RFR: 8371380: [LOOM] The debug agent should avoid enabling VIRTUAL_THREAD_START/END events when possible

[Chris Plummer]

On Tue, 25 Nov 2025 16:32:11 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> src/jdk.jdwp.agent/share/native/libjdwp/eventFilter.c line 1444:
>> 
>>> 1442:                 } else {
>>> 1443:                     error = threadControl_setEventMode(JVMTI_DISABLE, EI_VIRTUAL_THREAD_END, thread);
>>> 1444:                 }
>> 
>> Q: Why is the `THREAD_START` event disabled globally (for all threads) but the `THREAD_END` event is disabled for specific thread?
>
> THREAD_START can't have a thread filter. THREAD_END can. "thread" == requestThread(node) == the thread filter if there is one, so when disabling THREAD_START it could be an actual thread or it could just be NULL. For THREAD_START "thread" should always be NULL, so I suppose I could have just asserted that and used "thread" as the argument instead of "NULL".

Actually it appears that JDWP THREAD_START can have a filter but JVMTI THREAD_START does not allow you to set the thread to enable the events on. That means when JDWP THREAD_START events are requested and a thread filter is provided, JVMTI THREAD_START needs to be enabled for all threads. That's actually how it is working now. I tried adding the assert and it was triggered. So that means passing "thread" instead of NULL would sometimes end up passing an actual thread to JVMTI and it would produce a [JVMTI_ERROR_ILLEGAL_ARGUMENT in that case.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28485#discussion_r2560862724