ResponseWriter,
 	// Serving client and storing locally over same fetching request.
 	// This can prevent a redundant blob fetching.
 	multiWriter := io.MultiWriter(w, bw)
-	desc, err := pbs.copyContent(ctx, dgst, multiWriter)
+	desc, err := pbs.copyContent(ctx, dgst, multiWriter, w.Header())
 	if err != nil {
 		return err
 	}

registry/proxy/proxyblobstore_test.go

@@ -448,12 +448,22 @@ func testProxyStoreServe(t *testing.T, te *testEnv, numClients int) {
 					return
 				}
 
-				bodyBytes := w.Body.Bytes()
+				resp := w.Result()
+				bodyBytes, err := io.ReadAll(resp.Body)
+				resp.Body.Close()
+				if err != nil {
+					t.Errorf(err.Error())
+					return
+				}
 				localDigest := digest.FromBytes(bodyBytes)
 				if localDigest != remoteBlob.Digest {
 					t.Errorf("Mismatching blob fetch from proxy")
 					return
 				}
+				if resp.Header.Get("Docker-Content-Digest") != localDigest.String() {
+					t.Errorf("Mismatching digest in response header")
+					return
+				}
 
 				desc, err := te.store.localStore.Stat(te.ctx, remoteBlob.Digest)
 				if err != nil {