Jan Kiszka authored May 13, 2016 and Philippe Gerum committed Dec 27, 2016

Only if a signal was actually delivered to a thread that was blocked on
sleep, [clock_]nanosleep or select, those calls should return -EINTR.
Otherwise, they should resume with the timeout, accordingly adjusted in
case of relative timeout. So far we returned -EINTR immediately which
particularly disturbed the debugging of applications (SIGSTOP/CONT
terminated those syscalls).

This approach reuses the Linux restart mechanism to find out if those
syscalls should be restarted or actually terminated after the signal
was handled: Linux sets current->restart_block.fn in case a termination
is required, unconditionally, thus also when the syscall did not return
ERESTART_RESTARTBLOCK. We also use the restart_block.nanosleep.expires
to transfer the remaining timeout to the restarted syscall.

We can't use the original restart mechanism of Linux because it directs
all ERESTART_RESTARTBLOCK through a special, Linux-only syscall. In our
case, we would have to migrate the caller in that context to primary in
order to resume the sleep, but this is not possible under Xenomai (we
need to migration from within the syscall hooks).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>