Update prebuilt.py to handle files not existing. Add a unittest to test the case

prebuilt_unittest.py

 #!/usr/bin/python
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import copy
 import mox
 import os
 import prebuilt
 import shutil
@@ skipping 27 matching lines @@
                          'portage portage-20100310.tar.bz2',
                          'COMPILE_FLAGS="some_value=some_other"',
                          ]
     temp_fd, self.version_file = tempfile.mkstemp()
     os.write(temp_fd, '\n'.join(self.contents_str))
     os.close(temp_fd)
 
   def tearDown(self):
     os.remove(self.version_file)
 
-  def _read_version_file(self):
+  def _read_version_file(self, version_file=None):
     """Read the contents of self.version_file and return as a list."""
-    version_fh = open(self.version_file)
+    if not version_file:
+      version_file = self.version_file
+
+    version_fh = open(version_file)
     try:
       return [line.strip() for line in version_fh.readlines()]
     finally:
       version_fh.close()
 
   def _verify_key_pair(self, key, val):
     file_contents = self._read_version_file()
     # ensure key for verify is wrapped on quotes
     if '"' not in val:
       val = '"%s"' % val
@@ skipping 18 matching lines @@
     print self.version_file
 
   def testUpdateVariable(self):
     """Test updating a variable that already exists."""
     key, val = self.contents_str[2].split('=')
     new_val = 'test_update'
     self._verify_key_pair(key, val)
     prebuilt.UpdateLocalFile(self.version_file, new_val)
     self._verify_key_pair(key, new_val)
 
+  def testUpdateNonExistentFile(self):
+    key = 'PORTAGE_BINHOST'
+    value = '1234567'
+    non_existent_file = tempfile.mktemp()
+    try:
+      prebuilt.UpdateLocalFile(non_existent_file, value)
+      file_contents = self._read_version_file(non_existent_file)
+      self.assertEqual(file_contents, ['%s=%s' % (key, value)])
+    finally:
+      if os.path.exists(non_existent_file):
+        os.remove(non_existent_file)
+
 
 class TestPrebuiltFilters(unittest.TestCase):
 
   def setUp(self):
     self.tmp_dir = tempfile.mkdtemp()
     self.private_dir = os.path.join(self.tmp_dir,
                                     prebuilt._PRIVATE_OVERLAY_DIR)
     self.private_structure_base = 'chromeos-overlay/chromeos-base'
     self.private_pkgs = ['test-package/salt-flavor-0.1.r3.ebuild',
                          'easy/alpha_beta-0.1.41.r3.ebuild',
@@ skipping 217 matching lines @@
     pkgindex = SimplePackageIndex()
     self.mox.StubOutWithMock(pkgindex, 'Write')
     pkgindex.Write(mox.IgnoreArg())
     self.mox.ReplayAll()
     f = pkgindex.WriteToNamedTemporaryFile()
     self.assertEqual(f.read(), '')
 
 
 if __name__ == '__main__':
   unittest.main()