Accept defeat when select(2)ing without a monotonic clock

We prefer to use the monotonic clock because it's never affected by time
jumps (such as the user changing the date, or the system adjusting for
any other reasons, including automatic leap seconds). But if a system
doesn't have a monotonic clock, we simply accept the regular, real time
clock and hope it doesn't jump.

This is better than the current code that never restarts a call. The
side-effect is that a 30-second select may become a 3630-second select
if someone sets the clock back one hour.

Task-number: QTBUG-22301
Change-Id: Ia5a3bb453cd475f45b03637e2549165589fd2524
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/corelib/kernel/qcore_unix.cpp

@@ -63,12 +63,8 @@ QT_BEGIN_NAMESPACE
 static inline bool time_update(struct timespec *tv, const struct timespec &start,
                                const struct timespec &timeout)
 {
-    if (!QElapsedTimer::isMonotonic()) {
+    // clock source is (hopefully) monotonic, so we can recalculate how much timeout is left;
-        // we cannot recalculate the timeout without a monotonic clock as the time may have changed
+    // if it isn't monotonic, we'll simply hope that it hasn't jumped, because we have no alternative
-        return false;
-    }
-
-    // clock source is monotonic, so we can recalculate how much timeout is left
     struct timespec now = qt_gettime();
     *tv = timeout + start - now;
     return tv->tv_sec >= 0;