Implement MSC3925: changes to bundling of edits (#14811)

Two parts to this:

 * Bundle the whole of the replacement with any edited events. This is backwards-compatible so I haven't put it behind a flag.
 * Optionally, inhibit server-side replacement of edited events. This has scope to break things, so it is currently disabled by default.

5 files changed, 159 insertions, 63 deletions

diff --git a/changelog.d/14811.feature b/changelog.d/14811.feature
new file mode 100644
index 0000000000..87542835c3
--- /dev/null
+++ b/changelog.d/14811.feature
@@ -0,0 +1 @@
+Per [MSC3925](https://github.com/matrix-org/matrix-spec-proposals/pull/3925), bundle the whole of the replacement with any edited events, and optionally inhibit server-side replacement.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 0f3870bfe1..a8b2db372d 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -139,3 +139,6 @@ class ExperimentalConfig(Config):
 
         # MSC3391: Removing account data.
         self.msc3391_enabled = experimental.get("msc3391_enabled", False)
+
+        # MSC3925: do not replace events with their edits
+        self.msc3925_inhibit_edit = experimental.get("msc3925_inhibit_edit", False)
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 13fa93afb8..ae57a4df5e 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -403,6 +403,14 @@ class EventClientSerializer:
     clients.
     """
 
+    def __init__(self, inhibit_replacement_via_edits: bool = False):
+        """
+        Args:
+            inhibit_replacement_via_edits: If this is set to True, then events are
+               never replaced by their edits.
+        """
+        self._inhibit_replacement_via_edits = inhibit_replacement_via_edits
+
     def serialize_event(
         self,
         event: Union[JsonDict, EventBase],
@@ -422,6 +430,8 @@ class EventClientSerializer:
                into the event.
             apply_edits: Whether the content of the event should be modified to reflect
                any replacement in `bundle_aggregations[<event_id>].replace`.
+               See also the `inhibit_replacement_via_edits` constructor arg: if that is
+               set to True, then this argument is ignored.
         Returns:
             The serialized event
         """
@@ -495,7 +505,8 @@ class EventClientSerializer:
                 again for additional events in a recursive manner.
             serialized_event: The serialized event which may be modified.
             apply_edits: Whether the content of the event should be modified to reflect
-               any replacement in `aggregations.replace`.
+               any replacement in `aggregations.replace` (subject to the
+               `inhibit_replacement_via_edits` constructor arg).
         """
 
         # We have already checked that aggregations exist for this event.
@@ -518,15 +529,21 @@ class EventClientSerializer:
         if event_aggregations.replace:
             # If there is an edit, optionally apply it to the event.
             edit = event_aggregations.replace
-            if apply_edits:
+            if apply_edits and not self._inhibit_replacement_via_edits:
                 self._apply_edit(event, serialized_event, edit)
 
             # Include information about it in the relations dict.
-            serialized_aggregations[RelationTypes.REPLACE] = {
-                "event_id": edit.event_id,
-                "origin_server_ts": edit.origin_server_ts,
-                "sender": edit.sender,
-            }
+            #
+            # Matrix spec v1.5 (https://spec.matrix.org/v1.5/client-server-api/#server-side-aggregation-of-mreplace-relationships)
+            # said that we should only include the `event_id`, `origin_server_ts` and
+            # `sender` of the edit; however MSC3925 proposes extending it to the whole
+            # of the edit, which is what we do here.
+            serialized_aggregations[RelationTypes.REPLACE] = self.serialize_event(
+                edit,
+                time_now,
+                config=config,
+                apply_edits=False,
+            )
 
         # Include any threaded replies to this event.
         if event_aggregations.thread:
diff --git a/synapse/server.py b/synapse/server.py
index 5baae2325e..f4ab94c4f3 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -743,7 +743,7 @@ class HomeServer(metaclass=abc.ABCMeta):
 
     @cache_in_self
     def get_event_client_serializer(self) -> EventClientSerializer:
-        return EventClientSerializer()
+        return EventClientSerializer(self.config.experimental.msc3925_inhibit_edit)
 
