Pass ninja -n in the ninja -d explain step to avoid more work

scripts/slave/compile.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """A tool to build chrome, executed by buildbot.
 
   When this is run, the current directory (cwd) should be the outer build
   directory (e.g., chrome-release/build/).
 
@@ skipping 414 matching lines @@
       assert options.goma_jobs
       command.append('-j%d' % options.goma_jobs)
 
     # Run the build.
     env.print_overrides()
     exit_status = chromium_utils.RunCommand(command, env=env)
     if exit_status == 0 and options.ninja_ensure_up_to_date:
       # Run the build again if we want to check that the no-op build is clean.
       filter_obj = EnsureUpToDateFilter()
       # Append `-d explain` to help diagnose in the failure case.
-      command += ['-d', 'explain']
+      command += ['-d', 'explain', '-n']
       chromium_utils.RunCommand(command, env=env, filter_obj=filter_obj)
       if not filter_obj.was_up_to_date:
         print 'Failing build because ninja reported work to do.'
         print 'This means that after completing a compile, another was run and'
         print 'it resulted in still having work to do (that is, a no-op build'
         print 'wasn\'t a no-op). Consult the first "ninja explain:" line for a'
         print 'likely culprit.'
         return 1
     return exit_status
   finally:
@@ skipping 120 matching lines @@
   exit_status = main_ninja(options, args, env)
 
   # stop goma
   goma_teardown(options, env, exit_status, goma_cloudtail)
 
   return exit_status
 
 
 if '__main__' == __name__:
   sys.exit(real_main())