Support HEAD /v2/<team>/<artifact>/manifests/<name>

Adds support for responding to `HEAD /v2/<team>/<artifact>/manifests/<name>` requests.
This is required by containerd and described in the opencontainers spec: https://github.com/opencontainers/distribution-spec/blob/master/spec.md

Signed-off-by: Mikkel Oscar Lyderik Larsen <m@moscar.net>

resources/api/pierone-api.yaml

@@ -281,6 +281,21 @@ paths:
       responses:
         "200":
           description: OK
+    head:
+      summary: check for existing image manifest
+      tags:
+        - Docker Registry API v2
+      operationId: org.zalando.stups.pierone.api-v2/head-manifest
+      security:         # remove for HTTP_ALLOW_PUBLIC_READ
+        - oauth2: [uid] # remove for HTTP_ALLOW_PUBLIC_READ
+        - iid:          # remove for HTTP_ALLOW_PUBLIC_READ
+      parameters:
+        - $ref: '#/parameters/team'
+        - $ref: '#/parameters/artifact'
+        - $ref: '#/parameters/name'
+      responses:
+        "200":
+          description: OK
   /v2/{team}/{artifact}/tags/list:
     get:
       summary: list tags

src/org/zalando/stups/pierone/api_v2.clj

@@ -405,6 +405,20 @@
         (log/info "docker pull: manifest not found: %s" pretty-name)
         (resp (get-error-response :MANIFEST_UNKNOWN {"Parameters" parameters}) request :status 404)))))
 
+(defn head-manifest
+  "Check whether image metadata (a.k.a. manifest) exists."
+  [parameters request db _ _ _]
+  (if-let [manifest (load-manifest parameters db)]
+    (let [parsed-manifest (adjust-manifest (json/decode manifest))
+          size (get-in parsed-manifest ["config", "size"])
+          digest (get-in parsed-manifest ["config", "digest"])]
+      (log/info "getting head manifest %s - %s" size digest)
+      (-> (resp "OK" request)
+          (ring/header "Docker-Content-Digest" digest)
+          (ring/header "Content-Length" size)))
+    (do
+      (resp "manifest not found" request :status 404))))
+
 (defn list-tags
   [{:keys [team artifact] :as parameters} request db _ _ _]
   (let [tags (map :name (sql/cmd-list-tag-names parameters {:connection db}))]

test/org/zalando/stups/pierone/v2_test.clj

@@ -147,6 +147,10 @@
                      (client/get (u/v2-url "/myteam/myart/manifests/1.0")
                                  (u/http-opts)))))
 
+      (expect 200
+             (client/head (u/v2-url "/myteam/myart/manifests/1.0")
+                         (u/http-opts))))
+
       (is (= (:pretty d/manifest-v1)
              (expect 200
                      (client/get (u/v2-url "/myteam/myart/manifests/sha256:b0ebea73273b4d5a334d74cd826a2327f260fe5212613937add8b5e171bf49bd")