After a hive-ci restart the act_runner re-registers and immediately claims
any queued jobs, which can beat the in-container nix daemon coming up: its
nix-daemon.socket carries ConditionPathIsReadWrite=/nix/var/nix/daemon-socket
and is skipped until /nix/var is read-write, so the first nix-dependent
build dispatches into a cold daemon and hangs/retries (observed: a 55m48s
nix flake check vs the normal ~30s — a build-offload stall, not a code
failure; a non-nix step on the same runner passed in 1s, masking it).
Ordering the runner after/wants nix-daemon.socket does not help — a
condition-skipped unit satisfies systemd ordering immediately. Instead add
a blocking ExecStartPre that polls until the daemon actually answers
(nix store ping), placed with mkBefore so it runs ahead of the upstream
module's runner-registration ExecStartPre. The runner therefore cannot
register or claim jobs until nix is usable. Topology-agnostic (works
whether the daemon is in-container or a shared host socket); ~180s ceiling
then the unit fails cleanly rather than claiming jobs into a dead daemon.
Eval-proven: gitea-runner-hive's ExecStartPre is
[wait-nix-daemon, gitea-register-runner-hive] in that order.
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