On 30/10/2018 19:10, Matthew Flatt wrote:
> At Tue, 30 Oct 2018 17:54:46 +0100, "'Paulo Matos' via Racket Users" wrote:
>> On 30/10/2018 16:39, Matthew Flatt wrote:
>>> You need to close the output port after `gunzip-through-ports` finishes.
>> OK, that was a mistake of writing this smallish example. The problem
>> persists, if I write (close-output-port out) after (gunzip-through-ports
>> out):
>> ➜  zipping-through-ports wc -l foo1
>> 12987 foo1
>> ➜  zipping-through-ports wc -l foo2
>> 12987 foo2
>> ➜  zipping-through-ports wc -l foo3
>> 12987 foo3
>> ➜  zipping-through-ports wc -l foo.txt
>> 12988 foo.txt
>> I was expecting foo.txt to be the same as
>> $ cat foo1 foo2 foo3 > foo.txt
> Oh, I see. The gzip format lets `gunzip-through-ports` stop when it
> gets to the end of a compressed stream, so if you want to decompress
> multiple concatenated streams until an EOF:
>       (let loop ()
>         (unless (eof-object? (peek-byte cin))
>           (gunzip-through-ports cin out)
>           (loop)))

Yes! That makes sense. :)

This works:

#lang racket

(require file/gzip

(define paths '("foo1" "foo2" "foo3"))

;; compress
(printf "compressing~n")
(call-with-atomic-output-file "foo.gz"
  (lambda (op p)
    (for ([f (in-list paths)])
      (call-with-input-file f
        (lambda (i) (gzip-through-ports i op #false (current-seconds)))
        #:mode 'binary))))

;; decompress
(printf "decompressing~n")
(define-values (in out) (make-pipe))
   (call-with-input-file "foo.gz"
     (lambda (cin)
       (let loop ()
         (unless (eof-object? (peek-byte cin))
           (gunzip-through-ports cin out)
       (close-output-port out))
     #:mode 'binary))))
(call-with-atomic-output-file "foo.txt"
  (lambda (op p)
    (let loop ([l (read-line in)])
      (unless (eof-object? l)
        (display l op)
        (newline op)
        (loop (read-line in))))))

>> Still, I should point out that the reason I didn't close it originally
>> was because the documentation mentions that it's not necessary to close
>> make-pipe ports:
>> "Unlike some other kinds of ports, pipe ports do not need to be
>> explicitly closed to be reclaimed by garbage collection."
> It isn't necessary to close the pipe for the pipe to be GCed when it's
> no longer referenced. But if you want to get an EOF from the input end
> of the pipe, then the write end needs to be closed.

Thanks for the clarification.

Paulo Matos

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