[plt-scheme] writing to subprocess's stdin and then...

Hi Eli,

I'm glad that this topic came up, because I was playing with just
these issues last weekend.  I had been using process rather than
subprocess (so I had to make up some amusing string quoting to
pass in complicated, space-containing file names), and it mostly
worked, but I got the occasional hang, which I suspect was
because of the blocking/deadlock issues that you mentioned.

Anyway, I've just tried a variation on your version:

On Tue, 17 Jun 2008 04:49:10 -0400
Eli Barzilay <eli at barzilay.org> wrote:
> Here's the revised example -- with no use of threads.
> 
>  | #lang scheme
>  | 
>  | (require scheme/port)
>  | (define (with-input-from-subprocess exe thunk)
>  |   (define-values (p pout pin perr)
>  |     (subprocess #f #f (current-error-port)
>  |                 (find-executable-path exe)))
>  |   (close-output-port pin)
>  |   (parameterize ([current-input-port pout])
>  |     (begin0 (thunk)
>  |       (subprocess-wait p))))
>  | 
>  | (with-input-from-subprocess "du"
>  |   (lambda ()
>  |     (for ([line (in-lines)])
>  |       (printf ">> ~s\n" (regexp-split #rx"[/\t]" line)))))

Specifically, my version looks like:

(define (with-input-from-subprocess thunk exe . args)
  (define-values (p pout pin perr)
    (apply subprocess (append (list #f #f (current-error-port)
                                    (find-executable-path exe))
                              args)))
  (close-output-port pin)
  (parameterize ((current-input-port pout))
    (begin0 (thunk)
            (subprocess-wait p))))

; process individual tracks: do the real work

(define (get-flac-meta file)
  (make-immutable-hash
   (with-input-from-subprocess
    (lambda ()
      (let all ()
        (match (read-line)
          ((regexp #rx"(.*)=(.*)" (list _ tag val)) (cons (cons tag val) (all)))
          (_ '()))))
    "metaflac" "--export-tags-to=-" (path->string file))))

The problem is that this produces an error at run-time:
subprocess: expects type <file-stream-output-port> as 3rd
argument, given: #<output-port>; other arguments were: #f #f
#<path:/usr/local/bin/metaflac> "--export-tags-to=-" "some file
name..."

Should subprocess really be so picky?  I suppose that it has to
be, because the unix process wants a real file descriptor.  The
current-error-port in this case is that of the drscheme
environment.  Seems to work OK from mzscheme on the command line.

Cheers,

-- 
Andrew