1 files changed, 59 insertions, 1 deletions
diff --git a/synapse/storage/relations.py b/synapse/storage/relations.py
index de67e305a1..6e216066ab 100644
--- a/synapse/storage/relations.py
+++ b/synapse/storage/relations.py
@@ -17,11 +17,13 @@ import logging
 
 import attr
 
+from twisted.internet import defer
+
 from synapse.api.constants import RelationTypes
 from synapse.api.errors import SynapseError
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.stream import generate_pagination_where_clause
-from synapse.util.caches.descriptors import cached
+from synapse.util.caches.descriptors import cached, cachedInlineCallbacks
 
 logger = logging.getLogger(__name__)
 
@@ -312,6 +314,59 @@ class RelationsWorkerStore(SQLBaseStore):
             "get_aggregation_groups_for_event", _get_aggregation_groups_for_event_txn
         )
 
+    @cachedInlineCallbacks()
+    def get_applicable_edit(self, event_id):
+        """Get the most recent edit (if any) that has happened for the given
+        event.
+
+        Correctly handles checking whether edits were allowed to happen.
+
+        Args:
+            event_id (str): The original event ID
+
+        Returns:
+            Deferred[EventBase|None]: Returns the most recent edit, if any.
+        """
+
+        # We only allow edits for `m.room.message` events that have the same sender
+        # and event type. We can't assert these things during regular event auth so
+        # we have to do the checks post hoc.
+
+        # Fetches latest edit that has the same type and sender as the
+        # original, and is an `m.room.message`.
+        sql = """
+            SELECT edit.event_id FROM events AS edit
+            INNER JOIN event_relations USING (event_id)
+            INNER JOIN events AS original ON
+                original.event_id = relates_to_id
+                AND edit.type = original.type
+                AND edit.sender = original.sender
+            WHERE
+                relates_to_id = ?
+                AND relation_type = ?
+                AND edit.type = 'm.room.message'
+            ORDER by edit.origin_server_ts DESC, edit.event_id DESC
+            LIMIT 1
+        """
+
+        def _get_applicable_edit_txn(txn):
+            txn.execute(
+                sql, (event_id, RelationTypes.REPLACES,)
+            )
+            row = txn.fetchone()
+            if row:
+                return row[0]
+
+        edit_id = yield self.runInteraction(
+            "get_applicable_edit", _get_applicable_edit_txn
+        )
+
+        if not edit_id:
+            return
+
+        edit_event = yield self.get_event(edit_id, allow_none=True)
+        defer.returnValue(edit_event)
+
 
 class RelationsStore(RelationsWorkerStore):
     def _handle_event_relations(self, txn, event):
@@ -357,3 +412,6 @@ class RelationsStore(RelationsWorkerStore):
         txn.call_after(
             self.get_aggregation_groups_for_event.invalidate_many, (parent_id,)
         )
+
+        if rel_type == RelationTypes.REPLACES:
+            txn.call_after(self.get_applicable_edit.invalidate, (parent_id,))