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/utils.py

@@ -1176,18 +1176,24 @@ def sanitize_hostname(hostname):
     return hostname
 
 
-def read_cached_file(filename, cache_info):
+def read_cached_file(filename, cache_info, reload_func=None):
-    """Return the contents of a file. If the file hasn't changed since the
+    """Read from a file if it has been modified.
-    last invocation, a cached version will be returned.
+
+    :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):
+    if not cache_info or mtime != cache_info.get('mtime'):
-        return cache_info['data']
+        with open(filename) as fap:
-
+            cache_info['data'] = fap.read()
-    data = open(filename).read()
+        cache_info['mtime'] = mtime
-    cache_info['data'] = data
+        if reload_func:
-    cache_info['mtime'] = mtime
+            reload_func(cache_info['data'])
-    return data
+    return cache_info['data']
 
 
 @contextlib.contextmanager