RFR: 8319244: implement JVMTI handshakes support for virtual threads [v4]

[Patricio Chilano Mateo]

On Thu, 16 Nov 2023 22:44:12 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> Sounds good. Is there a reason why not have the check inside JvmtiEnvBase::is_vthread_alive()?
>
> If it is a part of the `JvmtiEnvBase::is_vthread_alive()` then it is racy for mounted virtual threads.
> It is not racy for unmounted virtual threads.

So we should only see that a continuation is done for an unmounted vthread. The last place where we could see a mounted vthread is at notifyJvmtiEnd(), blocked in start_VTMS_transition(), but the continuation is not marked done yet. 
Also I realize the window for the problematic case I mentioned starts even earlier at notifyJvmtiEnd(), not notifyJvmtiUnmount(), because blocking due to JvmtiVTMSTransitionDisabler happens in start_VTMS_transition() not finish_VTMS_transition(). Once the vthread executed notifyJvmtiEnd() any JvmtiVTMSTransitionDisabler that happens afterwards will fall into this case. So maybe the first sentence of the comment should instead be: "The vthread could have already executed the last unmount but might not have changed state to TERMINATED yet."

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PR Review Comment: https://git.openjdk.org/jdk/pull/16460#discussion_r1396592975