add Response.max_cookie_size config

CHANGES.rst

@@ -145,11 +145,14 @@ unreleased
   (`#2635`_)
 - A single trailing slash is stripped from the blueprint ``url_prefix``
   when it is registered with the app. (`#2629`_)
-- :meth:`Request.get_json() <flask.Request.get_json>` doesn't cache the
+- :meth:`Request.get_json` doesn't cache the
   result if parsing fails when ``silent`` is true. (`#2651`_)
-- :func:`request.get_json <flask.Request.get_json>` no longer accepts
-  arbitrary encodings. Incoming JSON should be encoded using UTF-8 per
-  :rfc:`8259`, but Flask will autodetect UTF-8, -16, or -32. (`#2691`_)
+- :func:`Request.get_json` no longer accepts arbitrary encodings.
+  Incoming JSON should be encoded using UTF-8 per :rfc:`8259`, but Flask
+  will autodetect UTF-8, -16, or -32. (`#2691`_)
+- Added :data:`MAX_COOKIE_SIZE` and :attr:`Response.max_cookie_size` to
+  control when Werkzeug warns about large cookies that browsers may
+  ignore. (`#2693`_)
 
 .. _pallets/meta#24: https://github.com/pallets/meta/issues/24
 .. _#1421: https://github.com/pallets/flask/issues/1421
@@ -196,6 +199,7 @@ unreleased
 .. _#2629: https://github.com/pallets/flask/pull/2629
 .. _#2651: https://github.com/pallets/flask/issues/2651
 .. _#2691: https://github.com/pallets/flask/pull/2691
+.. _#2693: https://github.com/pallets/flask/pull/2693
 
 
 Version 0.12.2

docs/api.rst

@@ -85,7 +85,7 @@ Response Objects
 ----------------
 
 .. autoclass:: flask.Response
-   :members: set_cookie, data, mimetype, is_json, get_json
+   :members: set_cookie, max_cookie_size, data, mimetype, is_json, get_json
 
    .. attribute:: headers
 

docs/config.rst

@@ -343,6 +343,12 @@ The following configuration values are used internally by Flask:
 
     Default: ``False``
 
+.. py:data:: MAX_COOKIE_SIZE
+
+    Warn if cookie headers are larger than this many bytes. Defaults to
+    ``4093``. Larger cookies may be silently ignored by browsers. Set to
+    ``0`` to disable the warning.
+
 .. versionadded:: 0.4
    ``LOGGER_NAME``
 
@@ -381,6 +387,8 @@ The following configuration values are used internally by Flask:
     Added :data:`SESSION_COOKIE_SAMESITE` to control the session
     cookie's ``SameSite`` option.
 
+    Added :data:`MAX_COOKIE_SIZE` to control a warning from Werkzeug.
+
 
 Configuring from Files
 ----------------------

flask/app.py

@@ -305,6 +305,7 @@ class Flask(_PackageBoundObject):
         'JSONIFY_PRETTYPRINT_REGULAR':          False,
         'JSONIFY_MIMETYPE':                     'application/json',
         'TEMPLATES_AUTO_RELOAD':                None,
+        'MAX_COOKIE_SIZE': 4093,
     })
 
     #: The rule object to use for URL rules created.  This is used by

flask/wrappers.py

@@ -191,9 +191,26 @@ class Response(ResponseBase, JSONMixin):
     .. versionchanged:: 1.0
         JSON support is added to the response, like the request. This is useful
         when testing to get the test client response data as JSON.
+
+    .. versionchanged:: 1.0
+
+        Added :attr:`max_cookie_size`.
     """
 
     default_mimetype = 'text/html'
 
     def _get_data_for_json(self, cache):
         return self.get_data()
+
+    @property
+    def max_cookie_size(self):
+        """Read-only view of the :data:`MAX_COOKIE_SIZE` config key.
+
+        See :attr:`~werkzeug.wrappers.BaseResponse.max_cookie_size` in
+        Werkzeug's docs.
+        """
+        if current_app:
+            return current_app.config['MAX_COOKIE_SIZE']
+
+        # return Werkzeug's default when not in an app context
+        return super(Response, self).max_cookie_size

tests/test_basic.py

@@ -1917,3 +1917,33 @@ def test_run_from_config(monkeypatch, host, port, expect_host, expect_port, app)
     monkeypatch.setattr(werkzeug.serving, 'run_simple', run_simple_mock)
     app.config['SERVER_NAME'] = 'pocoo.org:8080'
     app.run(host, port)
+
+
+def test_max_cookie_size(app, client, recwarn):
+    app.config['MAX_COOKIE_SIZE'] = 100
+
+    # outside app context, default to Werkzeug static value,
+    # which is also the default config
+    response = flask.Response()
+    default = flask.Flask.default_config['MAX_COOKIE_SIZE']
+    assert response.max_cookie_size == default
+
+    # inside app context, use app config
+    with app.app_context():
+        assert flask.Response().max_cookie_size == 100
+
+    @app.route('/')
+    def index():
+        r = flask.Response('', status=204)
+        r.set_cookie('foo', 'bar' * 100)
+        return r
+
+    client.get('/')
+    assert len(recwarn) == 1
+    w = recwarn.pop()
+    assert 'cookie is too large' in str(w.message)
+
+    app.config['MAX_COOKIE_SIZE'] = 0
+
+    client.get('/')
+    assert len(recwarn) == 0