From 6ccfee63af057a4ee7fe6c2420a127a60a3fa704 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Mon, 5 Oct 2020 16:46:28 +0100
Subject: [PATCH] Resolve state after event against current room state when
 determining latest state changes

---
 .../internal/input/input_latest_events.go     | 23 ++++++++++++++++---
 roomserver/state/state.go                     |  8 ++++---
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/roomserver/internal/input/input_latest_events.go b/roomserver/internal/input/input_latest_events.go
index 5c2a1de6a..2e9f3b4e4 100644
--- a/roomserver/internal/input/input_latest_events.go
+++ b/roomserver/internal/input/input_latest_events.go
@@ -215,10 +215,27 @@ func (u *latestEventsUpdater) latestState() error {
 	var err error
 	roomState := state.NewStateResolution(u.api.DB, *u.roomInfo)
 
-	// Get a list of the current latest events.
-	latestStateAtEvents := make([]types.StateAtEvent, len(u.latest))
+	// Get a list of the current room state events if available.
+	var currentState []types.StateEntry
+	if u.roomInfo.StateSnapshotNID != 0 {
+		currentState, _ = roomState.LoadStateAtSnapshot(u.ctx, u.roomInfo.StateSnapshotNID)
+	}
+
+	// Get a list of the current latest events. This will include both
+	// the current room state and the latest events after the input event.
+	// The idea is that we will perform state resolution on this set and
+	// any conflicting events will be resolved properly.
+	latestStateAtEvents := make([]types.StateAtEvent, len(u.latest)+len(currentState))
+	offset := 0
+	for i := range currentState {
+		latestStateAtEvents[i] = types.StateAtEvent{
+			BeforeStateSnapshotNID: u.roomInfo.StateSnapshotNID,
+			StateEntry:             currentState[i],
+		}
+		offset++
+	}
 	for i := range u.latest {
-		latestStateAtEvents[i] = u.latest[i].StateAtEvent
+		latestStateAtEvents[offset+i] = u.latest[i].StateAtEvent
 	}
 
 	// Takes the NIDs of the latest events and creates a state snapshot
diff --git a/roomserver/state/state.go b/roomserver/state/state.go
index 0663499e7..2944f71c1 100644
--- a/roomserver/state/state.go
+++ b/roomserver/state/state.go
@@ -118,7 +118,7 @@ func (v StateResolution) LoadCombinedStateAfterEvents(
 	// the snapshot of the room state before them was the same.
 	stateBlockNIDLists, err := v.db.StateBlockNIDs(ctx, uniqueStateSnapshotNIDs(stateNIDs))
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("v.db.StateBlockNIDs: %w", err)
 	}
 
 	var stateBlockNIDs []types.StateBlockNID
@@ -131,7 +131,7 @@ func (v StateResolution) LoadCombinedStateAfterEvents(
 	// multiple snapshots.
 	stateEntryLists, err := v.db.StateEntries(ctx, uniqueStateBlockNIDs(stateBlockNIDs))
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("v.db.StateEntries: %w", err)
 	}
 	stateBlockNIDsMap := stateBlockNIDListMap(stateBlockNIDLists)
 	stateEntriesMap := stateEntryListMap(stateEntryLists)
@@ -623,7 +623,7 @@ func (v StateResolution) calculateAndStoreStateAfterManyEvents(
 		v.calculateStateAfterManyEvents(ctx, v.roomInfo.RoomVersion, prevStates)
 	metrics.algorithm = algorithm
 	if err != nil {
-		return metrics.stop(0, err)
+		return metrics.stop(0, fmt.Errorf("v.calculateStateAfterManyEvents: %w", err))
 	}
 
 	// TODO: Check if we can encode the new state as a delta against the
@@ -642,6 +642,7 @@ func (v StateResolution) calculateStateAfterManyEvents(
 	// First stage: load the state after each of the prev events.
 	combined, err = v.LoadCombinedStateAfterEvents(ctx, prevStates)
 	if err != nil {
+		err = fmt.Errorf("v.LoadCombinedStateAfterEvents: %w", err)
 		algorithm = "_load_combined_state"
 		return
 	}
@@ -672,6 +673,7 @@ func (v StateResolution) calculateStateAfterManyEvents(
 		var resolved []types.StateEntry
 		resolved, err = v.resolveConflicts(ctx, roomVersion, notConflicted, conflicts)
 		if err != nil {
+			err = fmt.Errorf("v.resolveConflits: %w", err)
 			algorithm = "_resolve_conflicts"
 			return
 		}