Questions about select for sockets

[Ken Brown]

Some time ago I did a preliminary implementation of select for AF_UNIX sockets 
on the topic/af_unix branch.  I copied some things from the 
fhandler_socket_wsock case without really understanding them.  I'm taking a 
second look now, and would like to clean this up.  I'll start with just a few 
questions, but there may be more to follow.

1. What's the purpose of 'except_on_write'?  AFAICS, its only use is in 
set_bits, to indicate that the readfds entry should be set in case of a failed 
connect.  But shouldn't that entry always be set if there's a failed connect 
since a read won't block?

2. In that same place in set_bits where UNIX_FD_SET (me->fd, readfds) is called, 
shouldn't ready be incremented?

3. In fhandler_socket_wsock::select_write where except_on_write is set, 
except_ready is also set.  Why is that done there?  It seems to me that this is 
redundant, because the same setting of except_ready is done in 
fhandler_socket_wsock::select_except.

4. Why do we set

   except_ready = saw_shutdown_write () || saw_shutdown_read () ?

I can't find any documentation that says that a shutdown should be considered an 
exceptional condition.  On the other hand, POSIX does say, "Other circumstances 
under which a socket may be considered to have an exceptional condition pending 
are protocol-specific and implementation-defined."  So maybe there's some 
Cygwin-specific reason for doing this?

Thanks.

Ken