Add Etag header for manifests.

Return 304 (Not Modified) if retrieved with If-None-Match header

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/handlers/api_test.go

@@ -449,6 +449,7 @@ func TestManifestAPI(t *testing.T) {
 	checkResponse(t, "fetching uploaded manifest", resp, http.StatusOK)
 	checkHeaders(t, resp, http.Header{
 		"Docker-Content-Digest": []string{dgst.String()},
+		"ETag":                  []string{dgst.String()},
 	})
 
 	var fetchedManifest manifest.SignedManifest
@@ -470,6 +471,7 @@ func TestManifestAPI(t *testing.T) {
 	checkResponse(t, "fetching uploaded manifest", resp, http.StatusOK)
 	checkHeaders(t, resp, http.Header{
 		"Docker-Content-Digest": []string{dgst.String()},
+		"ETag":                  []string{dgst.String()},
 	})
 
 	var fetchedManifestByDigest manifest.SignedManifest
@@ -482,6 +484,33 @@ func TestManifestAPI(t *testing.T) {
 		t.Fatalf("manifests do not match")
 	}
 
+	// Get by name with etag, gives 304
+	etag := resp.Header.Get("Etag")
+	req, err := http.NewRequest("GET", manifestURL, nil)
+	if err != nil {
+		t.Fatalf("Error constructing request: %s", err)
+	}
+	req.Header.Set("If-None-Match", etag)
+	resp, err = http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Error constructing request: %s", err)
+	}
+
+	checkResponse(t, "fetching layer with etag", resp, http.StatusNotModified)
+
+	// Get by digest with etag, gives 304
+	req, err = http.NewRequest("GET", manifestDigestURL, nil)
+	if err != nil {
+		t.Fatalf("Error constructing request: %s", err)
+	}
+	req.Header.Set("If-None-Match", etag)
+	resp, err = http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Error constructing request: %s", err)
+	}
+
+	checkResponse(t, "fetching layer with etag", resp, http.StatusNotModified)
+
 	// Ensure that the tag is listed.
 	resp, err = http.Get(tagsURL)
 	if err != nil {

docs/handlers/images.go

@@ -60,6 +60,10 @@ func (imh *imageManifestHandler) GetImageManifest(w http.ResponseWriter, r *http
 	if imh.Tag != "" {
 		sm, err = manifests.GetByTag(imh.Tag)
 	} else {
+		if etagMatch(r, imh.Digest.String()) {
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
 		sm, err = manifests.Get(imh.Digest)
 	}
 
@@ -75,6 +79,10 @@ func (imh *imageManifestHandler) GetImageManifest(w http.ResponseWriter, r *http
 			imh.Errors = append(imh.Errors, v2.ErrorCodeDigestInvalid.WithDetail(err))
 			return
 		}
+		if etagMatch(r, dgst.String()) {
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
 
 		imh.Digest = dgst
 	}
@@ -82,9 +90,19 @@ func (imh *imageManifestHandler) GetImageManifest(w http.ResponseWriter, r *http
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.Header().Set("Content-Length", fmt.Sprint(len(sm.Raw)))
 	w.Header().Set("Docker-Content-Digest", imh.Digest.String())
+	w.Header().Set("Etag", imh.Digest.String())
 	w.Write(sm.Raw)
 }
 
+func etagMatch(r *http.Request, etag string) bool {
+	for _, headerVal := range r.Header["If-None-Match"] {
+		if headerVal == etag {
+			return true
+		}
+	}
+	return false
+}
+
 // PutImageManifest validates and stores and image in the registry.
 func (imh *imageManifestHandler) PutImageManifest(w http.ResponseWriter, r *http.Request) {
 	ctxu.GetLogger(imh).Debug("PutImageManifest")