D560 - An event queue wait never blocks

Status: assumed Date: 2026-09-04

Waiting on an event queue delivers what is ready and reports zero delivered when nothing is, even for an indefinite timeout. A delivered event uses the FreeBSD struct kevent layout, with udata taken from the registration and filter written as zero.

Why: a blocked wait with no poster on another thread stops the run forever, and a hang destroys a run's evidence where a busy loop only costs time. Zero delivered is what kevent reports on a timeout, so the answer is still lawful. The target kernel is FreeBSD-derived, which makes kevent the citable layout; udata echoes the caller's word, so only the registration knows it.

Rejected:

• Blocking on a null timeout: the thread that would post the completion is often the blocked one.
• Leaving delivery unimplemented: the guest waits on a flip completion that was performed and never announced.