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

[Zhengyu Gu]

On Fri, 24 Jun 2022 03:32:13 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Zhengyu Gu 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 18 additional commits since the last revision:
>> 
>>  - Improve naming and cleanup
>>  - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event
>>  - v4
>>  - v3
>>  - v2
>>  - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event
>>  - Merge branch 'jdi_tmp' into JDK-8256811-jdi-missing-class-unloading-event
>>  - v0
>>  - v2
>>  - v1
>>  - ... and 8 more: https://git.openjdk.org/jdk/compare/d5eafb33...559b4bf1
>
> src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c line 627:
> 
>> 625: 
>> 626:     if (evinfo->ei == EI_CLASS_UNLOAD) {
>> 627:         synthesizeUnloadEvent((char*)jlong_to_ptr(evinfo->tag), env);
> 
> You're in uncharted waters here. Previously `event_callback()` was always called with a `thread` argument except for VMDeath. Now it is being called during an ObjectFree, which has no thread associated with it. That means all the code below that is executed when we have a thread is no longer being executed after the ClassUnload events are generated by the `synthesizeUnloadEvent()` call. Have you thought this part through?
> 
> My guess is that for the most part it is harmless. The "invoke" support is not relevant in this case. I think/hope that means there is no need for calling `threadControl_onEventHandlerEntry()`.  The `filterAndHandleEvent()` probably amounts to an (expensive) nop, although it seems silly to be calling it for the `ObjectFree` event. I'd suggest just returning at this point in the code, but you do still have the restoration of the pending exception to handle. Maybe that should just be done here and then return. It will make it a lot more clear that the rest of the code below is not needed.

I did not thought through this part. I hooked up `ObjectFree` event in `event_callback()` to behave like other events, now I see it is probably not a good idea. Reverted.

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