RFR: 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM [v2]

[Alan Bateman]

On Sat, 19 Nov 2022 07:19:16 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> The can_support_virtual_thread was initially implemented as an onload capability.
>> It is why this capability does not work for the agents loaded into running VM.
>> The fix is to move it from `onload` to `always`capabilities list.
>> 
>> Testing:
>> New test is added: VirtualStartThreadTest.
>> TBD: mach5 jvmti, jdi and tier1-6 tests.
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   roll back unintended VirtualThread.java file update

The update to allow can_support_virtual_threads in the live phase looks okay, I'm just surprised we missed that in JDK 19.

The test looks okay too but it's not a complete unit test for the VirtualThreadEvent event. It only tests the late binding agent case (which is what this bug is about). Maybe it should be extended to run with -agentlib so that it starts in the onload phase or maybe rename so that it's clear what the test is for.

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PR: https://git.openjdk.org/jdk/pull/11246