A held port made 'hermes serve' print only uvicorn's bare
'ERROR: [Errno 98/10048] error while attempting to bind on address'
and exit 1 — indistinguishable from a broken backend for the desktop
spawn and wrapping scripts.
- Preflight bind probe (matching uvicorn's SO_REUSEADDR bind flags)
before uvicorn.Server; on conflict print machine-readable
'BACKEND_PORT_IN_USE port=<port>' + a human hint naming likely
holders, exit 75 (EX_TEMPFAIL — existing repo convention, see
gateway/restart.py, kanban_db.py).
- Probe-to-bind race covered: SystemExit(1) from uvicorn's own bind
failure is re-checked and translated on both POSIX and Windows
runner paths.
- --port 0 (ephemeral) short-circuits the probe: unchanged behavior.
- HERMES_BACKEND_READY contract untouched.
- Tests: real held-socket repro (sentinel + exit 75, sabotage-proven
to fail as bare exit 1 without the fix), free-port boot regression,
ephemeral-port regression, probe/classification units.
- Docs: port-conflict paragraph under 'hermes serve' in
reference/cli-commands.md.
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