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

[Serguei Spitsyn]

On Mon, 6 Nov 2023 23:22:03 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> The handshakes support for virtual threads is needed to simplify the JVMTI implementation for virtual threads. There is a significant duplication in the JVMTI code to differentiate code intended to support platform, virtual threads or both. The handshakes are unified, so it is enough to define just one handshake for both platform and virtual thread.
>> At the low level, the JVMTI code supporting platform and virtual threads still can be different.
>> This implementation is based on the `JvmtiVTMSTransitionDisabler` class.
>> 
>> The internal API includes two new classes:
>>   - `JvmtiHandshake` and `JvmtiUnifiedHandshakeClosure`
>>   
>>   The `JvmtiUnifiedHandshakeClosure` defines two different callback functions: `do_thread()` and `do_vthread()`.
>> 
>> The first JVMTI functions are picked first to be converted to use the `JvmtiHandshake`:
>>  - `GetStackTrace`, `GetFrameCount`, `GetFrameLocation`, `NotifyFramePop`
>> 
>> To get the test results clean, the update also fixes the test issue:
>> [8318631](https://bugs.openjdk.org/browse/JDK-8318631): GetStackTraceSuspendedStressTest.java failed with "check_jvmti_status: JVMTI function returned error: JVMTI_ERROR_THREAD_NOT_ALIVE (15)" 
>> 
>> Testing:
>>  - the mach5 tiers 1-6 are all passed
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review: get rid of the VM_HandshakeUnmountedVirtualThread

I've pushed an update which removes newly introduced VM_op and its use: `VM_HandshakeUnmountedVirtualThread`.
Patricio convinced me that it has to be handshake-safe to execute a `HanshakeClosure` callback on the current (handshake requesting) thread when target thread is an unmounted virtual threads. At an earlier development stage I saw various intermittent crashes and concluded it is not handshake-safe. It is why there was a decision to use the `VM_HandshakeUnmountedVirtualThread`. I do not see these crashes anymore after a full testing cycle.

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PR Comment: https://git.openjdk.org/jdk/pull/16460#issuecomment-1797026470