The `flush()` method unconditionally sets `_active.flag = True` and then resets it to `False` in its finally block. If `flush()` is called while `add()` is already executing on the same thread (e.g., via a callback triggered during `_ensure_thread()` or buffer operations), the `finally` block in `flush()` will reset the flag to `False` while `add()` is still running. This breaks the re-entry guard for the remainder of `add()`'s execution, allowing a subsequent re-entrant call to proceed and potentially cause the deadlock this PR aims to prevent.