allow HEAD on ACI images

Fixes #911.

storage/basestorage.py

@@ -46,7 +46,7 @@ class BaseStorage(StoragePaths):
         client to use for any external calls. """
     pass
 
-  def get_direct_download_url(self, path, expires_in=60, requires_cors=False):
+  def get_direct_download_url(self, path, expires_in=60, requires_cors=False, head=False):
     return None
 
   def get_direct_upload_url(self, path, mime_type, requires_cors=True):

storage/cloud.py

@@ -116,10 +116,12 @@ class _CloudStorage(BaseStorageV2):
   def get_supports_resumable_downloads(self):
     return True
 
-  def get_direct_download_url(self, path, expires_in=60, requires_cors=False):
+  def get_direct_download_url(self, path, expires_in=60, requires_cors=False, head=False):
     self._initialize_cloud_conn()
     path = self._init_path(path)
     k = self._key_class(self._cloud_bucket, path)
+    if head:
+      return k.generate_url(expires_in, 'HEAD')
     return k.generate_url(expires_in)
 
   def get_direct_upload_url(self, path, mime_type, requires_cors=True):

storage/fakestorage.py

@@ -15,7 +15,7 @@ class FakeStorage(BaseStorageV2):
   def _init_path(self, path=None, create=False):
     return path
 
-  def get_direct_download_url(self, path, expires_in=60, requires_cors=False):
+  def get_direct_download_url(self, path, expires_in=60, requires_cors=False, head=False):
     try:
       if self.get_content('supports_direct_download') == 'true':
         return 'http://somefakeurl'

storage/swift.py

@@ -114,7 +114,7 @@ class SwiftStorage(BaseStorage):
       logger.exception('Could not head object: %s', path)
       return None
 
-  def get_direct_download_url(self, object_path, expires_in=60, requires_cors=False):
+  def get_direct_download_url(self, object_path, expires_in=60, requires_cors=False, head=False):
     if requires_cors:
       return None
 
@@ -137,7 +137,7 @@ class SwiftStorage(BaseStorage):
     object_path = self._normalize_path(object_path)
 
     # Generate the signed HMAC body.
-    method = 'GET'
+    method = 'HEAD' if head else 'GET'
     expires = int(time() + expires_in)
     full_path = '%s/%s/%s' % (path, self._swift_container, object_path)