[Android] Only write the ordered libraries file when it changes

This action is triggered whenever the top-level library is re-linked. This file
being touched then triggers generating a Java file, and then
rebuilding/installing the APK. This is all unnecessary.

Instead, only write this file if the output has changed. Then, since the ninja
generator uses restat=1, dependent actions will only be retriggered when this
actual changes (almost never).

BUG=158821

Review URL: https://chromiumcodereview.appspot.com/13891010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194780 0039d316-1c4b-4281-b951-d872f2087c98

build/android/gyp/util/build_utils.py

@@ -61,6 +61,17 @@ def CheckOptions(options, parser, required=[]):
     if not getattr(options, option_name):
       parser.error('--%s is required' % option_name.replace('_', '-'))
 
+def WriteJson(obj, path, only_if_changed=False):
+  old_dump = None
+  if os.path.exists(path):
+    with open(path, 'r') as oldfile:
+      old_dump = oldfile.read()
+
+  new_dump = json.dumps(obj)
+
+  if not only_if_changed or old_dump != new_dump:
+    with open(path, 'w') as outfile:
+      outfile.write(new_dump)
 
 def ReadJson(path):
   with open(path, 'r') as jsonfile:

build/android/gyp/write_ordered_libraries.py

@@ -107,8 +107,7 @@ def main(argv):
 
   libraries = GetSortedTransitiveDependencies(libraries)
 
-  with open(_options.output, 'w') as outfile:
-    json.dump(libraries, outfile)
+  build_utils.WriteJson(libraries, _options.output, only_if_changed=True)
 
   if _options.stamp:
     build_utils.Touch(_options.stamp)