RFR: 8296492: Remove ObjectLocker in JVMTI get_subgroups call [v5]

[Serguei Spitsyn]

On Tue, 8 Nov 2022 14:55:17 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> The JVM code took a ThreadGroup lock before poking into ThreadGroup fields.  Call a method in the ThreadGroup to call the synchronized method instead.
>> Tested with tier 1-4.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Forgot a null check.

src/hotspot/share/prims/jvmtiEnvBase.cpp line 557:

> 555: JvmtiEnvBase::new_jthreadGroupArray(int length, objArrayHandle groups) {
> 556:   if (length == 0) {
> 557:     return NULL;

I do not think returning NULL is allowed for JVMTI `GetThreadGroupChildren()`.
Please, see: [GetThreadGroupChildren](https://docs.oracle.com/en/java/javase/19/docs/specs/jvmti.html#GetThreadGroupChildren)

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

PR: https://git.openjdk.org/jdk/pull/11033