Re: [Qemu-devel] [Bug 1776920] Re: qemu-img convert on Mac OSX creates corrupt images

[Eric Blake]

[redirecting to gnulib]

On 09/07/2018 01:04 PM, Yan-Jie Wang wrote:
> I have done some experiments and find out that
> the behavior of lseek with whence set to SEEK_DATA is different from the 
> behavior of Linux's lseek.
>
> If the supplied offset is in the middle of a data region, it returns the
> start of the next data region.  There may be many data regions in a big
> file even though it has no hole.
>
> return value of lseek with whence set to SEEK_DATA:
>
> |--(offset)--Data----|(return value)----Data----|
> |--(offset)--Data----|----Hole----|(return value)----Data----|

Ouch. That's a nasty bug in MacOS, and gnulib should be taught to work 
around broken lseek(SEEK_DATA) (qemu doesn't use gnulib, but this bug is 
nasty enough that I'm worried about what other gnulib clients may also 
encounter the problem).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org