Since https://github.com/qt/qtbase/commit/eaee1209f0ead5be786e81db8aee604ccfea85b0#diff-30e4174ed07523635a22648c06274e0e , QSemaphore::tryAcquire(int n, int timeout) will call return tryAcquire when timeout < 0 . But tryAcquire will immediately return false if no resource is availble, whereas it should wait for ever as documented (and implemented in previous version)

This is causing crashing bugs to the QGIS project: https://lists.osgeo.org/pipermail/qgis-developer/2017-November/050456.html