Deadlock/Race is as follows:
  1. get_task() reads m_shutdown == false and enters loop body
  2. shutdown() is called; sets m_shutdown = true
  3. shutdown() calls m_cond.notify_all()
  4. get_task() finds no task in try_get_task()
  5. get_task() calls m_cond.wait(), missing the notification
  6. solve() waits forever on join()

Provided patch wraps (2) and (3) with the condition variable lock so that
step (5) cannot miss the notification.