Optimize check_gn_headers.py for speed

build/check_gn_headers.py

 #!/usr/bin/env python
 # Copyright 2017 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.
 
 """Find header files missing in GN.
 
 This script gets all the header files from ninja_deps, which is from the true
 dependency generated by the compiler, and report if they don't exist in GN.
 """
 
 import argparse
 import json
 import os
 import re
 import subprocess
 import sys
 
 
 def GetHeadersFromNinja(out_dir):
   """Return all the header files from ninja_deps"""
-  ninja_out = subprocess.check_output(['ninja', '-C', out_dir, '-t', 'deps'])
+
+  def NinjaSource():
+    cmd = ['ninja', '-C', out_dir, '-t', 'deps']
+    # A negative bufsize means to use the system default, which usually
+    # means fully buffered.
+    popen = subprocess.Popen(cmd, stdout=subprocess.PIPE, bufsize=-1)
+    for line in iter(popen.stdout.readline, ''):
+      yield line
+
+    popen.stdout.close()
+    return_code = popen.wait()
+    if return_code:
+      raise subprocess.CalledProcessError(return_code, cmd)
+
+  ninja_out = NinjaSource()
   return ParseNinjaDepsOutput(ninja_out)
 
 
 def ParseNinjaDepsOutput(ninja_out):
   """Parse ninja output and get the header files"""
   all_headers = set()
 
   prefix = '..' + os.sep + '..' + os.sep
 
   is_valid = False
-  for line in ninja_out.split('\n'):
+  for line in ninja_out:
     if line.startswith('    '):
       if not is_valid:
         continue
       if line.endswith('.h') or line.endswith('.hh'):
         f = line.strip()
         if f.startswith(prefix):
           f = f[6:]  # Remove the '../../' prefix
           # build/ only contains build-specific files like build_config.h
           # and buildflag.h, and system header files, so they should be
           # skipped.
@@ skipping 78 matching lines @@
     return 0
 
   print 'The following files should be included in gn files:'
   for i in missing:
     print i
   return 1
 
 
 if __name__ == '__main__':
   sys.exit(main())