[QGIS-Developer] Crashes in qgsogrfeatureiterator.cpp:52 on master

[Nyall Dawson]

Hi all (specifically bugfixing devs)

While doing a bunch of mapping tasks yesterday on master (i.e. being a
QGIS user, not a developer!) I hit a bunch of frequent crashes on
Windows. Nathan's awesome crash report tool identified the issue as
occurring at qgsogrfeatureiterator.cpp:52.

Looking into that some more it seems that the previous line is failing
to acquire the connection to the datasource. I can't see anything
which would obviously explain this (failing a bug in Qt's
QSemaphore::tryAcquire e.g. possibly exposed by 49de489c).

So this is just a heads up for other devs/testers to be on the lookout
for this and to try and identify any common triggers to encountering
it. I suspect this bug may be the cause of the recent reports of
fragile master builds encountered by testers.

Nyall