Instead of initializing the whole QRandomGenerator::system(), which in
turn gets to checking CPUID and whether the HWRNG works, trust the
operating system functions for an initial value. On Linux, we'll use 4
or 8 of the 16 bytes of random data that the kernel populates for us on
AT_RANDOM.
This should make Qt applications not stall on an early system launch
without an RNG daemon, if compiled without getentropy() support. And
avoids silly mistakes causing recursion, like QTBUG-78007 found.
Additionally, qt_random_initial_value() will most likely not throw
either. It's marked noexcept, even though SystemGenerator::fillBuffer
could throw on Linux, if the current thread is canceled, but Linux also
has AT_RANDOM. That leaves the other Unix systems without getentropy()
(read: macOS, since the BSDs have getentropy()).
Fixes: QTBUG-69555
Change-Id: Id2983978ad544ff79911fffd1671fca1a9f9044d
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
928ce402a4