Follow-up to @benbarclay's Docker s6 PR (#30136). The Phase 4 hooks
`_maybe_register_gateway_service` and `_maybe_unregister_gateway_service`
were already documented as "no-op on host", but they reached that no-op
by:
1. importing `hermes_cli.service_manager`
2. calling `get_service_manager()` (which calls `detect_service_manager()`)
3. checking `mgr.supports_runtime_registration()` and returning False
If anything in step 1 or 2 raised an unexpected exception (e.g. a host
machine with a partial s6 install — `/proc/1/comm == s6-svscan` somehow,
but `/run/s6/basedir` absent, or vice versa), the `except Exception`
in the hook would print a confusing "⚠ Could not register s6 gateway
service: ..." warning on a non-container machine that has never touched
the container.
Reorder so `detect_service_manager() != "s6"` is checked FIRST, and
return silently for any detection failure. Host machines now:
- never import the s6 backend
- never call get_service_manager()
- never print an s6-shaped warning under any failure mode
E2E confirmed on host Linux (systemd):
`_maybe_register_gateway_service(...)` produces empty stdout,
detect_service_manager() returns "systemd".
Existing tests updated to patch `detect_service_manager` for the s6
call-through cases (they previously relied on get_service_manager
being the only gate, which is no longer true). Added one new test —
`test_register_silent_when_detect_throws` — asserting that a broken
detector cannot leak a warning to host users.
cc @benbarclay — visible behavior change vs. your branch is one
fewer code path on host. Test changes are minimal (one helper +
`_patch_detect_s6` opt-in per s6 test). Happy to revert if you
prefer the original shape.
a4092ab217