sigwait() cancellation point

[Jilles Tjoelker]

Our sigwait() implementation may not be POSIX-compliant as it returns
EINTR when it is interrupted by a caught signal. (Unfortunately I can
only find this in SUSv4 in the Rationale, B.2.3 Error Numbers,
Disallowing Return of the [EINTR] Error Code; the sigwait() page in XSH
does not list an [EINTR] error condition, but does not prohibit one
either like pthread_mutex_lock() and various others do.)

However, libthr's wrapper for sigwait() relies on EINTR. To keep
the possibility of changing this to be what POSIX intends, it would be
nice to remove this dependency.

One way is to include SIGCANCEL in the set of waited signals, but this
requires two additional syscalls to mask/unmask SIGCANCEL.

Another way is to use sigwaitinfo() in the wrapper, which is permitted
to return [EINTR]. If [EINTR] is removed from the kernel sigwait(),
looping on [EINTR] can then be implemented in the wrapper.

Calling pthread_exit from the SIGCANCEL handler is not possible as this
leaves the program in doubt about any signal consumed by sigwait().

-- 
Jilles Tjoelker