Perform HEAD request, fall back to GET

registry/docker/manifest.go

@@ -15,22 +15,24 @@ func (r *Registry) ManifestDigest(repository, reference string) (digest.Digest,
 	url := r.url("/v2/%s/manifests/%s", repository, reference)
 	r.Logf("registry.manifest.head url=%s repository=%s reference=%s", url, repository, reference)
 
-	req, err := http.NewRequest("GET", url, nil)
+	// Try HEAD request first because it's free
+	resp, err := r.request("HEAD", url)
 	if err != nil {
 		return "", err
 	}
 
-	req.Header.Set("Accept", strings.Join([]string{manifestv2.MediaTypeManifest, oci.MediaTypeImageIndex, oci.MediaTypeImageManifest}, ","))
-	resp, err := r.Client.Do(req)
-	if err != nil {
-		return "", err
-	}
-	defer resp.Body.Close()
-
 	if hdr := resp.Header.Get("Docker-Content-Digest"); hdr != "" {
 		return digest.Parse(hdr)
 	}
 
+	// HEAD request didn't return a digest, attempt to fetch digest from body
+	r.Logf("registry.manifest.get url=%s repository=%s reference=%s", url, repository, reference)
+	resp, err = r.request("GET", url)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
 	// Try to get digest from body instead, should be equal to what would be presented
 	// in Docker-Content-Digest
 	body, err := ioutil.ReadAll(resp.Body)
@@ -39,3 +41,19 @@ func (r *Registry) ManifestDigest(repository, reference string) (digest.Digest,
 	}
 	return digest.FromBytes(body), nil
 }
+
+// request performs a request against a url
+func (r *Registry) request(method string, url string) (*http.Response, error) {
+	req, err := http.NewRequest(method, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("Accept", strings.Join([]string{manifestv2.MediaTypeManifest, oci.MediaTypeImageIndex, oci.MediaTypeImageManifest}, ","))
+	resp, err := r.Client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}

registry/docker/manifest_test.go

@@ -1,7 +1,9 @@
 package docker
 
 import (
+	"bytes"
 	"io"
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -24,9 +26,13 @@ func TestGetDigest(t *testing.T) {
 	}
 	defer resp.Body.Close()
 
+	bodyBytes, _ := ioutil.ReadAll(resp.Body)
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Add("content-type", "application/vnd.docker.distribution.manifest.v2+json; charset=ISO-8859-1")
 		io.Copy(w, resp.Body)
+
+		// Reset body for additional calls
+		resp.Body = ioutil.NopCloser(bytes.NewBuffer(bodyBytes))
 	}))
 	defer ts.Close()