D256 - A failed trylock reports busy

Status: decided Date: 2026-08-25

A try-lock that cannot take the mutex answers a distinct busy result, never success and never the invalid-argument placeholder.

Why: reporting that the lock could not be taken is the whole purpose of a try-lock, and a guest branches on it. Success would send the guest into a critical section it does not hold, with nothing in a trace to say so. A held lock is the ordinary outcome, not a caller error, so a trace must tell it apart from a bad pointer.

Rejected:

• Answering success: silently breaks mutual exclusion.
• Reusing the invalid-argument code: a trace cannot tell a busy lock from a bad handle.