Recreating the Docker anchor left the supervised Core and host stranded in
the dead namespace while systemd still reported them active — serving
nobody, invisible to any monitoring that trusts unit state. Reproduced on
staging: netns 4026539938 -> 4026540033, both pids unchanged in the old
one, both units "active", traffic ConnectionResetError.
There is no systemd-native edge signal to bind to. Containers do appear as
units, but the scope name embeds the container ID (docker-<id>.scope), which
changes on every recreate, so BindsTo= has no stable target;
NetworkNamespacePath= resolves once at start; a .path unit on /run/netns
would watch the file this tooling maintains. So: a level-triggered reconcile
on a 10s timer, comparing the namespace the services are ACTUALLY in against
the anchor's CURRENT one, acting only on a real difference. That cannot miss
an event while the watcher restarts or dockerd is down, and needs no
debounce — a burst of three recreations produced exactly one rebind. The
trigger stays separable: a docker-events unit could invoke the same script.
Anchor absent stops the dependants rather than falling back to host
networking; docker unavailable logs once and retries on the next tick.
Two defects found while testing and fixed here:
- mount --bind STACKS when the old mount is busy, silently leaking nsfs
entries; the bind helper now drains stale mounts in a loop.
- reconcile must stop -> rebind -> start, not rebind -> restart: a running
service holds the old namespace open and makes the umount fail busy.
Staging also gained a faithful anchor container so the reproduction is
structural rather than mocked. Economy state was byte-identical across every
lifecycle test. Production units are templates only and remain uninstalled.
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