Switch V2 pagination back to using IDs, which should be much faster and easier on the DB

Also adds a test for the tags endpoint

test/registry/protocol_v2.py

@@ -347,6 +347,48 @@ class V2Protocol(RegistryProtocol):
     return PullResult(manifests=manifests, image_ids=image_ids)
 
 
+  def tags(self, session, namespace, repo_name, page_size=2, credentials=None, options=None,
+           expected_failure=None):
+    options = options or ProtocolOptions()
+    scopes = options.scopes or ['repository:%s:pull' % self.repo_name(namespace, repo_name)]
+
+    # Ping!
+    self.ping(session)
+
+    # Perform auth and retrieve a token.
+    headers = {}
+    if credentials is not None:
+      token, _ = self.auth(session, credentials, namespace, repo_name, scopes=scopes,
+                           expected_failure=expected_failure)
+      if token is None:
+        return None
+
+      headers = {
+        'Authorization': 'Bearer ' + token,
+      }
+
+    results = []
+    url = '/v2/%s/tags/list' % (self.repo_name(namespace, repo_name))
+    params = {}
+    if page_size is not None:
+      params['n'] = page_size
+
+    while True:
+      response = self.conduct(session, 'GET', url, headers=headers, params=params)
+      data = response.json()
+
+      assert len(data['tags']) <= page_size
+      results.extend(data['tags'])
+
+      if not response.headers.get('Link'):
+        return results
+
+      link_url = response.headers['Link']
+      v2_index = link_url.find('/v2/')
+      url = link_url[v2_index:]
+
+    return results
+
   def catalog(self, session, page_size=2, credentials=None, options=None, expected_failure=None,
               namespace=None, repo_name=None):
     options = options or ProtocolOptions()