Only write to xmodule asset files that have changed

Use md5 checksumming to verify that we only write out xmodule asset
files whose contents differ from what we are about to write. This
minimizes thrashing of the other watchers.

Fixes LMS-452

CHANGELOG.rst

@@ -5,6 +5,8 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+XModule: Only write out assets files if the contents have changed.
+
 XModule: Don't delete generated xmodule asset files when compiling (for
 instance, when XModule provides a coffeescript file, don't delete
 the associated javascript)

common/lib/xmodule/xmodule/static_content.py

@@ -4,6 +4,7 @@ This module has utility functions for gathering up the static content
 that is defined by XModules and XModuleDescriptors (javascript and css)
 """
 
+import logging
 import hashlib
 import os
 import errno
@@ -15,6 +16,9 @@ from path import path
 from xmodule.x_module import XModuleDescriptor
 
 
+LOG = logging.getLogger(__name__)
+
+
 def write_module_styles(output_root):
     return _write_styles('.xmodule_display', output_root, _list_modules())
 
@@ -140,7 +144,13 @@ def _write_files(output_root, contents, generated_suffix_map=None):
         (output_root / extra_file).remove_p()
 
     for filename, file_content in contents.iteritems():
-        (output_root / filename).write_bytes(file_content)
+        output_file = output_root / filename
+
+        if not output_file.isfile() or output_file.read_md5() != hashlib.md5(file_content).digest():
+            LOG.debug("Writing %s", output_file)
+            output_file.write_bytes(file_content)
+        else:
+            LOG.debug("%s unchanged, skipping", output_file)
 
 
 def main():