Qt concurrent blockingFilter() runs in random thread

It looks like blockingFilter() invokes a working function from:

• a newly created thread from a global thread pool
• or directly from main thread

In order to test it I set maxThreadCount for the global QThreadPool to 1 and run blockingFilter method. The test case is here:

https://codereview.qt-project.org/c/qt/qtbase/+/298910

The output is:

PASS : tst_QtConcurrentFilter::initTestCase()
QDEBUG : tst_QtConcurrentFilter::filterThreadPool() Main thread QThread(0x555f03411e30)
QDEBUG : tst_QtConcurrentFilter::filterThreadPool() current working thread: QThread(0x555f03411e30)
QDEBUG : tst_QtConcurrentFilter::filterThreadPool() current working thread: QThread(0x555f034148d0, name = "Thread (pooled)")
QDEBUG : tst_QtConcurrentFilter::filterThreadPool() current working thread: QThread(0x555f03411e30)
QDEBUG : tst_QtConcurrentFilter::filterThreadPool() current working thread: QThread(0x555f034148d0, name = "Thread (pooled)")
FAIL! : tst_QtConcurrentFilter::filterThreadPool() Compared values are not the same
Actual (workingThreads.size()): 2
Expected (1) : 1
Loc: [/home/jarek/dev/qt-dev/qtbase/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp(127)]
FAIL! : tst_QtConcurrentFilter::filterThreadPool() failed one line above on functor
Loc: [/home/jarek/dev/qt-dev/qtbase/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp(143)]
PASS : tst_QtConcurrentFilter::cleanupTestCase()
Totals: 2 passed, 2 failed, 0 skipped, 0 blacklisted, 8123ms

The call stack when working function is called directly from the main thread:

1 KeepOddIntegers::operator() tst_qtconcurrentfilter.cpp 106 0x555555585f53
2 std::__invoke_impl<bool, KeepOddIntegers&, int const&> invoke.h 60 0x5555555bb967
3 std::__invoke<KeepOddIntegers&, int const&> invoke.h 96 0x5555555b9076
4 std::invoke<KeepOddIntegers&, int const&> functional 81 0x5555555b3149
5 QtConcurrent::FilterKernel<QVector<int>, KeepOddIntegers, QtPrivate::PushBackWrapper>::runIterations qtconcurrentfilterkernel.h 122 0x5555555ab56d
6 QtConcurrent::IterateKernel<int const *, void>::forThreadFunction qtconcurrentiteratekernel.h 231 0x5555555b2e6c
7 QtConcurrent::IterateKernel<int const *, void>::threadFunction qtconcurrentiteratekernel.h 193 0x5555555ab2ba
8 QtConcurrent::ThreadEngineBase::startBlocking qtconcurrentthreadengine.cpp 189 0x7ffff7949a6c
9 QtConcurrent::ThreadEngine<void>::startBlocking qtconcurrentthreadengine.h 153 0x555555586420
10 QtConcurrent::ThreadEngineStarter<void>::startBlocking qtconcurrentthreadengine.h 256 0x555555585c45
11 QtConcurrent::blockingFilter<QVector<int>, KeepOddIntegers> qtconcurrentfilter.h 197 0x55555558f40f
12 testFilterGlobalPool<int, int, KeepOddIntegers> tst_qtconcurrentfilter.cpp 123 0x555555587099
13 tst_QtConcurrentFilter::filterThreadPool tst_qtconcurrentfilter.cpp 142 0x555555566ce4
14 tst_QtConcurrentFilter::qt_static_metacall tst_qtconcurrentfilter.moc 108 0x55555556ab61
15 QMetaMethod::invoke qmetaobject.cpp 2290 0x7ffff72844b3
16 QMetaMethod::invoke qmetaobject.h 120 0x7ffff7b727d1
17 QTest::TestMethods::invokeTestOnData qtestcase.cpp 938 0x7ffff7b6a86f
18 QTest::TestMethods::invokeTest qtestcase.cpp 1166 0x7ffff7b6b43c
19 QTest::TestMethods::invokeTests qtestcase.cpp 1507 0x7ffff7b6c7ee
20 QTest::qRun qtestcase.cpp 1934 0x7ffff7b6d091
... <More>

The call stack when it's called from a thread comming from a thread pool:

1 KeepOddIntegers::operator() tst_qtconcurrentfilter.cpp 106 0x555555585f53
2 std::__invoke_impl<bool, KeepOddIntegers&, int const&> invoke.h 60 0x5555555bb967
3 std::__invoke<KeepOddIntegers&, int const&> invoke.h 96 0x5555555b9076
4 std::invoke<KeepOddIntegers&, int const&> functional 81 0x5555555b3149
5 QtConcurrent::FilterKernel<QVector<int>, KeepOddIntegers, QtPrivate::PushBackWrapper>::runIterations qtconcurrentfilterkernel.h 122 0x5555555ab56d
6 QtConcurrent::IterateKernel<int const *, void>::forThreadFunction qtconcurrentiteratekernel.h 231 0x5555555b2e6c
7 QtConcurrent::IterateKernel<int const *, void>::threadFunction qtconcurrentiteratekernel.h 193 0x5555555ab2ba
8 QtConcurrent::ThreadEngineBase::run qtconcurrentthreadengine.cpp 302 0x7ffff7949e5e
9 QThreadPoolThread::run qthreadpool.cpp 98 0x7ffff6fd448a
10 QThreadPrivate::start qthread_unix.cpp 342 0x7ffff6fcc886
11 start_thread pthread_create.c 463 0x7ffff63096db
12 clone clone.S 95 0x7ffff664288f