QHash & QRandomGenerator: cooperate to provide a simpler initial seed

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>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/corelib/global/qrandom.h b/src/corelib/global/qrandom.h
index d6b8bd742e..c59ae4d42d 100644
--- a/src/corelib/global/qrandom.h
+++ b/src/corelib/global/qrandom.h
@@ -217,6 +217,7 @@ protected:
 private:
     Q_CORE_EXPORT quint64 _fillRange(void *buffer, qptrdiff count);
 
+    friend quintptr qt_initial_random_value() noexcept;
     friend class QRandomGenerator64;
     struct SystemGenerator;
     struct SystemAndGlobalGenerators;