RFR: 8256811: Delayed/missed jdwp class unloading events

[Zhengyu Gu]

On Wed, 15 Jun 2022 17:42:42 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> One issue I have with this fix is that is doesn't fix the "delay" issue. It helps it, and ensures the CLASS_UNLOAD events are sent before the debuggee exits, but it doesn't guarantee that they are sent in a time manner.
> 
Yes, it improves timing in some cases, but not for all.

> Your test is somewhat contrived in that the debuggee only runs for as long as it takes to look up all the test classes. Then it exits, generating a VM_DEATH, and guaranteeing that by that point all CLASS_UNLOAD events have been sent (and without delay). However, we also have the issue of a [JDK-8257705](https://bugs.openjdk.org/browse/JDK-8257705), which is a test that times out waiting for a CLASS_UNLOAD event, because there isn't another event to trigger sending it. Since the debuggee does not exit until after the test is done (and has timed out), the VM_DEATH comes too late.
> 
> [JDK-8256811](https://bugs.openjdk.org/browse/JDK-8256811) offers other suggestions for the fix, albeit more complex than your. Please consider the one mentioned in the first comment, which is basically adding a watchdog thread.

I could not seem to find the other solution in [JDK-8256811](https://bugs.openjdk.org/browse/JDK-8256811) comments.

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