     @cache_in_self
     def get_password_policy_handler(self) -> PasswordPolicyHandler:
diff --git a/tests/rest/client/test_relations.py b/tests/rest/client/test_relations.py
index b86f341ff5..c8a6911d5e 100644
--- a/tests/rest/client/test_relations.py
+++ b/tests/rest/client/test_relations.py
@@ -30,6 +30,7 @@ from tests import unittest
 from tests.server import FakeChannel
 from tests.test_utils import make_awaitable
 from tests.test_utils.event_injection import inject_event
+from tests.unittest import override_config
 
 
 class BaseRelationsTestCase(unittest.HomeserverTestCase):
@@ -355,30 +356,67 @@ class RelationsTestCase(BaseRelationsTestCase):
         self.assertEqual(200, channel.code, channel.json_body)
         self.assertNotIn("m.relations", channel.json_body["unsigned"])
 
+    def _assert_edit_bundle(
+        self, event_json: JsonDict, edit_event_id: str, edit_event_content: JsonDict
+    ) -> None:
+        """
+        Assert that the given event has a correctly-serialised edit event in its
+        bundled aggregations
+
+        Args:
+            event_json: the serialised event to be checked
+            edit_event_id: the ID of the edit event that we expect to be bundled
+            edit_event_content: the content of that event, excluding the 'm.relates_to`
+               property
+        """
+        relations_dict = event_json["unsigned"].get("m.relations")
+        self.assertIn(RelationTypes.REPLACE, relations_dict)
+
+        m_replace_dict = relations_dict[RelationTypes.REPLACE]
+        for key in [
+            "event_id",
+            "sender",
+            "origin_server_ts",
+            "content",
+            "type",
+            "unsigned",
+        ]:
+            self.assertIn(key, m_replace_dict)
+
+        expected_edit_content = {
+            "m.relates_to": {
+                "event_id": event_json["event_id"],
+                "rel_type": "m.replace",
+            }
+        }
+        expected_edit_content.update(edit_event_content)
+
+        self.assert_dict(
+            {
+                "event_id": edit_event_id,
+                "sender": self.user_id,
+                "content": expected_edit_content,
+                "type": "m.room.message",
+            },
+            m_replace_dict,
+        )
+
     def test_edit(self) -> None:
         """Test that a simple edit works."""
 
         new_body = {"msgtype": "m.text", "body": "I've been edited!"}
+        edit_event_content = {
+            "msgtype": "m.text",
+            "body": "foo",
+            "m.new_content": new_body,
+        }
         channel = self._send_relation(
             RelationTypes.REPLACE,
             "m.room.message",
-            content={"msgtype": "m.text", "body": "foo", "m.new_content": new_body},
+            content=edit_event_content,
         )
         edit_event_id = channel.json_body["event_id"]
 
-        def assert_bundle(event_json: JsonDict) -> None:
-            """Assert the expected values of the bundled aggregations."""
-            relations_dict = event_json["unsigned"].get("m.relations")
-            self.assertIn(RelationTypes.REPLACE, relations_dict)
-
-            m_replace_dict = relations_dict[RelationTypes.REPLACE]
-            for key in ["event_id", "sender", "origin_server_ts"]:
-                self.assertIn(key, m_replace_dict)
-
-            self.assert_dict(
-                {"event_id": edit_event_id, "sender": self.user_id}, m_replace_dict
-            )
-
         # /event should return the *original* event
         channel = self.make_request(
             "GET",
@@ -389,7 +427,7 @@ class RelationsTestCase(BaseRelationsTestCase):
         self.assertEqual(
             channel.json_body["content"], {"body": "Hi!", "msgtype": "m.text"}
         )
-        assert_bundle(channel.json_body)
+        self._assert_edit_bundle(channel.json_body, edit_event_id, edit_event_content)
 
