Neither half of the validation was covered. Dropping `&& -c $cached` from the wrapper left the whole file green: all three poison values fail on the pathname prefix, so none of them ever reached the character-device test. A path that matches the hiddev glob but is not a device now covers it, and it is the real case rather than a synthetic one -- the display replugs, the interface renumbers, and the cached node is gone. It is added only when the host has no such node, so a machine with the display attached cannot fail there spuriously.
The no-XDG_RUNTIME_DIR assertion had the same problem for the opposite reason: its decoy held a path the validation rejects on its own, so restoring the `${XDG_RUNTIME_DIR:-/tmp}` fallback left it passing. It asserts on the open now instead of on the contents -- a FIFO with no writer blocks whoever opens it, so a wrapper that consults the path hangs and one that ignores it exits. mkfifo is atomic and fails outright if the path is taken, so it still neither overwrites a file nor follows a symlink at the fixed path.
Clearing created_tmp_cache as soon as the decoy is removed keeps this run's EXIT trap from deleting a concurrent run's decoy at the same fixed path, which would have let that run pass against the old code.
🤖 Generated by Opus 5 in Claude Code.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Codex XHigh <noreply@openai.com>
