matrix-sync-fix.md

Matrix Sync Token Bug Fix Plan

Problem

The sync token advances even when no messages are extracted from allowed rooms. This causes messages to be missed when:

1. Sync returns events only for non-allowed rooms
2. Sync returns non-message events (typing, read receipts) for allowed rooms
3. Messages arrive during concurrent processing

Root Cause

// Currently: Always save token after sync, even if no messages extracted
self.save_sync_token(&next_batch);

Solution Options

Option 1: Only advance token when messages extracted (WRONG)

This would cause infinite re-processing of non-message events.

Option 2: Track "seen" event IDs (Complex)

Store event IDs of processed messages, skip duplicates. Memory grows over time.

Option 3: Per-room sync tokens (Best)

Matrix supports per-room prev_batch tokens. Track last seen event per room.

Option 4: Check if allowed rooms had ANY events (Simple fix)

Only advance token if allowed rooms were present in sync response, regardless of whether they had messages.

Recommended Fix: Option 4

// Track whether any allowed rooms appeared in this sync
let mut allowed_rooms_in_sync = false;

if let Some(rooms) = sync_response.rooms {
    if let Some(joined_rooms) = rooms.join {
        for (room_id, room_data) in joined_rooms {
            let in_allowed = self.allowed_chats.contains(&room_id) || dm_rooms.contains(&room_id);
            if in_allowed {
                allowed_rooms_in_sync = true;
                // ... process messages as before
            }
        }
    }
}

// Only advance token if:
// 1. We extracted messages, OR
// 2. Allowed rooms were in sync (even with no messages = they're caught up)
// 3. No allowed rooms configured (process everything)
if !messages.is_empty() || allowed_rooms_in_sync || (self.allowed_chats.is_empty() && dm_rooms.is_empty()) {
    self.save_sync_token(&next_batch);
}

Why This Works

• If allowed rooms appear in sync with no messages → token advances (caught up)
• If sync only has events for non-allowed rooms → token does NOT advance
• Next sync will include the same batch + any new events
• Eventually the allowed room will appear and token advances

Edge Case: Stuck Token

If we never get events for allowed rooms, token never advances. This is fine — we'll get them eventually when someone sends a message.

Implementation

File: crates/rustyclaw-core/src/messengers/matrix_cli.rs Lines: ~455-460 (save_sync_token section)