Make it possible to shard run_too.py invocations.

tools/clang/scripts/run_tool.py

 #!/usr/bin/env python
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Wrapper script to help run clang tools across Chromium code.
 
 How to use run_tool.py:
 If you want to run a clang tool across all Chromium code:
 run_tool.py <tool> <path/to/compiledb>
 
@@ skipping 177 matching lines @@
 
 def main():
   parser = argparse.ArgumentParser()
   parser.add_argument('tool', help='clang tool to run')
   parser.add_argument('--all', action='store_true')
   parser.add_argument(
       '--generate-compdb',
       action='store_true',
       help='regenerate the compile database before running the tool')
   parser.add_argument(
+      '--shard',
+      metavar='<n>-of-<count>')
+  parser.add_argument(
       'compile_database',
       help='path to the directory that contains the compile database')
   parser.add_argument(
       'path_filter',
       nargs='*',
       help='optional paths to filter what files the tool is run on')
   parser.add_argument(
       '--tool-args', nargs='*',
       help='optional arguments passed to the tool')
   args = parser.parse_args()
@@ skipping 11 matching lines @@
   if args.all:
     source_filenames = set(_GetFilesFromCompileDB(args.compile_database))
   else:
     git_filenames = set(_GetFilesFromGit(args.path_filter))
     # Filter out files that aren't C/C++/Obj-C/Obj-C++.
     extensions = frozenset(('.c', '.cc', '.cpp', '.m', '.mm'))
     source_filenames = [f
                         for f in git_filenames
                         if os.path.splitext(f)[1] in extensions]
 
+  if args.shard:
+    total_length = len(source_filenames)
+    match = re.match(r'(\d+)-of-(\d+)$', args.shard)
+    # Input is 1-based, but modular arithmetic is 0-based.
+    shard_number = int(match.group(1)) - 1
+    shard_count = int(match.group(2))
+    source_filenames = [
+        f[1] for f in enumerate(sorted(source_filenames))
+        if f[0] % shard_count == shard_number
+    ]
+    print 'Shard %d-of-%d will process %d entries out of %d' % (
+        shard_number, shard_count, len(source_filenames), total_length)
+
   dispatcher = _CompilerDispatcher(args.tool, args.tool_args,
                                    args.compile_database,
                                    source_filenames)
   dispatcher.Run()
   return -dispatcher.failed_count
 
 
 if __name__ == '__main__':
   sys.exit(main())