Method syncSessions

packages/server/src/ws/connection.ts:301–351

* Shared client_hello/subscribe session sync: * 1. register the subscription FIRST (live events flow immediately; * the client dedups overlap by seq), * 2. replay durable events past the client's cursor, or emit * `resync_required` when the cursor cannot be served, * 3

(
    sessionIds: readonly string[],
    cursors: CursorsBySession | undefined,
  )

   *      session so the client can adopt the current epoch.
   */
  private async syncSessions(
    sessionIds: readonly string[],
    cursors: CursorsBySession | undefined,
  ): Promise<{
    accepted: string[];
    resyncRequired: string[];
    serverCursors: CursorsBySession;
  }> {
    const accepted: string[] = [];
    const resyncRequired: string[] = [];
    const serverCursors: CursorsBySession = {};

    for (const sid of sessionIds) {
      if (!this.subscriptions.has(sid)) {
        this.subscribe(sid);
      }
      if (!accepted.includes(sid)) accepted.push(sid);
    }

    if (cursors) {
      for (const [sid, cursor] of Object.entries(cursors)) {
        this.cursorsBySession.set(sid, cursor);
        if (!this.subscriptions.has(sid)) {
          this.subscribe(sid);
        }
        if (!accepted.includes(sid)) accepted.push(sid);

        const result = await this.wsBroadcast.getBufferedSince(sid, cursor);
        if (result.resyncRequired !== false) {
          this.send(
            buildResyncRequired(sid, result.resyncRequired, result.currentSeq, result.epoch),
          );
          resyncRequired.push(sid);
        } else {
          for (const entry of result.events) {
            this.send(entry.envelope);
          }
        }
      }
    }

    for (const sid of accepted) {
      try {
        serverCursors[sid] = await this.wsBroadcast.getCursor(sid);
      } catch (err) {
        this.logger.warn({ sid, err: String(err) }, 'getCursor failed for ack');
      }
    }

    return { accepted, resyncRequired, serverCursors };
  }

  private async onSubscribe(msg: SubscribeMessage): Promise<void> {
    const { session_ids, cursors, watch_fs } = msg.payload;