Have build/landmines.py delete contents of build directory.

build/landmines.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.
 
 """
 This script runs every build as the first hook (See DEPS). If it detects that
-the build should be clobbered, it will remove the build directory.
+the build should be clobbered, it will delete the contents of the build
+directory.
 
 A landmine is tripped when a builder checks out a different revision, and the
 diff between the new landmines and the old ones is non-null. At this point, the
 build is clobbered.
 """
 
 import difflib
 import errno
 import gyp_environment
 import logging
@@ skipping 44 matching lines @@
     with open(landmines_path, 'r') as f:
       old_landmines = f.readlines()
     if old_landmines != new_landmines:
       old_date = time.ctime(os.stat(landmines_path).st_ctime)
       diff = difflib.unified_diff(old_landmines, new_landmines,
           fromfile='old_landmines', tofile='new_landmines',
           fromfiledate=old_date, tofiledate=time.ctime(), n=0)
       sys.stdout.write('Clobbering due to:\n')
       sys.stdout.writelines(diff)
 
-      # Clobber.
-      shutil.rmtree(out_dir)
+      # Clobber contents of build directory but not directory itself: some
+      # checkouts have the build directory mounted.
+      for f in os.listdir(out_dir):
+        path = os.path.join(out_dir, f)
+        if os.path.isfile(path):
+          os.unlink(path)
+        elif os.path.isdir(path):
+          shutil.rmtree(path)
 
   # Save current set of landmines for next time.
   with open(landmines_path, 'w') as f:
     f.writelines(new_landmines)
 
 
 def process_options():
   """Returns a list of landmine emitting scripts."""
   parser = optparse.OptionParser()
   parser.add_option(
@@ skipping 36 matching lines @@
     proc = subprocess.Popen([sys.executable, s], stdout=subprocess.PIPE)
     output, _ = proc.communicate()
     landmines.extend([('%s\n' % l.strip()) for l in output.splitlines()])
   clobber_if_necessary(landmines)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())