rebaseline_server: cache results in long-running ImageDiffDB instance

gm/rebaseline_server/compare_to_expectations.py

Index: gm/rebaseline_server/compare_to_expectations.py
diff --git a/gm/rebaseline_server/compare_to_expectations.py b/gm/rebaseline_server/compare_to_expectations.py
index e1a5f5f4744801370df0a1636bc10daca4714a9f..dad206d0c8fb57bc3e0763bacf3a29ccfd12f2c0 100755
--- a/gm/rebaseline_server/compare_to_expectations.py
+++ b/gm/rebaseline_server/compare_to_expectations.py
@@ -63,32 +63,30 @@ class ExpectationComparisons(results.BaseComparisons):
   are immutable.  If you want to update the results based on updated JSON
   file contents, you will need to create a new ExpectationComparisons object."""
 
-  def __init__(self, actuals_root=results.DEFAULT_ACTUALS_DIR,
+  def __init__(self, image_diff_db, actuals_root=results.DEFAULT_ACTUALS_DIR,
                expected_root=DEFAULT_EXPECTATIONS_DIR,
                ignore_failures_file=DEFAULT_IGNORE_FAILURES_FILE,
-               generated_images_root=results.DEFAULT_GENERATED_IMAGES_ROOT,
                diff_base_url=None, builder_regex_list=None):
     """
     Args:
+      image_diff_db: instance of ImageDiffDB we use to cache the image diffs
       actuals_root: root directory containing all actual-results.json files
       expected_root: root directory containing all expected-results.json files
       ignore_failures_file: if a file with this name is found within
           expected_root, ignore failures for any tests listed in the file
-      generated_images_root: directory within which to create all pixel diffs;
-          if this directory does not yet exist, it will be created
       diff_base_url: base URL within which the client should look for diff
           images; if not specified, defaults to a "file:///" URL representation
-          of generated_images_root
+          of image_diff_db's storage_root
       builder_regex_list: List of regular expressions specifying which builders
           we will process. If None, process all builders.
     """
     time_start = int(time.time())
     if builder_regex_list != None:
       self.set_match_builders_pattern_list(builder_regex_list)
-    self._image_diff_db = imagediffdb.ImageDiffDB(generated_images_root)
+    self._image_diff_db = image_diff_db
     self._diff_base_url = (
         diff_base_url or
-        url_utils.create_filepath_url(generated_images_root))
+        url_utils.create_filepath_url(image_diff_db.storage_root))
     self._actuals_root = actuals_root
     self._expected_root = expected_root
     self._ignore_failures_on_these_tests = []
@@ -402,11 +400,12 @@ def main():
       help='Directory within which to download images and generate diffs; '
       'defaults to \'%(default)s\' .')
   args = parser.parse_args()
+  image_diff_db = imagediffdb.ImageDiffDB(storage_root=args.workdir)
   results_obj = ExpectationComparisons(
+      image_diff_db=image_diff_db,
       actuals_root=args.actuals,
       expected_root=args.expectations,
-      ignore_failures_file=args.ignore_failures_file,
-      generated_images_root=args.workdir)
+      ignore_failures_file=args.ignore_failures_file)
   gm_json.WriteToFile(
       results_obj.get_packaged_results_of_type(results_type=args.results),
       args.outfile)