Re: [Bug-tar] bug in 'file has changed as we read it'

[Eric Blake]

On 09/09/2010 01:27 AM, Michal Svoboda wrote:
> When a hardlink to a file is created while tar is processing it, tar
> will bail out with 'file has changed as we read it' even if the file did
> not in fact change. The attached shell script has a pretty good chance
> of demonstrating the bug.
>
> I think the problem is that creating hardlinks updates ctime on the
> inode which fools tar into believing that the file has changed. But that
> new hardlink does not go into the archive so for all purposes it should
> be outside of the what-tar-cares-about domain.

_Any_ update to ctime means that at least file metadata has changed. 
Adding a hard link only changed the link count, but other changes 
include modifying timestamps, ownerships, permissions, or file contents. 
 It would require tar to stat() a file both before and after the ctime 
modification, and comparing those two stat structures, to rule which 
changes might be safe to ignore; it is much simpler and less 
memory-intensive to base the error solely off of whether ctime changed 
than off of whether the stat contents changed.

--
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org