Do not copy expectations file to itself.

slave/skia_slave_scripts/prerender.py

Index: slave/skia_slave_scripts/prerender.py
diff --git a/slave/skia_slave_scripts/prerender.py b/slave/skia_slave_scripts/prerender.py
index bc36c07a95a25603516ccfaed7382fe4941cc71b..799e422986d880b025880f78584bec277cd13465 100644
--- a/slave/skia_slave_scripts/prerender.py
+++ b/slave/skia_slave_scripts/prerender.py
@@ -133,8 +133,13 @@ class PreRender(BuildStep):
     if os.path.exists(skimage_host_expectations):
       skimage_device_expectations = self._flavor_utils.DevicePathJoin(
           self._device_dirs.SKImageExpectedDir(), skimage_expected_filename)
-      self._flavor_utils.PushFileToDevice(skimage_host_expectations,
-          skimage_device_expectations)
+      # For builders without an attached device, PushFileToDevice will fail
+      # when attempting to copy a file to itself. In this case, there is no
+      # need to copy. Only do the push when there is an attached device,
+      # which corresponds to the case that the filepaths are equal.
+      if not skimage_device_expectations == skimage_host_expectations:

[borenet, 2013/08/28 13:00:46]

I would prefer "skimage_device_expectations != skimage_host_expectations"

[scroggo, 2013/08/28 13:01:57]

Done.

[scroggo, 2013/08/28 13:03:39]

I could actually compare self._device_dirs.SKImageExpectedDir against
self._skimage_expected_dir, which would skip the DevicePathJoin on Win/Lin/Mac.
Better?

[borenet, 2013/08/28 13:12:40]

I think maybe the best thing is to modify DefaultBuildStepUtils.PushFileToDevice
to be a no-op which raises an exception if src and dst are different (see
DefaultBuildStepUtils.CopyDirectoryContentsTo*)

[scroggo, 2013/08/28 13:45:01]

Currently, GM uses PushFileToDevice (up above, in this same file;
PushFileToDevice is called by no one else except by other android and chromeos
flavor_utils) to copy an expectations file from one of two places to the right
place, and modifying PushFileToDevice to behave like
CopyDirectoryContentsToDevice would break that copy. I can think of a few
possible solutions:
- depending on where GM is in the transition to checksums, it may be ready to
grab the expectations from one place, in which case that could always be the
same as device_gm_expectations_path (except of course, when there is a device,
which will call the custom version of PushFileToDevice).
- instead of changing PushFileToDevice to match the behavior of
CopyDirectoryContentsToDevice, change it to only do the copy if the two
locations are different.
- it seems that GM is moving locally because the expectations file is either
downloaded from SVN, or it's created in another place. Either way it's moved to
the right place. If it was not downloaded by SVN, what if we create it where it
would have been (repo_gm_expectations_path), set device_gm_expectations_path to
be repo_gm_expectations_path, and then the copy would be a no op (except for
attached devices).

+        self._flavor_utils.PushFileToDevice(skimage_host_expectations,
+            skimage_device_expectations)
 
     self._flavor_utils.CopyDirectoryContentsToDevice(
         self._skimage_in_dir, self._device_dirs.SKImageInDir())