From 942b2db448bdec534c6cdbd0a44fb4aeee7cc426 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Wed, 1 Apr 2020 10:30:47 +0100
Subject: [PATCH] Limit prev_events and auth_events

---
 common/events.go | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/common/events.go b/common/events.go
index b79998a73..385ac0c95 100644
--- a/common/events.go
+++ b/common/events.go
@@ -105,17 +105,32 @@ func AddPrevEventsToEvent(
 		return err
 	}
 
+	// The limits here feel a bit arbitrary but they are currently here
+	// because of https://github.com/matrix-org/matrix-doc/issues/2307
+	// and because Synapse will just drop events that don't comply.
 	switch eventFormat {
 	case gomatrixserverlib.EventFormatV1:
 		builder.AuthEvents = refs
 		builder.PrevEvents = queryRes.LatestEvents
+		if len(builder.AuthEvents) > 10 {
+			builder.AuthEvents = builder.AuthEvents[:10]
+		}
+		if len(builder.PrevEvents) > 20 {
+			builder.PrevEvents = builder.PrevEvents[:20]
+		}
 	case gomatrixserverlib.EventFormatV2:
 		v2AuthRefs := []string{}
 		v2PrevRefs := []string{}
 		for _, ref := range refs {
+			if len(v2AuthRefs) == 10 {
+				break
+			}
 			v2AuthRefs = append(v2AuthRefs, ref.EventID)
 		}
 		for _, ref := range queryRes.LatestEvents {
+			if len(v2PrevRefs) == 20 {
+				break
+			}
 			v2PrevRefs = append(v2PrevRefs, ref.EventID)
 		}
 		builder.AuthEvents = v2AuthRefs