Fall back to credential refresh on EDEADLK

Fixes https://github.com/google/oauth2client/issues/335

This change fixes a bug where multiple threads and/or processes
using multistore_file to access the same backing store could
raise IOError errno.EDEADLK to the calling application. Since
EDEADLK is a possibility with concurrent access, this change
instead causes a fallback to read only mode and refresh
credentials if necessary.

oauth2client/multistore_file.py

@@ -290,6 +290,9 @@ class _MultiStore(object):
             elif e.errno == errno.ENOLCK:
                 logger.warn('File system is out of resources for writing the '
                             'credentials file (is your disk full?).')
+            elif e.errno == errno.EDEADLK:
+                logger.warn('Lock contention on multistore file, opening '
+                            'in read-only mode.')
             else:
                 raise
         if not self._file.is_locked():

tests/test_multistore_file.py

@@ -0,0 +1,43 @@
+"""Unit tests for oauth2client.multistore_file."""
+
+import errno
+import os
+import tempfile
+import unittest
+
+from oauth2client import multistore_file
+
+
+class _MockLockedFile(object):
+
+    def __init__(self, filename_str, error_code):
+        self.filename_str = filename_str
+        self.error_code = error_code
+        self.open_and_lock_called = False
+
+    def open_and_lock(self):
+        self.open_and_lock_called = True
+        raise IOError(self.error_code, '')
+
+    def is_locked(self):
+        return False
+
+    def filename(self):
+        return self.filename_str
+
+
+class MultistoreFileTests(unittest.TestCase):
+
+    def test_lock_file_raises_ioerror(self):
+        filehandle, filename = tempfile.mkstemp()
+        os.close(filehandle)
+
+        try:
+            for error_code in (errno.EDEADLK, errno.ENOSYS, errno.ENOLCK):
+                multistore = multistore_file._MultiStore(filename)
+                multistore._file = _MockLockedFile(filename, error_code)
+                # Should not raise even though the underlying file class did.
+                multistore._lock()
+                self.assertTrue(multistore._file.open_and_lock_called)
+        finally:
+            os.unlink(filename)