[Riceman2000]

Summary

Adds first-class Nix and NixOS support without changing any Bend sources. The package is the official Linux release tarball (v0.11.1, x86_64-linux and aarch64-linux), not a from-source Bend build.

• packages.default / overlays.default install backplane and backplane-serve with web/ and the bun helpers beside the real binaries, so the hub's exe-dir lookup still finds them. autoPatchelf plus an rpath cover libX11 (NEEDED and dlopen'd) and the dlopen'd libfreetype.so.6 / libfontconfig.so.1. The wrappers set BACKPLANE_NO_UPDATE=1 and expand LD_LIBRARY_PATH in a way that is safe under set -u.
• nixosModules.default is services.backplane: user, state and cache dirs, listen address and port, firewall, optional tailnet listener ordered after tailscaled and (optionally) tailscaled-autoconnect, extra packages, environmentFile / LoadCredential with a store-path assertion, a KiStack seed, and a PathChanged restart when credential files change.
• checks cover module assertions, GET /hello as the stand-in for the missing --version flag, and a TCG-friendly nixosTest (accel=kvm:tcg, requiredFeatures.kvm = false).
• nix fmt runs alejandra. nix/update.sh rewrites nix/sources.nix from GitHub SHA256SUMS and that tag's KiStack.revision().
• Usage, the module example, and the bump procedure are in nix/README.md. The project README only links there.

The service does not filter the hub's stdout. When the tailnet listener binds, the pairing token line is written to the journal as the binary prints it. With --no-tailscale the binary does not print it.

Verified

nix flake check on x86_64-linux exited 0 (all checks passed) on this revision. Alejandra reports the Nix files comply. QEMU fell back to TCG (failed to initialize kvm / no kvm system feature).

The package check starts backplane --gpu off -- --headless --foreground --no-tailscale and asserts:

• GET /hello contains 0.11.1 and backplane (CBOR, not a --version flag)
• the pairing token file exists and is not printed when --no-tailscale is set
• the wrapper is set -u safe (${LD_LIBRARY_PATH:-})
• libfreetype.so.6, libfontconfig.so.1, and libX11.so.6 are mapped

The NixOS test boots the module and asserts:

• backplane.service is up and GET /hello on port 3787 contains 0.11.1
• the listen address is 127.0.0.1:3787 and not 0.0.0.0, [::], or *
• state and cache dirs are owned by backplane, and /var/lib/systemd/linger/backplane exists
• with --no-tailscale, the journal does not contain the pairing token (the binary does not print it; the unit does not filter stdout)
• LoadCredential and EnvironmentFile reach the process (BACKPLANE_TEST_TOKEN, BACKPLANE_FROM_FILE), along with HOME, XDG_CACHE_HOME, and BACKPLANE_NO_UPDATE=1
• the secret path unit uses PathChanged and not PathExists
• with tailscale disabled, the unit does not want tailscaled
• the seeded KiStack tree is readable by the service user and not writable, owned by root
• systemctl --failed is empty

Eval checks reject /nix/store secret paths and, with tailscale.enable, put tailscaled.service and tailscaled-autoconnect.service in Wants= and After= but not Requires=.

Not verified

• aarch64-linux is pinned (hash from the v0.11.1 SHA256SUMS) but not built. nix flake check omitted it: incompatible systems: aarch64-linux.
• The bun helpers (backplane-browser, backplane-step2glb, backplane-voice) are installed and autoPatchelf'd; they were not executed.
• Tailscale listen, --tailscale-https, firewall opening, and a real tailscaled-autoconnect.service were checked by evaluation only, not by booting tailscaled. The token line on a tailnet bind was not observed in a journal.
• Agent CLIs (claude, codex, grok), KiCad, and FreeCAD were not run under the unit. Hardening leaves ProtectHome off and does not set MemoryDenyWriteExecute, ProtectControlGroups, or a syscall filter, which is what those CLIs need, but that is from reading the sources, not from running them.
• The credential path unit's PathChanged lines are present. A live rewrite of a secret file was not performed.
• kistack.readOnly makes the cache non-writable after the seed. Upstream's background refresh still runs; its failed writes are ignored. That refresh was not observed against GitHub in the test (the skill file was already present).

Upstream behavior this module works around

Verified against the sources at this commit and the v0.11.1 binaries:

• No --version. Unknown flags are ignored. A non-numeric --port falls back to 3787. GET /hello is CBOR {"backplane": "<version>", "busy": N}.
• The window binary NEEDs libX11.so.6 and also dlopens it, plus libfreetype.so.6 and libfontconfig.so.1. backplane-serve NEEDs only libm and libc.
• backplane --headless detaches unless --foreground is also passed. The service runs backplane-serve, which stays in the foreground.
• BACKPLANE_NO_UPDATE (any non-empty value) disables self-update. There is no env var for the home directory, the tailscale binary, or the KiStack directory.
• The pairing token is printed in full on stdout when the tailnet listener binds. The unit does not filter it. With --no-tailscale it is not printed.
• KiStack lives at $HOME/.cache/backplane/kistack, not under XDG_CACHE_HOME. Startup can write plugin.json into the revision directory, and a background refresh records GitHub HEAD in current, which the next start prefers. kistack.readOnly is a module-side approximation (copy, point current at the pin, drop write permission). Upstream has no read-only or pin switch.
• Skills.kistack (lexicographic last directory name) is not on the startup path. Main.prepare uses KiStack.current.
• The tailnet address is decided once at start (tailscale ip -4). Peer discovery retries every two minutes. The binary runs bare tailscale from PATH.
• Agents inherit the hub environment at spawn and are not restarted when a credential file changes unless the hub itself restarts. systemd-run --user is used only when a user manager answers; otherwise the agent is unconfined. The module enables lingering.

Follow-ups that need Bend changes

• Retry tailscale ip -4 until it succeeds, and accept a path to the tailscale binary.
• A KiStack directory override and a switch to keep the pin (no HEAD refresh, no write into the tree), and honor XDG_CACHE_HOME for that cache.
• Re-read agent credentials at each spawn.
• Reject unknown flags and a bad --port. Add a real --version.