On Mon, 2010-02-08 at 22:31 -0600, Peter Keller wrote:

Hello,

I discovered in my boundary testing of with-open-socket that it sort of has a funny behavior I didn't expect.

Basically, I had a top level code like this in a blocking i/o tcp ipv4 client talking to a server:

(with-open-socket (socket :stuff :things)

;; set up i/o handlers, each wrap their reads and writes around ;; handler-case. A disconnector on the sockets removes the io handlers, but ;; doesn't close the socket since with-open-socket should do it.

(handler-case ;; run forever until empty (meaning we disconnected from the server), ;; then return. (event-dispatch *base*)

(hangup () (format t "Hangup while writing to server~%"))

(end-of-file () (format t "End of file from server~%"))))

Now, what happened was, under certain conditions where there is inflight data on the sockets, a registered handler could get a signaled condition, specifically hangup, that tells me the server went away on a write. This is expected. I disconnect the socket which removes the i/o handlers, which causes event-dispatch to return since there are no registered handlers.... and then bam! The with-open-stream performs a (finish-output) and (close) on the socket and I get _another_ hangup condition!

So I had to rewrite the code like this moving the handler-case up one scope:

(handler-case (with-open-socket (socket :things :stuff) ;; set up i/o handlers, each wrap their reads and writes around ;; handler-case. A disconnector on the sockets removes the io handlers, but ;; doesn't close the socket since with-open-socket should do it. (event-dispatch *base*))

(hangup () (format t "Hangup while writing to server~%"))

(end-of-file () (format t "End of file from server~%"))))

Is this a to be expected scenario in condition handling with respect to with-open-socket? Is this idiomatic code?

This is to be expected. This is what happens: one of the handlers signals a HANGUP(which is Posix EPIPE), the handler-case handles it and returns. At his point, since the code wrapped by with-open-socket returns, w-o-s closes the socket with :abort NIL, which tries again to flush the output buffer(:abort T would just close the file descriptor), which causes another HANGUP to be signaled.

Should the disconnector function close the socket even though with-open-socket also closes it? I'm not sure of the right thing to do here.

My opinion is that it's not a good idea to mix with-open-socket and the event loop because w-o-s is thought for a synchronous data flow. The only exception to that is if you use w-o-s to open the server socket. With active sockets, it's better to close them from within the event loop.