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, 16 insertions, 0 deletions

diff --git a/roomserver/api/input.go b/roomserver/api/input.go
index 651c0e9f..862a6fa1 100644
--- a/roomserver/api/input.go
+++ b/roomserver/api/input.go
@@ -16,6 +16,8 @@
 package api
 
 import (
+	"fmt"
+
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
@@ -87,4 +89,18 @@ type InputRoomEventsRequest struct {
 
 // InputRoomEventsResponse is a response to InputRoomEvents
 type InputRoomEventsResponse struct {
+	ErrMsg     string // set if there was any error
+	NotAllowed bool   // true if an event in the input was not allowed.
+}
+
+func (r *InputRoomEventsResponse) Err() error {
+	if r.ErrMsg == "" {
+		return nil
+	}
+	if r.NotAllowed {
+		return &gomatrixserverlib.NotAllowed{
+			Message: r.ErrMsg,
+		}
+	}
+	return fmt.Errorf("InputRoomEventsResponse: %s", r.ErrMsg)
 }