[gn] Fix isolate_driver for gn

tools/isolate_driver.py

 #!/usr/bin/env python
 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Adaptor script called through build/isolate.gypi.
 
 Slimmed down version of chromium's isolate driver that doesn't process dynamic
 dependencies.
 """
@@ skipping 12 matching lines @@
 
   Dumps it as JSON to |output| file.
   """
   with open(output, 'wb') as f:
     json.dump({
       'args': args,
       'dir': os.getcwd(),
       'version': 1,
     }, f, indent=2, sort_keys=True)
 
+def rebase_directories(args, abs_base):
+  """Rebases all paths to be relative to abs_base."""
+  def replace(index):
+    args[index] = os.path.relpath(os.path.abspath(args[index]), abs_base)
+  for i, arg in enumerate(args):
+    if arg in ['--isolate', '--isolated']:
+      replace(i + 1)
+    if arg == '--path-variable':
+      # Path variables have a triple form: --path-variable NAME <path>.
+      replace(i + 2)
 
 def main():
   logging.basicConfig(level=logging.ERROR, format='%(levelname)7s %(message)s')
   if len(sys.argv) < 2:
     print >> sys.stderr, 'Internal failure; mode required'
     return 1
   mode = sys.argv[1]
   args = sys.argv[1:]
   isolate = None
   isolated = None
   for i, arg in enumerate(args):
     if arg == '--isolate':
       isolate = i + 1
     if arg == '--isolated':
       isolated = i + 1
   if not isolate or not isolated:
     print >> sys.stderr, 'Internal failure'
     return 1
 
+  # Make sure all paths are relative to the isolate file. This is an
+  # expectation of the go binaries. In gn, this script is not called
+  # relative to the isolate file, but relative to the product dir.
+  new_base = os.path.abspath(os.path.dirname(args[isolate]))
+  rebase_directories(args, new_base)
+  assert args[isolate] == os.path.basename(args[isolate])
+  os.chdir(new_base)
+
   # In 'prepare' mode just collect all required information for postponed
   # isolated.py invocation later, store it in *.isolated.gen.json file.
   if mode == 'prepare':
     prepare_isolate_call(args[1:], args[isolated] + '.gen.json')
     return 0
 
   swarming_client = os.path.join(TOOLS_DIR, 'swarming_client')
   sys.stdout.flush()
   return subprocess.call(
       [sys.executable, os.path.join(swarming_client, 'isolate.py')] + args)
 
 
 if __name__ == '__main__':
   sys.exit(main())