[usocket-devel] SBCL data flush problem

[Robert Brown]

I use SBCL on Linux and have noticed a problem when writing
lots of data to socket streams created with usocket.

In file backend/sbcl.lisp, the function socket-connect creates a
stream as follows:

(setf (socket-stream usocket)
      (sb-bsd-sockets:socket-make-stream socket
                                                             :input t
                                                             :output t
                                                             :buffering :full

:element-type element-type))

If you look at the code for sb-bsd-sockets:socket-make-stream in
sbcl/contrib/sb-bsd-sockets/sockets.lisp, you'll see that it specifies
a default value of T for the keyword argument serve-events:

 (defmethod socket-make-stream ((socket socket)
                              &key input output
                              (element-type 'character)
                              (buffering :full)
                              (external-format :default)
                              timeout
                              auto-close
                              (serve-events t))

This means that SBCL streams created by usocket have a true
serve-events property.  When writing large amounts of data to several
streams, the kernel will eventually stop accepting data from SBCL.
When this happens, SBCL either waits for I/O to be possible on
the file descriptor it's writing to or queues the data to be flushed later.
Because usocket streams specify serve-events as true, SBCL
always queues.  Instead, it should wait for I/O to be available and
write the remaining data to the socket.  That's what serve-events
equal to NIL gets you.

Usocket should add

  :serve-events NIL

to the call to socket-make-stream.

bob