RFR: 8376568: Change Thread::getStackTrace to use handshake op for all cases [v3]

[David Holmes]

On Fri, 30 Jan 2026 08:07:44 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Still not clear to me why any new thread is not already filtered out long before now; nor why we have not needed this in the past.
>
> We want ThreadSnapshot.of(Thread) to accept a Thread in any state. Existing behavior is to return null for platform threads that are not alive. For virtual threads it will return a snapshot so we want to change that. The ThreadNotAlive test in the PR allows us to test these cases as they are hard to demonstrate with the thread dump.
> 
> ThreadSnapshot.of(Thread) does not filter out the "not alive" cases. It could, in which case ThreadSnapshotFactory::get_thread_snapshot would need to assert if called with a new/unstarted thread. The terminating thread case would still need to be handled by ThreadSnapshotFactory::get_thread_snapshot. For platform threads there is no JavaThread so it bails easy. For virtual threads it needs to examine the state. Would you prefer if ThreadSnapshot.of(Thread) pre-checked the state so that get_thread_snapshot could be guaranteed to never see a new/unstarted thread?
>  
> Update: I changed ThreadSnapshot.of(Thread) to filter before calling get_thread_snapshot, hopefully this will be easier to understand.

I was assuming/expecting that the top-level code in `ThreadDumper` would filter out not-alive threads the same way `Thread.getStackTrace` does. You don't want lower-level code to have to worry about NEW threads, though of course they still have to deal with races against termination.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29461#discussion_r2757501486