Refactors utils.load_cached_file

* adds a boolean return representing whether file was reloaded
 * ensures file is actually closed by using a context manager

Change-Id: I4d998c34caa6dde65aaf780c188778477b7f6753

nova/tests/test_utils.py

@@ -345,20 +345,32 @@ class GenericUtilsTestCase(test.TestCase):
     def test_read_modified_cached_file(self):
         self.mox.StubOutWithMock(os.path, "getmtime")
         self.mox.StubOutWithMock(__builtin__, 'open')
-
         os.path.getmtime(mox.IgnoreArg()).AndReturn(2)
 
         fake_contents = "lorem ipsum"
         fake_file = self.mox.CreateMockAnything()
         fake_file.read().AndReturn(fake_contents)
-        __builtin__.open(mox.IgnoreArg()).AndReturn(fake_file)
+        fake_context_manager = self.mox.CreateMockAnything()
+        fake_context_manager.__enter__().AndReturn(fake_file)
+        fake_context_manager.__exit__(mox.IgnoreArg(),
+                                      mox.IgnoreArg(),
+                                      mox.IgnoreArg())
+
+        __builtin__.open(mox.IgnoreArg()).AndReturn(fake_context_manager)
 
         self.mox.ReplayAll()
         cache_data = {"data": 1123, "mtime": 1}
-        data = utils.read_cached_file("/this/is/a/fake", cache_data)
-        self.mox.VerifyAll()
+        self.reload_called = False
+
+        def test_reload(reloaded_data):
+            self.assertEqual(reloaded_data, fake_contents)
+            self.reload_called = True
+
+        data = utils.read_cached_file("/this/is/a/fake", cache_data,
+                                                reload_func=test_reload)
         self.mox.UnsetStubs()
         self.assertEqual(data, fake_contents)
+        self.assertTrue(self.reload_called)
 
     def test_generate_password(self):
         password = utils.generate_password()

nova/utils.py

@@ -1176,18 +1176,24 @@ def sanitize_hostname(hostname):
     return hostname
 
 
-def read_cached_file(filename, cache_info):
-    """Return the contents of a file. If the file hasn't changed since the
-    last invocation, a cached version will be returned.
+def read_cached_file(filename, cache_info, reload_func=None):
+    """Read from a file if it has been modified.
+
+    :param cache_info: dictionary to hold opaque cache.
+    :param reload_func: optional function to be called with data when
+                        file is reloaded due to a modification.
+
+    :returns: data from file
+
     """
     mtime = os.path.getmtime(filename)
-    if cache_info and mtime == cache_info.get('mtime', None):
-        return cache_info['data']
-
-    data = open(filename).read()
-    cache_info['data'] = data
-    cache_info['mtime'] = mtime
-    return data
+    if not cache_info or mtime != cache_info.get('mtime'):
+        with open(filename) as fap:
+            cache_info['data'] = fap.read()
+        cache_info['mtime'] = mtime
+        if reload_func:
+            reload_func(cache_info['data'])
+    return cache_info['data']
 
 
 @contextlib.contextmanager