From 6940c7c7dd9cefc3f7fa0bec00926e96ee669b56 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Wed, 25 May 2022 16:40:31 +0100
Subject: [PATCH] Try to spot state deletions when they happen (#2489)

---
 roomserver/internal/input/input_events.go     |  4 ----
 .../internal/input/input_latest_events.go     | 19 ++++++++++++-------
 .../storage/tables/events_table_test.go       |  1 -
 roomserver/types/types.go                     |  4 ----
 4 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 3ab9ba4f0..d437d7768 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -535,8 +535,6 @@ func (r *Inputer) calculateAndSetState(
 	roomState := state.NewStateResolution(updater, roomInfo)
 
 	if input.HasState {
-		stateAtEvent.Overwrite = true
-
 		// We've been told what the state at the event is so we don't need to calculate it.
 		// Check that those state events are in the database and store the state.
 		var entries []types.StateEntry
@@ -549,8 +547,6 @@ func (r *Inputer) calculateAndSetState(
 			return fmt.Errorf("updater.AddState: %w", err)
 		}
 	} else {
-		stateAtEvent.Overwrite = false
-
 		// We haven't been told what the state at the event is so we need to calculate it from the prev_events
 		if stateAtEvent.BeforeStateSnapshotNID, err = roomState.CalculateAndStoreStateBeforeEvent(ctx, event, isRejected); err != nil {
 			return fmt.Errorf("roomState.CalculateAndStoreStateBeforeEvent: %w", err)
diff --git a/roomserver/internal/input/input_latest_events.go b/roomserver/internal/input/input_latest_events.go
index e4c138d58..4e5b73420 100644
--- a/roomserver/internal/input/input_latest_events.go
+++ b/roomserver/internal/input/input_latest_events.go
@@ -27,6 +27,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
 )
 
 // updateLatestEvents updates the list of latest events for this room in the database and writes the
@@ -258,13 +259,6 @@ func (u *latestEventsUpdater) latestState() error {
 		return fmt.Errorf("roomState.CalculateAndStoreStateAfterEvents: %w", err)
 	}
 
-	// If we are overwriting the state then we should make sure that we
-	// don't send anything out over federation again, it will very likely
-	// be a repeat.
-	if u.stateAtEvent.Overwrite {
-		u.sendAsServer = ""
-	}
-
 	// Now that we have a new state snapshot based on the latest events,
 	// we can compare that new snapshot to the previous one and see what
 	// has changed. This gives us one list of removed state events and
@@ -277,6 +271,17 @@ func (u *latestEventsUpdater) latestState() error {
 		return fmt.Errorf("roomState.DifferenceBetweenStateSnapshots: %w", err)
 	}
 
+	if removed := len(u.removed) - len(u.added); removed > 0 {
+		logrus.WithFields(logrus.Fields{
+			"event_id":      u.event.EventID(),
+			"room_id":       u.event.RoomID(),
+			"old_state_nid": u.oldStateNID,
+			"new_state_nid": u.newStateNID,
+			"old_latest":    u.oldLatest,
+			"new_latest":    u.latest,
+		}).Errorf("Unexpected state deletion (removing %d events)", removed)
+	}
+
 	// Also work out the state before the event removes and the event
 	// adds.
 	u.stateBeforeEventRemoves, u.stateBeforeEventAdds, err = roomState.DifferenceBetweeenStateSnapshots(
diff --git a/roomserver/storage/tables/events_table_test.go b/roomserver/storage/tables/events_table_test.go
index 6f72a59b5..74502a31f 100644
--- a/roomserver/storage/tables/events_table_test.go
+++ b/roomserver/storage/tables/events_table_test.go
@@ -85,7 +85,6 @@ func Test_EventsTable(t *testing.T) {
 				assert.NoError(t, err)
 			}
 			stateAtEvent := types.StateAtEvent{
-				Overwrite:              false,
 				BeforeStateSnapshotNID: types.StateSnapshotNID(stateSnapshot),
 				IsRejected:             false,
 				StateEntry: types.StateEntry{
diff --git a/roomserver/types/types.go b/roomserver/types/types.go
index 62695aaee..cf12a3c1e 100644
--- a/roomserver/types/types.go
+++ b/roomserver/types/types.go
@@ -173,10 +173,6 @@ func DeduplicateStateEntries(a []StateEntry) []StateEntry {
 
 // StateAtEvent is the state before and after a matrix event.
 type StateAtEvent struct {
-	// Should this state overwrite the latest events and memberships of the room?
-	// This might be necessary when rejoining a federated room after a period of
-	// absence, as our state and latest events will be out of date.
-	Overwrite bool
 	// The state before the event.
 	BeforeStateSnapshotNID StateSnapshotNID
 	// True if this StateEntry is rejected. State resolution should then treat this