`punktfunk-client --check-update` fetches the same Ed25519-signed per-channel
manifest the host trusts, works out how this client was installed, and reports
what — if anything — can be done about a newer build. `--apply-update` does the
part it owns. `--version` prints the CI-stamped build string, which is both what
the comparison needs and what a packaging gate can run the binary for.
The engine lives here rather than in the Decky plugin for two reasons: verifying
a signature needs crypto that Decky's embedded Python does not have, and the
plugin is not the only surface that wants the answer. Every UI becomes a caller,
exactly as it already is for --pair, --library and --list-hosts.
What is possible depends on the install, and the module refuses to pretend
otherwise. A flatpak is a per-user install its launcher can update. A .deb/.rpm/
pacman client goes through the packaged root helper — a fixed, parameterless
systemd oneshot polkit authorises for members of the punktfunk-update group, so
the request carries nothing an attacker could influence and the payload comes
from the distro's own signed repositories. A sysext, a nix profile or a source
build gets the command and nothing else: the signed sysext feed carries the HOST
image, so a client sysext has no feed to update from, and a button that can only
fail is worse than one honest line.
A failed check reports the failure. "Could not tell" rendering as "up to date"
is the one answer that must never happen.
The box's capabilities are probed once into a `Caps` struct so the routing rules
are a pure function of (install kind, caps) — all seven of them are tested
without a box, including that the kill switch beats every kind and that pacman
needs its own root-owned full-sysupgrade opt-in on top of the group.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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