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2 qthread_unix fixes

Andreas Sturmlechner requested to merge asturmlechner/qtbase:QTBUG-96846 into kde/5.15

QTBUG: https://bugreports.qt.io/browse/QTBUG-96846

Searching for "Thread tried to wait on itself" on bugs.kde.org yields several results so we know this is visible in users' output at least.

QThread: Reset the system thread ID when thread exits on Unix

Unix QThread implementation stores pthread_t as a system thread ID
when the thread is created, but never resets the system ID when
those threads are destroyed. Some implementations may reuse the
same thread IDs for new threads, and this may cause QThread::wait()
to erroneously complain that "Thread tried to wait on itself".

This patch sets the system thread ID to nullptr when the thread is
about to exit and be destroyed by the system.

A regression test is added to tst_qthread.

Fixes: QTBUG-96846
Pick-to: 5.15 6.2
Change-Id: I0850425dd0e09af50e59c9038e7e662a2a624beb
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 52ad59f9eabbe1fc8ca49d117e4955f2d21d50a7)

QThread: Remove superfluous initialization of threadId on Unix

The thread ID is already initialized inside QThread::start() while
the thread lock is taken. This is completed before the attempted
initialization in QThreadPrivate::start() because it tries to take
the same lock.

Task-number: QTBUG-96846
Pick-to: 5.15 6.2
Change-Id: Ic9588f3e2e2f3c2180afbed8ec01155b33043eb3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 05870db7d0606e0a64115a523bf1366137f34700)

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