backup_existing snapshots the configuration before the update stages, but a
staging disk-headroom failure then exited and left that snapshot behind. On a
small root filesystem each automatic retry added another full backup and made
the low-space condition progressively worse until updates could no longer run
(#2127). Record the snapshot and remove it when download_pulse aborts at the
headroom check, before anything was replaced. Add regression coverage.
Change-source: pulse-maintainer
pulse-triage[bot]
07d54cd345