improved 4.2BSD signal(2) library routine

[gwyn%brl-vld at sri-unix.UUCP]

From:      Doug Gwyn (VLD/VMB) <gwyn at brl-vld>

It is easy enough to patch the previously posted signal(2) emulation
to at least make sure that EINTR is only returned for system calls
that are supposed to be able to return this indication; I can send the
amended routine to anyone who wants it.  It is true that some reads
or writes that should not be interruptible (disk file i/o) may return
EINTR on rare occasion, although that should not bother stdio users.

The real botch in the new signal handling, as I see it, is that there
is no reliable way (short of snooping in one's u_ area on architectures
that allow that) to determine if one is in the middle of a system call
when the signal is fielded.  It appears that the new facilities were
designed with a specific model of how programs do i/o and use signals,
and that model does not fit every application.  There are many times
when I DO NOT WANT terminal i/o restarted after a signal, especially
after a keyboard-generated signal.  This is difficult to arrange when
terminal i/o gets automatically restarted by the system!

I can't be as enthusiastic over people's "courage" to try to improve
things when they don't take sufficient care in the new design.