Add testclient test for domain restricted cookies (#2154)

tests/test_testclient.py

@@ -1,4 +1,5 @@
 import itertools
+import sys
 from asyncio import current_task as asyncio_current_task
 from contextlib import asynccontextmanager
 
@@ -273,3 +274,48 @@ def homepage(request):
     client = test_client_factory(app)
     response = client.get("/", params={"param": param})
     assert response.text == param
+
+
+@pytest.mark.parametrize(
+    "domain, ok",
+    [
+        pytest.param(
+            "testserver",
+            True,
+            marks=[
+                pytest.mark.xfail(
+                    sys.version_info < (3, 11),
+                    reason="Fails due to domain handling in http.cookiejar module (see "
+                    "#2152)",
+                ),
+            ],
+        ),
+        ("testserver.local", True),
+        ("localhost", False),
+        ("example.com", False),
+    ],
+)
+def test_domain_restricted_cookies(test_client_factory, domain, ok):
+    """
+    Test that test client discards domain restricted cookies which do not match the
+    base_url of the testclient (`http://testserver` by default).
+
+    The domain `testserver.local` works because the Python http.cookiejar module derives
+    the "effective domain" by appending `.local` to non-dotted request domains
+    in accordance with RFC 2965.
+    """
+
+    async def app(scope, receive, send):
+        response = Response("Hello, world!", media_type="text/plain")
+        response.set_cookie(
+            "mycookie",
+            "myvalue",
+            path="/",
+            domain=domain,
+        )
+        await response(scope, receive, send)
+
+    client = test_client_factory(app)
+    response = client.get("/")
+    cookie_set = len(response.cookies) == 1
+    assert cookie_set == ok