Get resumable downloads working in a way that the Docker CLI will actually understand. Also rename the method to conform the the name used in the Docker source code.

endpoints/registry.py

@@ -88,6 +88,43 @@ def set_cache_headers(f):
   return wrapper
 
 
+@registry.route('/images/<image_id>/layer', methods=['HEAD'])
+@process_auth
+@extract_namespace_repo_from_session
+@require_completion
+@set_cache_headers
+def head_image_layer(namespace, repository, image_id, headers):
+  permission = ReadRepositoryPermission(namespace, repository)
+
+  profile.debug('Checking repo permissions')
+  if permission.can() or model.repository_is_public(namespace, repository):
+    profile.debug('Looking up repo image')
+    repo_image = model.get_repo_image(namespace, repository, image_id)
+
+    # Add the Accept-Ranges header if the storage engine supports resumeable
+    # downloads.
+    extra_headers = {}
+
+    profile.debug('Looking up the layer path')
+    try:
+      path = store.image_layer_path(repo_image.storage.uuid)
+
+      if store.get_supports_resumeable_downloads(repo_image.storage.locations, path):
+        profile.debug('Storage supports resumeable downloads')
+        extra_headers['Accept-Ranges'] = 'bytes';
+
+      resp = make_response('')
+      resp.headers.extend(extra_headers)
+      return resp
+
+    except (IOError, AttributeError):
+      profile.debug('Image not found')
+      abort(404, 'Image %(image_id)s not found', issue='unknown-image',
+            image_id=image_id)
+  
+  abort(403)
+
+
 @registry.route('/images/<image_id>/layer', methods=['GET'])
 @process_auth
 @extract_namespace_repo_from_session
@@ -101,13 +138,6 @@ def get_image_layer(namespace, repository, image_id, headers):
     profile.debug('Looking up repo image')
     repo_image = model.get_repo_image(namespace, repository, image_id)
 
-    # Add the Accept-Ranges header if the storage engine supports resumable
-    # downloads.
-    extra_headers = {}
-
-    if store.get_supports_resumable_downloads():
-      extra_headers['Accept-Ranges'] = 'bytes';
-
     profile.debug('Looking up the layer path')
     try:
       path = store.image_layer_path(repo_image.storage.uuid)
@@ -118,7 +148,6 @@ def get_image_layer(namespace, repository, image_id, headers):
       if direct_download_url:
         profile.debug('Returning direct download URL')
         resp = redirect(direct_download_url)
-        resp.headers = dict(headers, **extra_headers)
         return resp
 
       profile.debug('Streaming layer data')