BusyThreadPolicy

How a submission treats a thread that already has an open entry or a live run (design 4.55; W3 #2136 gives each its full behaviour). Reject, the default, refuses the submission with GraphError.ThreadBusy, as a direct run on a busy thread is refused (§5.3). Enqueue waits behind the thread's open entries, in submission order. Interrupt goes ahead of the thread's queued entries, which keep their order behind it; of two Interrupt entries the later goes first and the earlier is closed EntryOutcome.Interrupted before it runs (Stage 5 rule 5). Stopping the thread's running entry at its next safe point is W3's.