Handle missing storage files (#576)

* Move `validate_file` to `oauth2client.util`
* Warn user if storage file is missing
* Raise an `IOError` exception if the given filename is a directory.
* Raise an `IOError` exception if the given filename is a symbolic link.
      (Previously raised `CredentialsFileSymbolicLinkError`)
* (test) Expanding single-letter variables
* (test) `assertEqual(None, <obj>)` -> `assertIsNone(<obj>)`
* (test) `assertNotEqual(None, <obj>)` -> `assertIsNotNone(<obj>)`

5 files changed, 28 insertions, 31 deletions

diff --git a/oauth2client/contrib/_fcntl_opener.py b/oauth2client/contrib/_fcntl_opener.py
index ae6c85b..c9777a9 100644
--- a/oauth2client/contrib/_fcntl_opener.py
+++ b/oauth2client/contrib/_fcntl_opener.py
@@ -16,6 +16,7 @@ import errno
 import fcntl
 import time
 
+from oauth2client import util
 from oauth2client.contrib import locked_file
 
 
@@ -32,15 +33,14 @@ class _FcntlOpener(locked_file._Opener):
         Raises:
             AlreadyLockedException: if the lock is already acquired.
             IOError: if the open fails.
-            CredentialsFileSymbolicLinkError: if the file is a symbolic
-                                              link.
+            IOError: if the file is a symbolic link.
         """
         if self._locked:
             raise locked_file.AlreadyLockedException(
                 'File {0} is already locked'.format(self._filename))
         start_time = time.time()
 
-        locked_file.validate_file(self._filename)
+        util.validate_file(self._filename)
         try:
             self._fh = open(self._filename, self._mode)
         except IOError as e:
diff --git a/oauth2client/contrib/_win32_opener.py b/oauth2client/contrib/_win32_opener.py
index 34b4f48..6fa0196 100644
--- a/oauth2client/contrib/_win32_opener.py
+++ b/oauth2client/contrib/_win32_opener.py
@@ -19,6 +19,7 @@ import pywintypes
 import win32con
 import win32file
 
+from oauth2client import util
 from oauth2client.contrib import locked_file
 
 
@@ -43,15 +44,14 @@ class _Win32Opener(locked_file._Opener):
         Raises:
             AlreadyLockedException: if the lock is already acquired.
             IOError: if the open fails.
-            CredentialsFileSymbolicLinkError: if the file is a symbolic
-                                              link.
+            IOError: if the file is a symbolic link.
         """
         if self._locked:
             raise locked_file.AlreadyLockedException(
                 'File {0} is already locked'.format(self._filename))
         start_time = time.time()
 
-        locked_file.validate_file(self._filename)
+        util.validate_file(self._filename)
         try:
             self._fh = open(self._filename, self._mode)
         except IOError as e:
diff --git a/oauth2client/contrib/locked_file.py b/oauth2client/contrib/locked_file.py
index 0d28ebb..9c880d7 100644
--- a/oauth2client/contrib/locked_file.py
+++ b/oauth2client/contrib/locked_file.py
@@ -45,21 +45,11 @@ __author__ = 'cache@google.com (David T McWherter)'
 logger = logging.getLogger(__name__)
 
 
-class CredentialsFileSymbolicLinkError(Exception):
-    """Credentials files must not be symbolic links."""
-
-
 class AlreadyLockedException(Exception):
     """Trying to lock a file that has already been locked by the LockedFile."""
     pass
 
 
-def validate_file(filename):
-    if os.path.islink(filename):
-        raise CredentialsFileSymbolicLinkError(
-            'File: {0} is a symbolic link.'.format(filename))
-
-
 class _Opener(object):
     """Base class for different locking primitives."""
 
@@ -119,14 +109,14 @@ class _PosixOpener(_Opener):
         Raises:
             AlreadyLockedException: if the lock is already acquired.
             IOError: if the open fails.
-            CredentialsFileSymbolicLinkError if the file is a symbolic link.
+            IOError: if the file is a symbolic link.
         """
         if self._locked:
             raise AlreadyLockedException(
                 'File {0} is already locked'.format(self._filename))
         self._locked = False
 
-        validate_file(self._filename)
+        util.validate_file(self._filename)
         try:
             self._fh = open(self._filename, self._mode)
         except IOError as e:
diff --git a/oauth2client/file.py b/oauth2client/file.py
index feede11..3d8e41a 100644
--- a/oauth2client/file.py
+++ b/oauth2client/file.py
@@ -22,15 +22,12 @@ import os
 import threading
 
 from oauth2client import client
+from oauth2client import util
 
 
 __author__ = 'jcgregorio@google.com (Joe Gregorio)'
 
 
-class CredentialsFileSymbolicLinkError(Exception):
-    """Credentials files must not be symbolic links."""
-
-
 class Storage(client.Storage):
     """Store and retrieve a single credential to and from a file."""
 
@@ -38,11 +35,6 @@ class Storage(client.Storage):
         super(Storage, self).__init__(lock=threading.Lock())
         self._filename = filename
 
-    def _validate_file(self):
-        if os.path.islink(self._filename):
-            raise CredentialsFileSymbolicLinkError(
-                'File: {0} is a symbolic link.'.format(self._filename))
-
     def locked_get(self):
         """Retrieve Credential from file.
 
@@ -50,10 +42,10 @@ class Storage(client.Storage):
             oauth2client.client.Credentials
 
         Raises:
-            CredentialsFileSymbolicLinkError if the file is a symbolic link.
+            IOError if the file is a symbolic link.
         """
         credentials = None
-        self._validate_file()
+        util.validate_file(self._filename)
         try:
             f = open(self._filename, 'rb')
             content = f.read()
@@ -89,10 +81,10 @@ class Storage(client.Storage):
             credentials: Credentials, the credentials to store.
 
         Raises:
-            CredentialsFileSymbolicLinkError if the file is a symbolic link.
+            IOError if the file is a symbolic link.
         """
         self._create_file_if_needed()
-        self._validate_file()
+        util.validate_file(self._filename)
         f = open(self._filename, 'w')
         f.write(credentials.to_json())
         f.close()
diff --git a/oauth2client/util.py b/oauth2client/util.py
index e3ba62b..18481c9 100644
--- a/oauth2client/util.py
+++ b/oauth2client/util.py
@@ -17,6 +17,8 @@
 import functools
 import inspect
 import logging
+import os
+import warnings
 
 import six
 from six.moves import urllib
@@ -44,6 +46,10 @@ POSITIONAL_SET = frozenset([POSITIONAL_WARNING, POSITIONAL_EXCEPTION,
 
 positional_parameters_enforcement = POSITIONAL_WARNING
 
+_SYM_LINK_MESSAGE = 'File: {0}: Is a symbolic link.'
+_IS_DIR_MESSAGE = '{0}: Is a directory'
+_MISSING_FILE_MESSAGE = 'Cannot access {0}: No such file or directory'
+
 
 def positional(max_positional_args):
     """A decorator to declare that only the first N arguments my be positional.
@@ -204,3 +210,12 @@ def _add_query_parameter(url, name, value):
         q[name] = value
         parsed[4] = urllib.parse.urlencode(q)
         return urllib.parse.urlunparse(parsed)
+
+
+def validate_file(filename):
+    if os.path.islink(filename):
+        raise IOError(_SYM_LINK_MESSAGE.format(filename))
+    elif os.path.isdir(filename):
+        raise IOError(_IS_DIR_MESSAGE.format(filename))
+    elif not os.path.isfile(filename):
+        warnings.warn(_MISSING_FILE_MESSAGE.format(filename))