From 366befc6d0e545ee127e9fa7e7f357ef07172d89 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 6 Aug 2020 17:10:25 +0100
Subject: [PATCH 1/2] Add an assertion on prev_events in
 create_new_client_event

I think this would have caught all the cases in
https://github.com/matrix-org/synapse/issues/7642 - and I think a 500 makes
more sense here than a 403
---
 changelog.d/8041.misc       | 1 +
 synapse/handlers/message.py | 9 +++++++++
 2 files changed, 10 insertions(+)
 create mode 100644 changelog.d/8041.misc

diff --git a/changelog.d/8041.misc b/changelog.d/8041.misc
new file mode 100644
index 000000000000..eefa98d74462
--- /dev/null
+++ b/changelog.d/8041.misc
@@ -0,0 +1 @@
+Add an assertion on prev_events in create_new_client_event.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index e451d6dc86cb..1439cb1ff72d 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -768,6 +768,15 @@ async def create_new_client_event(
         else:
             prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
 
+        # we now ought to have some prev_events (unless it's a create event).
+        #
+        # do a quick sanity check here, rather than waiting until we've created the
+        # event and then try to auth it (which fails with a somewhat confusing "No
+        # create event in auth events")
+        assert (
+            builder.type == EventTypes.Create or len(prev_event_ids) > 0
+        ), "Attempting to create an event with no prev_events"
+
         event = await builder.build(prev_event_ids=prev_event_ids)
         context = await self.state.compute_event_context(event)
         if requester:

From de337090f42385f06d910a5b5b68864668e23e25 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Fri, 7 Aug 2020 15:07:17 +0100
Subject: [PATCH 2/2] fix broken test

---
 tests/storage/test_redaction.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tests/storage/test_redaction.py b/tests/storage/test_redaction.py
index 0f0e1cd09b6e..164d3006b359 100644
--- a/tests/storage/test_redaction.py
+++ b/tests/storage/test_redaction.py
@@ -251,6 +251,10 @@ def build(self, prev_event_ids):
             def room_id(self):
                 return self._base_builder.room_id
 
+            @property
+            def type(self):
+                return self._base_builder.type
+
         event_1, context_1 = self.get_success(
             self.event_creation_handler.create_new_client_event(
                 EventIdManglingBuilder(