Enable caching of blobs in V2 registry protocol, to avoid DB connections after the cache has been loaded

This should help for bursty pull traffic, as it will avoid DB connections on a huge % of requests

endpoints/v2/test/test_blob.py

@@ -0,0 +1,59 @@
+import hashlib
+import pytest
+
+from mock import patch
+from flask import url_for
+from playhouse.test_utils import assert_query_count
+
+from app import instance_keys, app as realapp
+from data import model
+from data.cache import InMemoryDataModelCache
+from data.database import ImageStorageLocation
+from endpoints.test.shared import conduct_call
+from util.security.registry_jwt import generate_bearer_token, build_context_and_subject
+from test.fixtures import *
+
+@pytest.mark.parametrize('method, endpoint', [
+  ('GET', 'download_blob'),
+  ('HEAD', 'check_blob_exists'),
+])
+def test_blob_caching(method, endpoint, client, app):
+  digest = 'sha256:' + hashlib.sha256("a").hexdigest()
+  location = ImageStorageLocation.get(name='local_us')
+  model.blob.store_blob_record_and_temp_link('devtable', 'simple', digest, location, 1, 10000000)
+
+  params = {
+    'repository': 'devtable/simple',
+    'digest': digest,
+  }
+
+  user = model.user.get_user('devtable')
+  access = [{
+    'type': 'repository',
+    'name': 'devtable/simple',
+    'actions': ['pull'],
+  }]
+
+  context, subject = build_context_and_subject(user=user)
+  token = generate_bearer_token(realapp.config['SERVER_HOSTNAME'], subject, context, access, 600,
+                                instance_keys)
+
+  headers = {
+    'Authorization': 'Bearer %s' % token,
+  }
+
+  # Run without caching to make sure the request works. This also preloads some of
+  # our global model caches.
+  conduct_call(client, 'v2.' + endpoint, url_for, method, params, expected_code=200,
+               headers=headers)
+
+  with patch('endpoints.v2.blob.model_cache', InMemoryDataModelCache()):
+    # First request should make a DB query to retrieve the blob.
+    with assert_query_count(1):
+      conduct_call(client, 'v2.' + endpoint, url_for, method, params, expected_code=200,
+                   headers=headers)
+
+    # Subsequent requests should use the cached blob.
+    with assert_query_count(0):
+      conduct_call(client, 'v2.' + endpoint, url_for, method, params, expected_code=200,
+                   headers=headers)