RFR: 8256811: Delayed/missed jdwp class unloading events [v14]

[Chris Plummer]

On Mon, 11 Jul 2022 15:00:41 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Currently, jdi only check and process class unloading event when it detects a new GC cycle.
>> 
>> After [JDK-8212879](https://bugs.openjdk.org/browse/JDK-8212879), posting class events can overlap with GC finish event, that results, sometimes, it only captures partial or even empty unloaded class list. The pending list usually can be flushed out at next GC cycle. But for the classes unloaded during the last GC cycle, the class unloading events may lost forever.
>> 
>> This patch checks and processes class unloading events unconditionally, suggested by @kbarrett, the last pending unloaded class list can be flushed by other events, such as `VM_DEATH`.
>> 
>> It also performs `commonRef_compact()` only when there are classes unloaded.
>> 
>> New test failed about 20% without patch, none with patch.
>> 
>> **Update**
>> There are significant changes from early patch. 
>> 
>> The new approach:
>> No longer removing dead objects and post events on VM thread. I believe it was implemented this way to workaround the following issues:
>> - JDI event handler uses JVMTI raw monitor, it requires thread in `_in_native` state
>> - The thread can not hold lock, which is needed to protect `JvmtiTagMap` while walking, when transition to `_in_native` state
>> 
>> The new solution breaks up into two steps:
>> - Collect all dead object tags with lock
>> - Transition to `_in_native` state and post object free events in one batch
>> 
>> This way, JDI event handler can process object free events upon arrivals without delay.
>> 
>> **Update 2**
>> There is a comment for ` JvmtiTagMap::check_hashmap()` that states `ObjectFree` events are posted before heap walks.
>> 
>> // This checks for posting and rehashing before operations that
>> // this tagmap table.  The calls from a JavaThread only rehash, posting is
>> // only done before heap walks.
>> void JvmtiTagMap::check_hashmap(bool post_events) {
>> 
>> Now, the events are actually posted after heap walks, but I don't think it makes any material material difference. 
>> Even the events are posted earlier in old code, but they are only processed after next GC cycle.
>
> Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 31 commits:
> 
>  - Merge
>  - plummercj's comment
>  - Fix test to ensure class unloading
>  - Incorporated plummercj's test changes
>  - Merge branch 'master' into JDK-8256811-jdi-missing-class-unloading-event
>  - Moved TestClassUnloadEvents.java to new location
>  - Fix test
>  - Use Shenandoah GC for debuggee for deterministic
>  - debug test
>  - Adding log for debugging test failure on Windows
>  - ... and 21 more: https://git.openjdk.org/jdk/compare/0c370089...c45dd040

Actually I have one minor request. Rename the test to ClassUnloadEventTest.java to be consistent with other test names in the directory.

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Changes requested by cjplummer (Reviewer).

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