set Vary: Cookie header consistently for session

CHANGES.rst

@@ -4,6 +4,7 @@ Version 2.2.5
 Unreleased
 
 -   Update for compatibility with Werkzeug 2.3.3.
+-   Set ``Vary: Cookie`` header when the session is accessed, modified, or refreshed.
 
 
 Version 2.2.4

src/flask/sessions.py

@@ -383,6 +383,10 @@ def save_session(
         samesite = self.get_cookie_samesite(app)
         httponly = self.get_cookie_httponly(app)
 
+        # Add a "Vary: Cookie" header if the session was accessed at all.
+        if session.accessed:
+            response.vary.add("Cookie")
+
         # If the session is modified to be empty, remove the cookie.
         # If the session is empty, return without setting the cookie.
         if not session:
@@ -395,13 +399,10 @@ def save_session(
                     samesite=samesite,
                     httponly=httponly,
                 )
+                response.vary.add("Cookie")
 
             return
 
-        # Add a "Vary: Cookie" header if the session was accessed at all.
-        if session.accessed:
-            response.vary.add("Cookie")
-
         if not self.should_set_cookie(app, session):
             return
 
@@ -417,3 +418,4 @@ def save_session(
             secure=secure,
             samesite=samesite,
         )
+        response.vary.add("Cookie")

tests/test_basic.py

@@ -560,6 +560,11 @@ def getitem():
     def setdefault():
         return flask.session.setdefault("test", "default")
 
+    @app.route("/clear")
+    def clear():
+        flask.session.clear()
+        return ""
+
     @app.route("/vary-cookie-header-set")
     def vary_cookie_header_set():
         response = flask.Response()
@@ -592,11 +597,29 @@ def expect(path, header_value="Cookie"):
     expect("/get")
     expect("/getitem")
     expect("/setdefault")
+    expect("/clear")
     expect("/vary-cookie-header-set")
     expect("/vary-header-set", "Accept-Encoding, Accept-Language, Cookie")
     expect("/no-vary-header", None)
 
 
+def test_session_refresh_vary(app, client):
+    @app.get("/login")
+    def login():
+        flask.session["user_id"] = 1
+        flask.session.permanent = True
+        return ""
+
+    @app.get("/ignored")
+    def ignored():
+        return ""
+
+    rv = client.get("/login")
+    assert rv.headers["Vary"] == "Cookie"
+    rv = client.get("/ignored")
+    assert rv.headers["Vary"] == "Cookie"
+
+
 def test_flashes(app, req_ctx):
     assert not flask.session.modified
     flask.flash("Zap")