POSIX: Keep original timeout from sleep call

The `dispatch_sema4_timedwait` would reset the `_timeout` to an absolute
time after the sleep was interrupted instead of when the sleep was
called. Interrupting the process while it was sleeping would result in
the new absolute timeout deadline being computed using the `timeout`
offset from the time of the interrupt. e.g. if the timeout is 10
seconds, it will be ten seconds from when the process was interrupted
because the absolute deadline was recomputed. Interrupting the process
repeatedly while sleeping would will make the process go back to sleep
instead of waking up when the original absolute deadline was reached.

`timeout` is a relative timeout offset. `nsec` and `_timeout` are
absolute times since the epoch specifying when the wait should stop.

Author: etcwilde

src/shims/lock.c

@@ -231,10 +231,10 @@ _dispatch_sema4_timedwait(_dispatch_sema4_t *sema, dispatch_time_t timeout)
 	struct timespec _timeout;
 	int ret;
 
+	uint64_t nsec = _dispatch_time_nanoseconds_since_epoch(timeout);
+	_timeout.tv_sec = (__typeof__(_timeout.tv_sec))(nsec / NSEC_PER_SEC);
+	_timeout.tv_nsec = (__typeof__(_timeout.tv_nsec))(nsec % NSEC_PER_SEC);
 	do {
-		uint64_t nsec = _dispatch_time_nanoseconds_since_epoch(timeout);
-		_timeout.tv_sec = (__typeof__(_timeout.tv_sec))(nsec / NSEC_PER_SEC);
-		_timeout.tv_nsec = (__typeof__(_timeout.tv_nsec))(nsec % NSEC_PER_SEC);
 		ret = sem_timedwait(sema, &_timeout);
 	} while (unlikely(ret == -1 && errno == EINTR));