kern: unix: raise POLLIN when the remote end has shutdown writes

[kevans]

Summary

Historically, select(2) and poll(2) will return POLLIN if the next
read(2) would return EOF to signal that the application should try.

Fix the new stream/seqpacket poll implementation to do the same to avoid
breaking applications that expect it, and select(2) which won't poll
for POLLRDHUP. select(2) arguably should poll for POLLRDHUP on the
read set as well, but poll(2) should also surface it with only
events=POLLIN.

Fixes: d15792780760e ("unix: new implementation of unix/stream [...]")

[kevans]

sys/kern/uipc_usrreq.c

1695 ↗

(On Diff #156284)

Looking at uipc_socket.c's sopoll_generic, I wonder if this should instead be this to be technically correct (not that I can find anything actually using POLLIGNEOF except for the implicit use in pipe stuff?)

[kib]

sys/kern/uipc_usrreq.c

1695 ↗

(On Diff #156284)

Gleb tried to get rid of POLLINIGNEOF. The bit is with us forever due to ABI reqs.

So most likely you are right.