Re: [PATCH] Add 'guix hash'.

[Ludovic Courtès]

Nikita Karetnikov <nik...@karetnikov.org> skribis:

>> +         (if (file-exists? file)
>> +             (format #t "~a~%"
>> +                     (call-with-input-file file
>> +                       (compose fmt sha256 get-bytevector-all)))
>
> Oh, Mark warned me about 'file-exists?' some time ago.
>
> How can I do the same but avoid a race condition?

Ah, TOCTTOU, right.

You could do:

  (let ((hash (catch 'system-error
                (lambda ()
                  (call-with-input-file ...))
                (lambda args
                  (leave (_ "error ..." (strerror (system-error-errno 
args))))))))
    ...)

Ludo’.