To make the program concrete, suppose we have "a.rkt" as
#lang racket/base
(define (epipe? x)
(and
(exn:fail:filesystem:errno? x)
(equal? '(32 . posix) (exn:fail:filesystem:errno-errno x))))
(define (epipe-handler x)
"ignore")
(with-handlers ([epipe? epipe-handler])
(let loop ()
(displayln "hi")
(loop)))
and then we run
racket a.rkt | more
and then we hit "q" to exit `more`.
In that case, we will get
error writing to stream port
system error: Broken pipe; errno=32
The problem is that `epipe-handler` returns a string that becomes the
result of the whole `with-handlers`. When Racket attempts to print that
string, it fails (outside the `with-handlers`, of course). In other
words, the exception reports the failure to print "ignore", not a
failure from `(displayln "hi")`.
In this little example, changing `epipe-handler` to
(define (epipe-handler x)
(void))
avoids the problem, because a void result isn't printed. Another solution is
(define (epipe-handler x)
(exit 0))
to immediately exit, instead of returning a value.
At Fri, 12 Sep 2014 21:57:30 +0200, Wolfgang Hukriede wrote:
> Hi Matthias, nice to meet you! Yes, this was what I was looking for.
>
> For the records:
>
> (define (epipe? x)
> (and
> (exn:fail:filesystem:errno? x)
> (equal? '(32 . posix) (exn:fail:filesystem:errno-errno x))))
>
> (define (epipe-handler x)
> "ignore")
>
> (with-handlers
> ((epipe? epipe-handler))
> transput...)
>
> Many thanks!
>
> Results are mixed though. Usage like
>
> $ mzscheme --no-init-file --load pipetest.scm | head
>
> mostly works okay, but depending on how and how much is written to
> standard out and on the parameters to "head" failures reproducibly
> occur: in some situations the signal is not caught, yielding a "system
> error: Broken pipe; errno=32" message (versions 6.1 and 5.93 on
> freebsd, and 4.1 on generic linux, 6.1 precompiled racket on
> archlinux), or sometimes the pipe even just hangs (6.1 on freebsd).
> These were the combinations I tried.
> (On 4.1 exn:fail:filesystem was used instead of non-existing
> exn:fail:filesystem:errno.)
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