Help test via snapshots: parallel threads

[mflatt]

@LiberalArtist

Potentially a #:keep 'result/exn option could be added later.

Yes, I agree. I intended to add that, but haven't gotten to it.

@LiberalArtist

It doesn't seem like there's a way to tell currently (without controlling the thread's thunk) if a thread terminated normally, with an exception, or by being killed.

The updated thread-wait does support this with an extra fail-thunk argument:

> (list (thread-wait (thread #:keep 'results (lambda () (+ "oops"))) 
                     (lambda () 'failed)))
+: contract violation
  expected: number?
  given: "oops"
 [,bt for context]
'(failed)

@LiberalArtist

From this experiment, it looks like parallel threads must also be blocked, but I'm not sure that behavior is desirable:

The intent is definitely that (system-idle-evt) waits until no parallel thread can make progress, the same a coroutine threads. That's important for the only use that I know for system-idle-evt, which is testing.