The previous commit restarted ssh and only checked `sshd -T` afterwards. On a
box whose only door is SSH, restarting against a config sshd refuses to parse
leaves no listener and no way back in — the same shape as the firewall-before-
bootstrap lockout this session already found in the migration runbook: commit
to the irreversible act, then verify.
Now `sshd -t` parses the MERGED config (our drop-in, cloud-init's, and any
third-party file) before the restart; on failure the drop-in is rolled back and
the daemon is left untouched. Verified: a bad neighbour drop-in exits 255 and
never reaches `systemctl restart`.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
claude-hdb
6e6bf0dceb