Commit 1ed0dd88a32cd2c5ae100b48e14ff55bcbb652e6 moved the finish()
functionality from immediately after run() returns to the time of
thread-local destruction, to make sure that user destructors didn't run
after our cleaning up. But as a side effect, it made other user code run
too late, after some thread-local statics had been destroyed.
This is a common practice, which causes the destructor for worker to run
too late:
worker->moveToThread(thread);
...
QObject::connect(thread, &QThread::finished, thread, &QObject::deleteLater);
QObject::connect(thread, &QThread::finished, worker, &QObject::deleteLater);
This commit splits the cleanup in two phases: QThreadPrivate::finish(),
which runs immediately after run() and will call back out to user code
(finished() signal and delivery of deleteLater()), and cleanup() that
cleans up the QThread{Private,Data} state and destroys the event
dispatcher. That destruction is the only call out to user code.
I've removed the complex mix of pre-C++11 pthread_setspecific() content
and C++11 thread_local variables in favor of using one or the other, not
both. We prefer the thread-local for future-proofing and simplicity, on
platforms where we can verify this C++11 feature works, and because it
allows us to clean up QThreadData and the event dispatcher as late as
possible. (There's some code that runs even later, such as pthread TLS
destructors, used by Glib's GMainLoop)
Unfortunately, we can't use it everywhere. The commit above had already
noticed QNX has a problem and recent bug reports have shown other
platforms (Solaris, MUSL libc) that, 13 years after the ratification of
the standard, still have broken support, so we use pthread for them and
we call cleanup() from within finish() (that is, no late cleaning-up,
retaining the status quo from Qt 4 and 5). See QTBUG-129846 for an
analysis.
Drive-by moving the resetting of thread priority to after finished() is
emitted.
[ChangeLog][QtCore][QThread] Restored the Qt 6.7 timing of when the
finished() signal is emitted relative to the destruction of thread_local
variables. Qt 6.8.0 contained a change that moved this signal to a later
time on most Unix systems, which has caused problems with the order in
which those variables were accessed. The destruction of the event
dispatcher is kept at this late stage, wherever possible.
Fixes: QTBUG-129927
Fixes: QTBUG-129846
Fixes: QTBUG-130341
Task-number: QTBUG-117996
Change-Id: Ie5e40dd18faa05d8f777fffdf7dc30fc4fe0c7e9
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 4fabde349f16b59f37568da2a4c050c6dd53a34e)
334a3922c0