Fix test data copying on iOS

build/copy_test_data_ios.gypi

 # 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 file is meant to be included into an action to copy test data files into
 # an iOS app bundle. To use this the following variables need to be defined:
 #   test_data_files: list: paths to test data files or directories
 #   test_data_prefix: string: a directory prefix that will be prepended to each
 #                             output path.  Generally, this should be the base
 #                             directory of the gypi file containing the unittest
@@ skipping 16 matching lines @@
 #           },
 #           'includes': ['path/to/this/gypi/file'],
 #         },
 #       ],
 #     }],
 # }
 #
 
 {
   'inputs': [
-    '<!@pymod_do_main(copy_test_data_ios --inputs <(test_data_files))',
+    # The |-o <(test_data_prefix)| is ignored; it is there to work around a
+    # caching bug in gyp (https://code.google.com/p/gyp/issues/detail?id=112).
+    # It caches command output when the string is the same, so if two copy
+    # steps have the same relative paths, there can be bogus cache hits that
+    # cause compile failures unless something varies.
+    '<!@pymod_do_main(copy_test_data_ios -o <(test_data_prefix) --inputs <(test_data_files))',
   ],
   'outputs': [
     '<!@pymod_do_main(copy_test_data_ios -o <(PRODUCT_DIR)/<(_target_name).app/<(test_data_prefix) --outputs <(test_data_files))',
   ],
   'action': [
     'python',
     '<(DEPTH)/build/copy_test_data_ios.py',
     '-o', '<(PRODUCT_DIR)/<(_target_name).app/<(test_data_prefix)',
     '<@(_inputs)',
   ],
 }