CPython's logging module is not reentrant-safe. `Logger.isEnabledFor`
caches level results in `Logger._cache`; under shutdown races the cache
can be cleared (`Logger._clear_cache`, triggered by logging config changes
from another thread) or mid-mutation when a signal fires, raising
`KeyError: <level_int>` (e.g. `KeyError: 10` for DEBUG) inside the signal
handler.
When that happens, the KeyError escapes before the `raise KeyboardInterrupt()`
on the next line can fire, which bypasses prompt_toolkit's normal interrupt
unwind and surfaces as the EIO cascade originally reported in #13710.
Issue #13710 shipped two defenses (asyncio exception handler + outer
`except (KeyError, OSError)` with EIO suppression) that cover the EIO
unwind path. This patch closes the remaining escape hatch: the
`logger.debug` call at the top of `_signal_handler` itself. Wrap it in a
bare `try/except Exception: pass` so logging can never raise through a
signal handler.
Observed in the wild: debug report on 0.12.0 (commit 8163d371) shows the
exact stack — KeyError: 10 at logging/__init__.py:1742 inside the
signal handler's `logger.debug`, followed by the EIO cascade from
prompt_toolkit's emergency flush.
Tests: adds `TestSignalHandlerLoggingRace` to
`tests/hermes_cli/test_suppress_eio_on_interrupt.py` with 6 new cases:
- normal path still raises KeyboardInterrupt
- KeyError(10) from logger.debug does not escape
- any Exception from logger.debug is swallowed
- agent.interrupt still fires when logger.debug raises
- agent.interrupt raising also does not escape
- BaseException (SystemExit) is NOT swallowed — guard uses `except Exception`
deliberately so real shutdown signals still propagate
Closes#13710 regression.
e70e49016f