[libfuzzer] support of custom libfuzzer options via .options file.

testing/libfuzzer/gen_fuzzer_config.py

Index: testing/libfuzzer/gen_fuzzer_config.py
diff --git a/testing/libfuzzer/gen_fuzzer_config.py b/testing/libfuzzer/gen_fuzzer_config.py
new file mode 100755
index 0000000000000000000000000000000000000000..5628a30f7b5d4ce2b977c81a05f209018ed9c97e
--- /dev/null
+++ b/testing/libfuzzer/gen_fuzzer_config.py
@@ -0,0 +1,47 @@
+#!/usr/bin/python2
+#
+# Copyright (c) 2015 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.
+
+"""Generate or update an existing config (.options file) for libfuzzer test.
+
+Invoked by GN from fuzzer_test.gni.
+"""
+
+import argparse
+import os
+import sys
+
+
+OPTIONS_FILE_TEMPLATE = '''# This is automatically generated fuzzer config.
+[libfuzzer]
+dict = %(dict)s
+'''
+
+def main():
+  parser = argparse.ArgumentParser(description="Generate fuzzer config.")
+  parser.add_argument('--config', required=True)
+  parser.add_argument('--dict', required=True)
+  parser.add_argument('--libfuzzer_options', required=False)
+  args = parser.parse_args()
+
+  config_path = args.config
+  # Dict will be copied into build directory, use only its basename for config.
+  dict_name = os.path.basename(args.dict)
+
+  if not args.libfuzzer_options:
+    # Generate .options file with initialized 'dict' option.
+    with open(config_path, 'w') as options_file:
+      options_file.write(OPTIONS_FILE_TEMPLATE % {'dict': dict_name})
+    return
+
+  # Append 'dict' option to an existing .options file.
+  initial_config = open(args.libfuzzer_options).read()
+  with open(config_path, 'w') as options_file:
+    options_file.write(initial_config)
+    options_file.write('\ndict = %s\n' % dict_name)
+
+
+if __name__ == '__main__':
+  main()