Stabilize flaky TransactionTest2.transactionsConcurrentWithPersistitClose

[vharseko]

Problem

TransactionTest2.transactionsConcurrentWithPersistitClose is flaky. It starts worker threads that run transactions in a loop, closes Persistit, and then immediately asserts that no worker thread was left stranded:

_persistit.close();
assertEquals("All threads should have exited correctly", 0, _strandedThreads.get());

Each worker increments _strandedThreads on entry and only decrements it once it catches one of the expected exceptions thrown when Persistit is closed. That unwinding is asynchronous: after close() returns, a worker still in the middle of a transaction needs a moment to reach its next Persistit operation, receive the exception, and decrement the counter. There is no wait between close() and the assertion, so on a loaded CI runner a worker that has not yet caught its exception leaves the counter at 1:

java.lang.AssertionError: All threads should have exited correctly expected:<0> but was:<1>
    at com.persistit.TransactionTest2.transactionsConcurrentWithPersistitClose(TransactionTest2.java:271)

Observed on build-maven (ubuntu-latest, 17); the same module passed on JDK 11/21/25/26 in the same run, confirming it is a timing flake rather than a real regression.

Fix

• Keep references to the worker threads and join() them with a single bounded deadline (TIMEOUT, 10s) before the assertion, so the threads get a chance to observe the closure and exit. On success the join returns almost immediately; on a genuine problem (a stuck worker, or one that exited via an unexpected path and therefore did not decrement the counter) the assertion still fails, so the test keeps its diagnostic value.
• Flatten the now-unnecessary launcher thread so the worker threads can be joined directly.
• Reset the shared static _strandedThreads counter at the start of the test for isolation from the other methods in the class.

Verification

• transactionsConcurrentWithPersistitClose run 5×: 5/5 pass.
• Full TransactionTest2 class: 5 tests, 0 failures.