From 8646edca6f47e2cd57464081b3911218d4734f8d Mon Sep 17 00:00:00 2001
From: David Lord <davidism@gmail.com>
Date: Mon, 1 May 2023 08:01:32 -0700
Subject: [PATCH] set `Vary: Cookie` header consistently for session

Origin:
https://github.com/pallets/flask/commit/8646edca6f47e2cd57464081b3911218d4734f8d
---
 src/flask/sessions.py | 10 ++++++----
 tests/test_basic.py   | 23 +++++++++++++++++++++++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/flask/sessions.py b/src/flask/sessions.py
index c57ba29..127bc62 100644
--- a/src/flask/sessions.py
+++ b/src/flask/sessions.py
@@ -354,6 +354,10 @@ class SecureCookieSessionInterface(SessionInterface):
         domain = self.get_cookie_domain(app)
         path = self.get_cookie_path(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:
@@ -361,13 +365,10 @@ class SecureCookieSessionInterface(SessionInterface):
                 response.delete_cookie(
                     app.session_cookie_name, domain=domain, path=path
                 )
+                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
 
@@ -386,3 +387,4 @@ class SecureCookieSessionInterface(SessionInterface):
             secure=secure,
             samesite=samesite,
         )
+        response.vary.add("Cookie")
diff --git a/tests/test_basic.py b/tests/test_basic.py
index 4d3b7b0..847c3f3 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -538,6 +538,11 @@ def test_session_vary_cookie(app, client):
     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()
@@ -570,11 +575,29 @@ def test_session_vary_cookie(app, client):
     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.route("/login", methods=["GET"])
+    def login():
+        flask.session["user_id"] = 1
+        flask.session.permanent = True
+        return ""
+
+    @app.route("/ignored", methods=["GET"])
+    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")
-- 
2.30.0