RFR: 8256811: Delayed/missed jdwp class unloading events [v17]

[Serguei Spitsyn]

On Thu, 21 Jul 2022 15:31:48 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> This becomes a problem when the test tries to free a raw monitor on exit. It first disable events, then it frees the raw monitor that the even handler synchronizes on. However, it's possible that an event can be in flight when doing this (an event that was triggered before disabling events, but not fully delivered yet). So now you have a chicken-and-egg problem because you need a 2nd raw monitor to synchronize around freeing the 1st raw monitor. So the end result is you can't always safely cleanup on test exit the way we currently are. I think we are getting away with it because the set of events we expect is fixed or predictable. So once the test decides it is done, it won't get more events, even if they are not disabled. Thus we can free the raw monitor after disabling events without worry of some spurious event causing an untimely entry into the event handler. However, I thought there was one test you fixed where we had this exact problem, and you fixed it by not freeing the raw
  monitor on exit. `ThreadStart` and `ThreadDeath` events come to mind, since these are now notoriously unpredictable.

I fixed it this way for simplicity. However, it is still possible to correctly sync if necessary.

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