RFR: 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM [v7]

[Daniel D. Daugherty]

On Tue, 22 Nov 2022 05:57:37 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
> 
>  - Merge branch 'master' into br19
>    Merge
>  - removed thread->vthread() != NULL from  JvmtiVirtualThreadEventMark constructor
>  - minor update for unnamed threads in jvmti_common.h
>  - fixed a trailing white space issue
>  - extended VirtualThreadStartTest to support more configs; fixed issue in jvmtiExport.cpp
>  - roll back unintended VirtualThread.java file update
>  - simplified VirtualThreadStartTest
>  - 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM

src/hotspot/share/prims/jvmtiExport.cpp line 202:

> 200:     JvmtiEventMark(thread) {
> 201:     _jthread = to_jobject(thread->vthread());
> 202:     assert(thread->vthread() != NULL || thread->threadObj() == NULL, "sanity check");

Seems a little strange to me that L202 is after L201. You're asserting
that `thread->vthread() != NULL` after passing it to a `to_jobject()` call.

-------------

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