RFR: 8352088: Call of com.sun.jdi.ThreadReference.threadGroups() can lock up target VM

[Jaikiran Pai]

On Tue, 25 Mar 2025 20:36:28 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> Calling ThreadGroupReference.groups() from an event handler can cause a deadlock. Details in first comment. Tested with :jdk_lang on all supported platforms and tier1, tier2, tier3, and tier5 svc testing.

src/java.base/share/classes/java/lang/ThreadGroup.java line 664:

> 662:     private ThreadGroup[] subgroupsAsArray() {
> 663:         List<ThreadGroup> groups = synchronizedSubgroups();
> 664:         return groups.toArray(new ThreadGroup[groups.size()]);

Hello Chris, would a comment on this line be necessary to prevent future (IDE encouraged [1]) refactoring of this code back to `toArray(new ThreadGroup[0])`?

[1] 
<img width="983" alt="toArray" src="https://github.com/user-attachments/assets/643e66b8-c18a-4a10-8961-2aa3b2869fcc" />

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PR Review Comment: https://git.openjdk.org/jdk/pull/24236#discussion_r2013272532