Add forget_forced_upon_leave capability as per MSC4267 (#18196)

This adds the capability from
https://github.com/matrix-org/matrix-spec-proposals/pull/4267 under an
experimental feature.

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelog.d/18196.feature

@@ -0,0 +1 @@
+Add `forget_forced_upon_leave` capability as per [MSC4267](https://github.com/matrix-org/matrix-spec-proposals/pull/4267).

synapse/config/experimental.py

@@ -567,5 +567,8 @@ class ExperimentalConfig(Config):
             False,
         )
 
+        # MSC4267: Automatically forgetting rooms on leave
+        self.msc4267_enabled: bool = experimental.get("msc4267_enabled", False)
+
         # MSC4155: Invite filtering
         self.msc4155_enabled: bool = experimental.get("msc4155_enabled", False)

synapse/config/room.py

@@ -85,4 +85,4 @@ class RoomConfig(Config):
 
         # When enabled, users will forget rooms when they leave them, either via a
         # leave, kick or ban.
-        self.forget_on_leave = config.get("forget_rooms_on_leave", False)
+        self.forget_on_leave: bool = config.get("forget_rooms_on_leave", False)

synapse/rest/client/capabilities.py

@@ -109,6 +109,11 @@ class CapabilitiesRestServlet(RestServlet):
                     "disallowed"
                 ] = disallowed
 
+        if self.config.experimental.msc4267_enabled:
+            response["capabilities"]["org.matrix.msc4267.forget_forced_upon_leave"] = {
+                "enabled": self.config.room.forget_on_leave,
+            }
+
         return HTTPStatus.OK, response
 
 

tests/rest/client/test_capabilities.py

@@ -264,3 +264,43 @@ class CapabilitiesTestCase(unittest.HomeserverTestCase):
 
         self.assertEqual(channel.code, HTTPStatus.OK)
         self.assertTrue(capabilities["m.get_login_token"]["enabled"])
+
+    @override_config(
+        {
+            "experimental_features": {"msc4267_enabled": True},
+            "forget_rooms_on_leave": True,
+        }
+    )
+    def test_get_forget_forced_upon_leave_with_auto_forget(self) -> None:
+        # Server auto-forgets on /leave, expect enabled client capability
+        access_token = self.get_success(
+            self.auth_handler.create_access_token_for_user_id(
+                self.user, device_id=None, valid_until_ms=None
+            )
+        )
+        channel = self.make_request("GET", self.url, access_token=access_token)
+        capabilities = channel.json_body["capabilities"]
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertTrue(
+            capabilities["org.matrix.msc4267.forget_forced_upon_leave"]["enabled"]
+        )
+
+    @override_config(
+        {
+            "experimental_features": {"msc4267_enabled": True},
+            "forget_rooms_on_leave": False,
+        }
+    )
+    def test_get_forget_forced_upon_leave_without_auto_forget(self) -> None:
+        # Server doesn't auto-forget on /leave, expect disabled client capability
+        access_token = self.get_success(
+            self.auth_handler.create_access_token_for_user_id(
+                self.user, device_id=None, valid_until_ms=None
+            )
+        )
+        channel = self.make_request("GET", self.url, access_token=access_token)
+        capabilities = channel.json_body["capabilities"]
+        self.assertEqual(channel.code, HTTPStatus.OK)
+        self.assertFalse(
+            capabilities["org.matrix.msc4267.forget_forced_upon_leave"]["enabled"]
+        )