D344 - Guest threads park at the trampoline

Status: decided Date: 2026-08-27

Suspending a title raises a process-wide park flag, and each guest thread parks at its next pass through the import trampoline. The check is a load and a branch, with no allocation or lock. The run report counts how many live threads parked, and ending a title clears the flag.

Why: suspending a thread at an arbitrary instruction can catch it holding the host runtime's heap lock and deadlock the worker. A thread stopped in the trampoline is in our code and holds no guest lock. A thread that never calls an import never parks, so the count makes that hole visible. A flag left set would park the next run's threads.

Rejected:

• Suspending threads from outside: an occasional unrecoverable deadlock.
• Reporting "suspended" without a count: hides threads that kept running.