RFR: 8256811: Delayed/missed jdwp class unloading events [v3]

[Zhengyu Gu]

On Mon, 27 Jun 2022 17:01:02 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> @sspitsyn  Thanks.
>> 
>> Yes, `JvmtiTagMap::check_hashmaps_for_heapwalk()` is where I saw the call from `VMThread`.
>> 
>> This patch is intended to **not** post `ObjectFree` events on `VMThread`, so I am going to move this PR back to draft state and take another look.
>
>> This patch is intended to not post ObjectFree events on VMThread, so I am going to move this PR back to draft state and take another look.
> 
> I'm not so sure it matters to the debug agent. I thought the main change here was to not force posting of ObjectFree events to be deferred to the VMThread, and to change the debug agent to send the CLASS_UNLOAD events right away rather than accumulating them and waiting for a GCFinishEvent. Is there any issue if the posting of ObjectFree and sending of CLASS_UNLOAD is done on the VMThread?

Hmm... I thought that is the whole purpose for delaying processing class unload events, because it can not be done on `VMThread`? Otherwise, we don't need all this complexity, just have `cbTrackingObjectFree()` calls `synthesizeUnloadEvent()` directly.

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PR: https://git.openjdk.org/jdk/pull/9168