Implement rejected events (#1426)

* WIP Event rejection

* Still send back errors for rejected events

Instead, discard them at the federationapi /send layer rather than
re-implementing checks at the clientapi/PerformJoin layer.

* Implement rejected events

Critically, rejected events CAN cause state resolution to happen
as it can merge forks in the DAG. This is fine, _provided_ we
do not add the rejected event when performing state resolution,
which is what this PR does. It also fixes the error handling
when NotAllowed happens, as we were checking too early and needlessly
handling NotAllowed in more than one place.

* Update test to match reality

* Modify InputRoomEvents to no longer return an error

Errors do not serialise across HTTP boundaries in polylith mode,
so instead set fields on the InputRoomEventsResponse. Add `Err()`
function to make the API shape basically the same.

* Remove redundant returns; linting

* Update blacklist

1 files changed, 3 insertions, 0 deletions

diff --git a/roomserver/types/types.go b/roomserver/types/types.go
index f5b45763..c0fcef65 100644
--- a/roomserver/types/types.go
+++ b/roomserver/types/types.go
@@ -101,6 +101,9 @@ type StateAtEvent struct {
 	Overwrite bool
 	// The state before the event.
 	BeforeStateSnapshotNID StateSnapshotNID
+	// True if this StateEntry is rejected. State resolution should then treat this
+	// StateEntry as being a message event (not a state event).
+	IsRejected bool
 	// The state entry for the event itself, allows us to calculate the state after the event.
 	StateEntry
 }