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

[Eric Blake <eblake@redhat.com>]

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----|
> 
> 
> ** Patch added: "macOS-lseek.patch"
>     https://bugs.launchpad.net/qemu/+bug/1776920/+attachment/5186138/+files/macOS-lseek.patch

While a developer can chase a URL, our CI tools can't.  Can you please 
also send that patch directly to qemu-devel@nongnu.org, so that it gets 
the same level of review as other patches?

But I am very reluctant to take your patch. MacOS is flat-out buggy and 
in violation of the specification of lseek(SEEK_DATA), so making all 
applications work around their bug is not going to scale as well as 
having them fix their bug in the first place.

Here's the proposed POSIX specification for what lseek(SEEK_DATA) is 
supposed to do:
http://austingroupbugs.net/view.php?id=415#c862

That text has been present for 7 years now - so anyone implementing 
SEEK_DATA should really be paying attention to interoperability with 
that text.

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