         # Request the room messages.
         channel = self.make_request(
@@ -398,7 +436,11 @@ class RelationsTestCase(BaseRelationsTestCase):
             access_token=self.user_token,
         )
         self.assertEqual(200, channel.code, channel.json_body)
-        assert_bundle(self._find_event_in_chunk(channel.json_body["chunk"]))
+        self._assert_edit_bundle(
+            self._find_event_in_chunk(channel.json_body["chunk"]),
+            edit_event_id,
+            edit_event_content,
+        )
 
         # Request the room context.
         # /context should return the edited event.
@@ -408,7 +450,9 @@ class RelationsTestCase(BaseRelationsTestCase):
             access_token=self.user_token,
         )
         self.assertEqual(200, channel.code, channel.json_body)
-        assert_bundle(channel.json_body["event"])
+        self._assert_edit_bundle(
+            channel.json_body["event"], edit_event_id, edit_event_content
+        )
         self.assertEqual(channel.json_body["event"]["content"], new_body)
 
         # Request sync, but limit the timeline so it becomes limited (and includes
@@ -420,7 +464,11 @@ class RelationsTestCase(BaseRelationsTestCase):
         self.assertEqual(200, channel.code, channel.json_body)
         room_timeline = channel.json_body["rooms"]["join"][self.room]["timeline"]
         self.assertTrue(room_timeline["limited"])
-        assert_bundle(self._find_event_in_chunk(room_timeline["events"]))
+        self._assert_edit_bundle(
+            self._find_event_in_chunk(room_timeline["events"]),
+            edit_event_id,
+            edit_event_content,
+        )
 
         # Request search.
         channel = self.make_request(
@@ -437,7 +485,45 @@ class RelationsTestCase(BaseRelationsTestCase):
                 "results"
             ]
         ]
-        assert_bundle(self._find_event_in_chunk(chunk))
+        self._assert_edit_bundle(
+            self._find_event_in_chunk(chunk),
+            edit_event_id,
+            edit_event_content,
+        )
+
+    @override_config({"experimental_features": {"msc3925_inhibit_edit": True}})
+    def test_edit_inhibit_replace(self) -> None:
+        """
+        If msc3925_inhibit_edit is enabled, then the original event should not be
+        replaced.
+        """
+
+        new_body = {"msgtype": "m.text", "body": "I've been edited!"}
+        edit_event_content = {
+            "msgtype": "m.text",
+            "body": "foo",
+            "m.new_content": new_body,
+        }
+        channel = self._send_relation(
+            RelationTypes.REPLACE,
+            "m.room.message",
+            content=edit_event_content,
+        )
+        edit_event_id = channel.json_body["event_id"]
+
+        # /context should return the *original* event.
+        channel = self.make_request(
+            "GET",
+            f"/rooms/{self.room}/context/{self.parent_id}",
+            access_token=self.user_token,
+        )
+        self.assertEqual(200, channel.code, channel.json_body)
+        self.assertEqual(
+            channel.json_body["event"]["content"], {"body": "Hi!", "msgtype": "m.text"}
+        )
+        self._assert_edit_bundle(
+            channel.json_body["event"], edit_event_id, edit_event_content
+        )
 
     def test_multi_edit(self) -> None:
         """Test that multiple edits, including attempts by people who
@@ -455,10 +541,15 @@ class RelationsTestCase(BaseRelationsTestCase):
         )
 
         new_body = {"msgtype": "m.text", "body": "I've been edited!"}
+        edit_event_content = {
+            "msgtype": "m.text",
+            "body": "foo",
+            "m.new_content": new_body,
+        }
         channel = self._send_relation(
             RelationTypes.REPLACE,
             "m.room.message",
-            content={"msgtype": "m.text", "body": "foo", "m.new_content": new_body},
+            content=edit_event_content,
         )
         edit_event_id = channel.json_body["event_id"]
 
