RFR: 8319244: implement JVMTI handshakes support for virtual threads [v8]

[Serguei Spitsyn]

On Sat, 18 Nov 2023 02:29:26 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   review: add jdk_internal_vm_Continuation::done(cont) check to JvmtiEnvBase::is_vthread_alive
>
> src/hotspot/share/prims/jvmtiEnvBase.cpp line 1989:
> 
>> 1987:     } else {
>> 1988:       Handshake::execute(hs_cl, tlh, target_jt); // delegate to Handshake implementation
>> 1989:     }
> 
> Every implementation of JvmtiUnitedHandshakeClosure has to check if the target thread is virtual and call do_vthread manually.
> I'd suggest to handle this by proxy class, something like
> Suggestion:
> 
>     class Adapter : public HandshakeClosure {
>       JvmtiUnitedHandshakeClosure* _hs_cl;
>       Handle _target_h;
>     public:
>       Adapter(JvmtiUnitedHandshakeClosure* hs_cl, Handle target_h)
>           : HandshakeClosure(hs_cl->name()), _hs_cl(hs_cl), _target_h(target_h) {}
>       virtual void do_thread(Thread* thread) {
>         if (java_lang_VirtualThread::is_instance(_target_h())) { // virtual thread
>           _hs_cl->do_vthread(_target_h);
>         } else {
>           _hs_cl->do_thread(target);
>         }
>       }
>     } adapter(hs_cl, target_h);
> 
>     if (self) {                    // target thread is current
>       adapter.do_thread(target_jt); // execute handshake closure callback on current thread directly
>     } else {
>       Handshake::execute(&adapter, tlh, target_jt); // delegate to Handshake implementation
>     }

Thank you for the suggestion! Agreed, this should help to get rid of this duplication/ugliness.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16460#discussion_r1398218934