Fix another bug in the gn_roll script.

tools/gn/bin/roll_gn.py

 # Copyright 2014 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.
 
 """An auto-roller for GN binaries into Chromium.
 
 This script is used to update the GN binaries that a Chromium
 checkout uses. In order to update the binaries, one must follow
 four steps in order:
 
@@ skipping 161 matching lines @@
   def MakeDummyDepsChange(self):
     with open('DEPS') as fp:
       deps_content = fp.read()
       new_deps = deps_content.replace("'buildtools_revision':",
                                       "'buildtools_revision':  ")
 
     with open('DEPS', 'w') as fp:
       fp.write(new_deps)
 
   def WaitForBuildToFinish(self):
+    ret = self.CheckoutBuildBranch()
+    if ret:
+      return ret
+
     print('Checking build')
     results = self.CheckBuild()
     while (len(results) < 3 or
            any(r['state'] in ('pending', 'started')
                for r in results.values())):
       print()
       print('Sleeping for 30 seconds')
       time.sleep(30)
       print('Checking build')
       results = self.CheckBuild()
 
     ret = 0 if all(r['state'] == 'success' for r in results.values()) else 1
     if ret:
       print('Build failed.')
     else:
       print('Builds ready.')
 
     # Close the build CL and move off of the build branch back to whatever
     # we were on before.
     self.Call('git-cl set-close')
     self.MoveToLastHead()
 
     return ret
 
+  def CheckoutBuildBranch(self):
+    ret, out, err = self.Call('git checkout build_gn_%s' % self.new_gn_version)
+    if ret:
+      print('Failed to check out build_gn_%s' % self.new_gn_version)
+      if out:
+        print(out)
+      if err:
+        print(err, file=sys.stderr)
+    return ret
+
   def CheckBuild(self):
     _, out, _ = self.Call('git-cl issue')
 
     issue = int(out.split()[2])
 
     _, out, _ = self.Call('git config user.email')
     email = ''
     rpc_server = upload.GetRpcServer(CODE_REVIEW_SERVER, email)
     try:
       props = json.loads(rpc_server.Send('/api/%d' % issue))
@@ skipping 53 matching lines @@
       print(platform)
       print('  sha1:  %s' % r['sha1'])
       print('  state: %s' % r['state'])
       print('  build: %s' % r['build'])
       print('  url:   %s' % r['url'])
       print()
 
     return results
 
   def RollBuildtools(self):
+    ret = self.CheckoutBuildBranch()
+    if ret:
+      return ret
+
     results = self.CheckBuild()
     if (len(results) < 3 or
         not all(r['state'] == 'success' for r in results.values())):
       print("Roll isn't done or didn't succeed, exiting:")
       return 1
 
     desc = self.GetBuildtoolsDesc()
 
     self.Call('git new-branch roll_buildtools_gn_%s' % self.new_gn_version,
               cwd=self.buildtools_dir)
@@ skipping 145 matching lines @@
   def Call(self, cmd, cwd=None):
     proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell=True,
                             cwd=(cwd or self.chromium_src_dir))
     out, err = proc.communicate()
     return proc.returncode, out, err
 
 
 if __name__ == '__main__':
   roller = GNRoller()
   sys.exit(roller.Roll())