Captures the symptom (Reset blew up on `umount target busy`), the
false starts (eager retry, lazy fallback, TimeoutStopSec bump — all
shipped briefly and reverted), the actual root cause (the helper's
own Python interpreter inheriting and pinning the unit's mount
namespace), and the fix (nsenter at the systemd Exec line).
The lessons section is the part future-me reads first: a retry loop
is a hint that something we own is the blocker; probe `/proc/*/ns/mnt`
before assuming kernel async; `+` Exec prefix doesn't escape the
unit's mount namespace.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
