[jdk17u-dev] RFR: 8256811: Delayed/missed jdwp class unloading events

[Andrew Dinn]

On Wed, 17 Aug 2022 18:42:08 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Backport of patch to fix class unloading event notification.
>> 
>> This backport was originally developed by @zhengyu123 and involved some minor changes to the original patch.
>> 
>> n.b. the original patch required a follow up fix (cf JDK-8290908) which also needs backporting after this has been included.
>
> This fix results a couple of followup CRs: JDK-8290908 and JDK-8291456. JDK-8291456 has yet been resolved, probably this backport should wait till JDK-8291456 is resolved.

@zhengyu123 @gnu-andrew 

I think we are now ready to queue and push this backport to jdk17u (JDK-8256811) as long as it is followed up by backports for these follow-up JIRAs. I propose the following sequence:

 - _JDK-8256811_ : Zhengyu's original fix
 - _JDK-8290908_ : Correct TCK failure from last patch
 - _JDK-8291456_ : Flush pending ObjectFree events at VM shutdown
 - _JDK-8291650_ : Add delay to ClassUnloadEventTest to allow ObjectFree events to propagate
 - _JDK-8292880_ : Enable debuggee logging in ClassUnloadEventTest
 - _JDK-8292879_ : Add delay to skew race between ClassPrepare event propagation and GC request initiation in debuggee

I already have a dependent PR queued for JDK-8290908 and am preparing further dependent PRs for the next 4 JIRAs.

A few comments:

JDK-8291456 is required to ensure that events are not missed but it is not enough in itself to fix the test

Two of the three following fixes to the test code (JDK-8291650, JDK-JDK-8292879) also do not guarantee to remove the problem since all they do is add delays to skew races in favour of the desired outcome. However, absent any proper fix to these timing problems this is the best option we have to remedy the failure.

Strictly, JDK-8292880 is not needed. However, it should not cause any harm and it will be very useful for diagnosis if we run into problems later on which demand a proper fix.

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PR: https://git.openjdk.org/jdk17u-dev/pull/635