rebaseline_server: add "prefetch" directive that just warms the cache without awaiting results

gm/rebaseline_server/imagepair.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 ImagePair class (see class docstring for details)
 """
@@ skipping 14 matching lines @@
 # image diff details yet.
 _DIFF_RECORD_STILL_LOADING = 'still_loading'
 
 
 class ImagePair(object):
   """Describes a pair of images, pixel difference info, and optional metadata.
   """
 
   def __init__(self, image_diff_db,
                base_url, imageA_relative_url, imageB_relative_url,
-               expectations=None, extra_columns=None):
+               expectations=None, extra_columns=None,
+               download_all_images=False):
     """
     Args:
       image_diff_db: ImageDiffDB instance we use to generate/store image diffs
       base_url: base of all image URLs
       imageA_relative_url: string; URL pointing at an image, relative to
           base_url; or None, if this image is missing
       imageB_relative_url: string; URL pointing at an image, relative to
           base_url; or None, if this image is missing
       expectations: optional dictionary containing expectations-specific
           metadata (ignore-failure, bug numbers, etc.)
       extra_columns: optional dictionary containing more metadata (test name,
           builder name, etc.)
+      download_all_images: if True, download any images associated with this
+          image pair, even if we don't need them to generate diffs
+          (imageA == imageB, or one of them is missing)
     """
     self._image_diff_db = image_diff_db
     self.base_url = base_url
     self.imageA_relative_url = imageA_relative_url
     self.imageB_relative_url = imageB_relative_url
     self.expectations_dict = expectations
     self.extra_columns_dict = extra_columns
     if not imageA_relative_url or not imageB_relative_url:
       self._is_different = True
       self._diff_record = None
     elif imageA_relative_url == imageB_relative_url:
       self._is_different = False
       self._diff_record = None
     else:
       # Tell image_diff_db to add an entry for this diff asynchronously.
       # Later on, we will call image_diff_db.get_diff_record() to find it.
       self._is_different = True
       self._diff_record = _DIFF_RECORD_STILL_LOADING
+
+    if self._diff_record != None or download_all_images:
       image_diff_db.add_image_pair(
           expected_image_locator=imageA_relative_url,
-          expected_image_url=posixpath.join(base_url, imageA_relative_url),
+          expected_image_url=self.posixpath_join(base_url, imageA_relative_url),
           actual_image_locator=imageB_relative_url,
-          actual_image_url=posixpath.join(base_url, imageB_relative_url))
+          actual_image_url=self.posixpath_join(base_url, imageB_relative_url))
 
   def as_dict(self):
     """Returns a dictionary describing this ImagePair.
 
     Uses the KEY__IMAGEPAIRS__* constants as keys.
     """
     asdict = {
         KEY__IMAGEPAIRS__IMAGE_A_URL: self.imageA_relative_url,
         KEY__IMAGEPAIRS__IMAGE_B_URL: self.imageB_relative_url,
     }
     asdict[KEY__IMAGEPAIRS__IS_DIFFERENT] = self._is_different
     if self.expectations_dict:
       asdict[KEY__IMAGEPAIRS__EXPECTATIONS] = self.expectations_dict
     if self.extra_columns_dict:
       asdict[KEY__IMAGEPAIRS__EXTRACOLUMNS] = self.extra_columns_dict
     if self._diff_record is _DIFF_RECORD_STILL_LOADING:
       # We have waited as long as we can to ask ImageDiffDB for details of
       # this image diff.  Now we must block until ImageDiffDB can provide
       # those details.
       #
       # TODO(epoger): Is it wasteful for every imagepair to have its own
       # reference to image_diff_db?  If so, we could pass an image_diff_db
       # reference into this method call instead...
       self._diff_record = self._image_diff_db.get_diff_record(
           expected_image_locator=self.imageA_relative_url,
           actual_image_locator=self.imageB_relative_url)
     if self._diff_record != None:
       asdict[KEY__IMAGEPAIRS__DIFFERENCES] = self._diff_record.as_dict()
     return asdict
+
+  @staticmethod
+  def posixpath_join(*args):
+    """Wrapper around posixpath.join().
+
+    Returns posixpath.join(*args), or None if any arg is None.
+    """
+    for arg in args:
+      if arg == None:
+        return None
+    return posixpath.join(*args)