@@ -480,16 +571,8 @@ class RelationsTestCase(BaseRelationsTestCase):
         self.assertEqual(200, channel.code, channel.json_body)
 
         self.assertEqual(channel.json_body["event"]["content"], new_body)
-
-        relations_dict = channel.json_body["event"]["unsigned"].get("m.relations")
-        self.assertIn(RelationTypes.REPLACE, relations_dict)
-
-        m_replace_dict = relations_dict[RelationTypes.REPLACE]
-        for key in ["event_id", "sender", "origin_server_ts"]:
-            self.assertIn(key, m_replace_dict)
-
-        self.assert_dict(
-            {"event_id": edit_event_id, "sender": self.user_id}, m_replace_dict
+        self._assert_edit_bundle(
+            channel.json_body["event"], edit_event_id, edit_event_content
         )
 
     def test_edit_reply(self) -> None:
@@ -502,11 +585,15 @@ class RelationsTestCase(BaseRelationsTestCase):
         )
         reply = channel.json_body["event_id"]
 
-        new_body = {"msgtype": "m.text", "body": "I've been edited!"}
+        edit_event_content = {
+            "msgtype": "m.text",
+            "body": "foo",
+            "m.new_content": {"msgtype": "m.text", "body": "I've been edited!"},
+        }
         channel = self._send_relation(
             RelationTypes.REPLACE,
             "m.room.message",
-            content={"msgtype": "m.text", "body": "foo", "m.new_content": new_body},
+            content=edit_event_content,
             parent_id=reply,
         )
         edit_event_id = channel.json_body["event_id"]
@@ -549,28 +636,22 @@ class RelationsTestCase(BaseRelationsTestCase):
 
             # We expect that the edit relation appears in the unsigned relations
             # section.
-            relations_dict = result_event_dict["unsigned"].get("m.relations")
-            self.assertIn(RelationTypes.REPLACE, relations_dict, desc)
-
-            m_replace_dict = relations_dict[RelationTypes.REPLACE]
-            for key in ["event_id", "sender", "origin_server_ts"]:
-                self.assertIn(key, m_replace_dict, desc)
-
-            self.assert_dict(
-                {"event_id": edit_event_id, "sender": self.user_id}, m_replace_dict
+            self._assert_edit_bundle(
+                result_event_dict, edit_event_id, edit_event_content
             )
 
     def test_edit_edit(self) -> None:
         """Test that an edit cannot be edited."""
         new_body = {"msgtype": "m.text", "body": "Initial edit"}
+        edit_event_content = {
+            "msgtype": "m.text",
+            "body": "Wibble",
+            "m.new_content": new_body,
+        }
         channel = self._send_relation(
             RelationTypes.REPLACE,
             "m.room.message",
-            content={
-                "msgtype": "m.text",
-                "body": "Wibble",
-                "m.new_content": new_body,
-            },
+            content=edit_event_content,
         )
         edit_event_id = channel.json_body["event_id"]
 
@@ -599,8 +680,7 @@ class RelationsTestCase(BaseRelationsTestCase):
         )
 
         # The relations information should not include the edit to the edit.
-        relations_dict = channel.json_body["unsigned"].get("m.relations")
-        self.assertIn(RelationTypes.REPLACE, relations_dict)
+        self._assert_edit_bundle(channel.json_body, edit_event_id, edit_event_content)
 
         # /context should return the event updated for the *first* edit
         # (The edit to the edit should be ignored.)
@@ -611,13 +691,8 @@ class RelationsTestCase(BaseRelationsTestCase):
         )
         self.assertEqual(200, channel.code, channel.json_body)
         self.assertEqual(channel.json_body["event"]["content"], new_body)
-
-        m_replace_dict = relations_dict[RelationTypes.REPLACE]
-        for key in ["event_id", "sender", "origin_server_ts"]:
-            self.assertIn(key, m_replace_dict)
-
-        self.assert_dict(
-            {"event_id": edit_event_id, "sender": self.user_id}, m_replace_dict
+        self._assert_edit_bundle(
+            channel.json_body["event"], edit_event_id, edit_event_content
         )
 
         # Directly requesting the edit should not have the edit to the edit applied.