Add notes on mutable values & sessions.

Using notes in 8445f0d939

CHANGES

@@ -56,7 +56,8 @@ Relase date to be decided, codename to be chosen.
   max-age for `send_static_file` can be configured through a new
   ``SEND_FILE_MAX_AGE_DEFAULT`` configuration variable, regardless of whether
   the `get_send_file_options` hook is used.
-
+- Fixed an assumption in sessions implementation which could break message
+  flashing on sessions implementations which use external storage.
 
 Version 0.8.1
 -------------

flask/helpers.py

@@ -283,6 +283,13 @@ def flash(message, category='message'):
                      messages and ``'warning'`` for warnings.  However any
                      kind of string can be used as category.
     """
+    # Original implementation:
+    #
+    #     session.setdefault('_flashes', []).append((category, message))
+    #
+    # This assumed that changes made to mutable structures in the session are
+    # are always in sync with the sess on object, which is not true for session
+    # implementations that use external storage for keeping their keys/values.
     flashes = session.get('_flashes', [])
     flashes.append((category, message))
     session['_flashes'] = flashes