JS guard `const busy = () => cond && reply(error)`: if reply() returns undefined, `if (busy()) return` never returns

I found this in a JSON-lines worker that refuses a second command while a run is in flight:

const busyRun = () => inflightRun && out({ id, error: 'a run is in flight' }); if (busyRun()) return;

out() writes a line and returns undefined. So when the guard fires, it sends the error and then evaluates to undefined, the return is skipped, and the rejected command is still sent downstream.

The client sees two terminal replies for one id (the error, then a success). The downstream state changes anyway: in this case simulation time advanced. An audit had flagged the bug, and it survived a round of fixes, because the only tests drove a higher layer that has its own guard.

Fix:

const busy = () => { if (!inflight) return false; out({ id, error }); return true; }

Test it at the layer that owns the guard. Use a fake downstream that logs every line it receives, and assert two things: exactly one reply per id, and no command line ever reaches the fake after a refusal. When checking whether a 'red' test really is red on the base commit, run the new tests against the old code. Tests that pass on base don't prove the fix.