8323792: ThreadSnapshot::initialize can cause assert in Thread::check_for_dangling_thread_pointer (possibility of dangling Thread pointer)

[kevinjwalls]

An assert can be triggered in Thread::check_for_dangling_thread_pointer.
This is with a testcase that has threads starting and terminating, while continually using ThreadMXBean.getThreadInfo() on the thread IDs.

jmm_GetThreadInfo runs without a safepoint/vm operation (when the stack depth requested is zero).

find_JavaThread_from_java_tid(tid) occasionally (with this test) returns a JavaThread for which Thread::is_JavaThread_protected_by_TLH(const JavaThread* target) is false. So use that as an additional condition to protect ourselves from trying to put such a thread in the ThreadDumpResult.

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Issue

• JDK-8323792: ThreadSnapshot::initialize can cause assert in Thread::check_for_dangling_thread_pointer (possibility of dangling Thread pointer) (Bug - P4)

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