RFR: 8269934: RunThese24H.java failed with EXCEPTION_ACCESS_VIOLATION in java_lang_Thread::get_thread_status

[David Holmes]

If a thread is attaching via JNI and has not yet created its Thread object it can be caught in a ThreadSnapshot during a thread dump (VM_DumpThreads) of all threads**, and the threadObj() will be NULL, so we can't pass it to get_thread_status().

** JMM dumpThreads API

The initial fix simply checks for a NULL threadObj() and reports thread status NEW in that case.

Alternatively we could filter the attaching thread out completely in VM_DumpThreads::doit by expanding:

      if (jt->is_exiting() ||
          jt->is_hidden_from_external_view())  {
        // skip terminating threads and hidden threads
        continue;
      }

to also check jt->is_attaching_via_jni().

Note that higher-level code already filters out ThreadSnapshots with NULL threadObj() anyway so we could go either way.

Testing: manual hacks - see bug report.
  - tier 1-3 sanity testing

Thanks,
David

-------------

Commit messages:
 - 8269934: RunThese24H.java failed with EXCEPTION_ACCESS_VIOLATION in java_lang_Thread::get_thread_status

Changes: https://git.openjdk.java.net/jdk/pull/4921/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4921&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269934
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4921.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4921/head:pull/4921

PR: https://git.openjdk.java.net/jdk/pull/4921