Using Q(Open)GLFunctions concurrently on multiple threads had some issues
due to incorrect synchronization: The m_groups list in the
QOpenGLMultiGroupSharedResource, in which the Q(Open)GLFunctions instance
is stored, became corrupted under certain scenarios, for example in the
tst_qglthreads autotest and any two threads that happen to enter an
initializeOpenGLFunctions() or QOpenGLContext::functions() call concurrently.
Locking in value() has been introduced in 666c25c089
to fix such issues, however using the context group's mutex is not enough: that still
allows two threads using two contexts with a different context group to concurrently
enter insert(). Instead, the MultiGroupSharedResource has to have its own mutex
to protect its own member variables.
Task-number: QTBUG-38771
Change-Id: If01c44c2084b95e487bc9146576ca180ed8044da
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
295786d946