Rewriting the FIFO code

[Ken Brown]

This is a followup to http://www.cygwin.org/ml/cygwin/2018-12/msg00104.html.

On 12/11/2018 2:40 PM, Corinna Vinschen wrote
 > Perhaps the unfinished, new AF_UNIX
 > sockets code can be reused or partially duplicated to implement FIFOs.

Here's my first idea.  Associated to each FIFO is an abstract listener socket, 
bound to the FIFO path name preceded by NUL.  This is created as soon as there's 
an attempt to open the FIFO.  It acts as a communication hub for all reads from 
or writes to the FIFO.  Each call to fhandler_fifo::open() creates a client 
socket that connects to the hub.  The client is designated as a reader, writer, 
or duplexer, depending on how it was opened.

The hub is created by a process that then monitors all the clients (via 
select()) until one is ready to read or write, as appropriate.  The process runs 
as long as there are are clients connected to the hub.

There are obviously many details to be worked out, but I hope the general idea 
is clear.  Does this seem like a reasonable approach?  Or is it unnecessarily 
complicated?

Ken