fast-stop ServeBlob if we're doing a HEAD request

A registry pointing to ECR is having issues if we try loading the blob

Signed-off-by: Damien Mathieu <dmathieu@salesforce.com>

registry/client/repository.go

@@ -672,17 +672,21 @@ func (bs *blobs) ServeBlob(ctx context.Context, w http.ResponseWriter, r *http.R
 		return err
 	}
 
+	w.Header().Set("Content-Length", strconv.FormatInt(desc.Size, 10))
+	w.Header().Set("Content-Type", desc.MediaType)
+	w.Header().Set("Docker-Content-Digest", dgst.String())
+	w.Header().Set("Etag", dgst.String())
+
+	if r.Method == http.MethodHead {
+		return nil
+	}
+
 	blob, err := bs.Open(ctx, dgst)
 	if err != nil {
 		return err
 	}
 	defer blob.Close()
 
-	w.Header().Set("Content-Length", strconv.FormatInt(desc.Size, 10))
-	w.Header().Set("Content-Type", desc.MediaType)
-	w.Header().Set("Docker-Content-Digest", dgst.String())
-	w.Header().Set("Etag", dgst.String())
-
 	_, err = io.CopyN(w, blob, desc.Size)
 	return err
 }

registry/client/repository_test.go

@@ -149,7 +149,55 @@ func TestBlobServeBlob(t *testing.T) {
 			t.Errorf("Unexpected %s. Got %s, expected %s", h.Name, resp.Header().Get(h.Name), h.Value)
 		}
 	}
+}
 
+func TestBlobServeBlobHEAD(t *testing.T) {
+	dgst, blob := newRandomBlob(1024)
+	var m testutil.RequestResponseMap
+	addTestFetch("test.example.com/repo1", dgst, blob, &m)
+
+	e, c := testServer(m)
+	defer c()
+
+	ctx := context.Background()
+	repo, _ := reference.WithName("test.example.com/repo1")
+	r, err := NewRepository(repo, e, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	l := r.Blobs(ctx)
+
+	resp := httptest.NewRecorder()
+	req := httptest.NewRequest("HEAD", "/", nil)
+
+	err = l.ServeBlob(ctx, resp, req, dgst)
+	if err != nil {
+		t.Errorf("Error serving blob: %s", err.Error())
+	}
+
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		t.Errorf("Error reading response body: %s", err.Error())
+	}
+	if string(body) != "" {
+		t.Errorf("Unexpected response body. Got %q, expected %q", string(body), "")
+	}
+
+	expectedHeaders := []struct {
+		Name  string
+		Value string
+	}{
+		{Name: "Content-Length", Value: "1024"},
+		{Name: "Content-Type", Value: "application/octet-stream"},
+		{Name: "Docker-Content-Digest", Value: dgst.String()},
+		{Name: "Etag", Value: dgst.String()},
+	}
+
+	for _, h := range expectedHeaders {
+		if resp.Header().Get(h.Name) != h.Value {
+			t.Errorf("Unexpected %s. Got %s, expected %s", h.Name, resp.Header().Get(h.Name), h.Value)
+		}
+	}
 }
 
 func TestBlobDelete(t *testing.T) {