Prevent landmines.py from deleting your source 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 delete the contents of the build
 directory.
 
@@ skipping 26 matching lines @@
     r'c:\b\build\slave\win\build\src\out'
     '/mnt/data/b/build/slave/linux/build/src/out'
     '/b/build/slave/ios_rel_device/build/src/xcodebuild'
 
   Keep this function in sync with tools/build/scripts/slave/compile.py
   """
   ret = None
   if build_tool == 'xcode':
     ret = os.path.join(SRC_DIR, 'xcodebuild')
   elif build_tool in ['make', 'ninja', 'ninja-ios']:  # TODO: Remove ninja-ios.
-    if ('CHROMIUM_OUT_DIR' not in os.environ and
-        'output_dir' in landmine_utils.gyp_generator_flags()):
-      output_dir = landmine_utils.gyp_generator_flags()['output_dir']
+    if 'CHROMIUM_OUT_DIR' in os.environ:
+      output_dir = os.environ.get('CHROMIUM_OUT_DIR').strip()
+      if not output_dir:
+        raise Error('CHROMIUM_OUT_DIR environment variable is set but blank!')
     else:
-      output_dir = os.environ.get('CHROMIUM_OUT_DIR', 'out')
+      output_dir = landmine_utils.gyp_generator_flags().get('output_dir', 'out')
     ret = os.path.join(SRC_DIR, output_dir)
   else:
     raise NotImplementedError('Unexpected GYP_GENERATORS (%s)' % build_tool)
   return os.path.abspath(ret)
 
 
 def extract_gn_build_commands(build_ninja_file):
   """Extracts from a build.ninja the commands to run GN.
 
   The commands to run GN are the gn rule and build.ninja build step at the
@@ skipping 144 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())