[racket-dev] BUG: busy-waiting

Can you provide an example?

Here's what I tried, but it blocks without busy-waiting:

----------------------------------------
#lang racket
(require file/gunzip file/gzip)

(define dest (open-output-bytes))
(deflate (open-input-bytes #"hello") dest)

(define bstr (get-output-bytes dest))

(define-values (i o) (make-pipe))

;; Omit the last byte:
(write-bytes bstr o 0 (- (bytes-length bstr) 1))

'inflating...
(inflate i (open-output-bytes))
----------------------------------------


At Wed, 3 Sep 2014 21:02:06 -0400, Marc Burns wrote:
> The offending function in my case seems to be peek-bytes-avail!
> 
> The busy wait is entered on line 284 of collects/file/gunzip.rkt. Here's
> what happens when I add some tracing to gunzip.rkt and then try to
> inflate a Racket pipe:
> 
> ...
> +           (displayln "before read-bytes!")
>             (read-bytes! buffer input-port 0 (max 0 (- buf-max MAX-LOOKAHEAD)))
> +           (displayln "after read-bytes!")
>             ;; Even though we won't actually use bytes that we "unwind",
>             ;; setting `buf-pos' to the number of unwound bytes lets us
>             ;; keep track of how much to not actually read at the end.
>             (set! buf-pos (min MAX-LOOKAHEAD buf-max)))
>           ;; Peek (not read) some available bytes:
> +         (displayln "before peek-bytes-avail!")
>           (let ([got (peek-bytes-avail! buffer buf-pos #f input-port buf-pos 
> BUFFER-SIZE)])
> +           (displayln "after peek-bytes-avail!")
> ...
> +  (trace READBITS)
> 
> Output:
> 
> ...
> >(READBITS 9)
> <#<void>
> >(READBITS 9)
> before read-bytes!
> after read-bytes!
> before peek-bytes-avail!
> 
> That's it. Racket is busy-waiting in peek-bytes-avail!
> 
> On Wed, Sep 03, 2014 at 10:52:18PM +0200, Jan Dvořák wrote:
> > Hello,
> > 
> > I am hitting a rather uncomfortable bug that causes runtime to start
> > internal busy-waiting at around:
> > 
> > #0  scheme_block_until(_f=<syncing_ready>,
> > fdf=<scheme_syncing_needs_wakeup>)
> >    at ../src/thread.c:5199
> > #1  do_sync (name="sync", with_break=0, with_timeout=0)
> >    at ../src/thread.c:7109
> > 
> > It goes through ../src/thread.c:5190, which Matthew mentions in one of his
> > recent patches.
> > 
> > The code that manages to trigger this have been written under NDA and cannot
> > be published. I have not yet managed to reproduce the issue separately, but
> > it seems that this might not be the only instance:
> > 
> >    (09:28:36 PM) m4burns: Mordae: I'm having a similar problem somewhere in
> >                           `inflate`. Haven't applied a debugger yet, but
> >                           there's certainly no busy waiting in the script.
> > 
> > I am stuck and would like to ask for your help.
> > 
> > Best regards,
> > Jan Dvorak
> > 
> > 
> > 
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> >  http://lists.racket-lang.org/dev
> > 
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