The helper used to call ``socket.shutdown(SHUT_RDWR)`` followed by
``socket.close()`` to drop CLOSE-WAIT entries immediately. On its own
``shutdown()`` is safe from any thread — it only sends FIN and breaks
pending ``recv``/``send`` — but ``close()`` releases the FD integer to
the kernel. When the helper runs on a stranger thread (the interrupt
loop, the stale-call detector) the FD release races the owning httpx
worker thread that still has the same integer cached inside the SSL
BIO. The kernel then recycles that integer to the next ``open()`` call
— in production, kanban dispatcher's ``kanban.db`` — and the worker's
delayed TLS flush writes a 24-byte TLS application-data record on top
of the SQLite header.
Restrict the helper to ``shutdown(SHUT_RDWR)`` only. The owning httpx
worker's own unwind will close the underlying socket via the same
Python ``socket.socket`` object, which atomically swaps ``_fd`` to -1
before issuing ``close(2)`` — no FD-aliasing window.
The log field ``tcp_force_closed=N`` is kept (now counts shutdowns) so
existing dashboards / log parsers keep working.
e2a7d73a66