rebaseline_server: fail fast if skpdiff binary is not available

gm/rebaseline_server/imagediffdb.py

Index: gm/rebaseline_server/imagediffdb.py
diff --git a/gm/rebaseline_server/imagediffdb.py b/gm/rebaseline_server/imagediffdb.py
index 8c010f8a2bbf364e010153a393cd15c5b03267af..8cec46bc9434fcb74ee7d1b1d2619e0411704618 100644
--- a/gm/rebaseline_server/imagediffdb.py
+++ b/gm/rebaseline_server/imagediffdb.py
@@ -31,7 +31,7 @@ sys.path.append(
                         'tools'))
 import find_run_binary
 
-SKPDIFF_BINARY_NAME = 'skpdiff'
+SKPDIFF_BINARY = find_run_binary.find_path_to_program('skpdiff')
 
 DEFAULT_IMAGE_SUFFIX = '.png'
 DEFAULT_IMAGES_SUBDIR = 'images'
@@ -115,13 +115,12 @@ class DiffRecord(object):
     skpdiff_csv_dir = tempfile.mkdtemp()
     try:
       skpdiff_csv_output = os.path.join(skpdiff_csv_dir, 'skpdiff-output.csv')
-      skpdiff_binary = find_run_binary.find_path_to_program(SKPDIFF_BINARY_NAME)
       expected_img = os.path.join(storage_root, expected_images_subdir,
                                   str(expected_image_locator) + image_suffix)
       actual_img = os.path.join(storage_root, actual_images_subdir,
                                 str(actual_image_locator) + image_suffix)
       find_run_binary.run_command(
-          [skpdiff_binary, '-p', expected_img, actual_img,
+          [SKPDIFF_BINARY, '-p', expected_img, actual_img,
            '--csv', skpdiff_csv_output, '-d', 'perceptual'])
       with contextlib.closing(open(skpdiff_csv_output)) as csv_file:
         for row in csv.DictReader(csv_file):
@@ -172,7 +171,10 @@ class DiffRecord(object):
 
   def get_weighted_diff_measure(self):
     """Returns a weighted measure of image diffs, as a float between 0 and 100
-    (inclusive)."""
+    (inclusive).
+
+    TODO(epoger): Delete this function, now that we have perceptual diff?
+    """
     return self._weighted_diff_measure
 
   def get_max_diff_per_channel(self):
@@ -265,6 +267,8 @@ def _calculate_weighted_diff_metric(histogram, num_pixels):
   pixel between two images), calculate the weighted diff metric (a
   stab at how different the two images really are).
 
+  TODO(epoger): Delete this function, now that we have perceptual diff?
+
   Args:
     histogram: PIL histogram of a per-channel diff between two images
     num_pixels: integer; the total number of pixels in the diff image
@@ -274,8 +278,6 @@ def _calculate_weighted_diff_metric(histogram, num_pixels):
   # TODO(epoger): As a wild guess at an appropriate metric, weight each
   # different pixel by the square of its delta value.  (The more different
   # a pixel is from its expectation, the more we care about it.)
-  # In the long term, we will probably use some metric generated by
-  # skpdiff anyway.
   assert(len(histogram) % VALUES_PER_BAND == 0)
   num_bands = len(histogram) / VALUES_PER_BAND
   max_diff = num_pixels * num_bands * (VALUES_PER_BAND - 1)**2