QThread::terminate(): don't depend on stack unwinding

Posix doesn't seem to specify whether the stack of cancelled threads
is unwound, and there's nothing preventing a QThread from
terminate()ing itself, so be extra careful to drop the mutex before
calling pthread_cancel.

We can't drop the mutex in general, as that would open a window for
the following race condition:

   T1                       T2
   t3->terminate()
     lock();
     read ID;
     terminated = true;
     unlock();
   ----------- t3 exits naturally -----------
                            t3->wait();
                            t4->start(); // gets ex-t3's ID
     pthread_cancel(ID) // oops, cancels new t4

But we can drop it when this == currentThread(), because said window
does not exist: While this_thread is executing terminate(), it cannot
at the same time exit naturally.

As drive-by, scope a variable tighter.

Pick-to: 6.8 6.7 6.5
Change-Id: I77a628e62d88e383d5aa91cfd97440186c997fc4
Reviewed-by: Thiago Macieira <[email protected]>

src/corelib/thread/qthread_unix.cpp

@@ -751,16 +751,27 @@ void QThread::terminate()
  Q_D(QThread);
  QMutexLocker locker(&d->mutex);
 
- if (!d->data->threadId.loadRelaxed())
+ const auto id = d->data->threadId.loadRelaxed();
+ if (!id)
  return;
 
  if (d->terminated) // don't try again, avoids killing the wrong thread on threadId reuse (ABA)
  return;
 
  d->terminated = true;
 
- int code = pthread_cancel(from_HANDLE<pthread_t>(d->data->threadId.loadRelaxed()));
- if (code) {
+ const bool selfCancelling = d->data == currentThreadData;
+ if (selfCancelling) {
+ // Posix doesn't seem to specify whether the stack of cancelled threads
+ // is unwound, and there's nothing preventing a QThread from
+ // terminate()ing itself, so drop the mutex before calling
+ // pthread_cancel():
+ locker.unlock();
+ }
+
+ if (int code = pthread_cancel(from_HANDLE<pthread_t>(id))) {
+ if (selfCancelling)
+ locker.relock();
  d->terminated = false; // allow to try again
  qErrnoWarning(code, "QThread::start: Thread termination error");
  }