Strange fstatat / stat behavour on directories causing tar "file changed as we read it" error

[Eric Blake]

On 01/12/2011 03:47 AM, Corinna Vinschen wrote:
> 
> On second thought, let's take a step back.
> 
> Actually, directories can change all the time.  Why on earth is tar
> checking the st_size member of a directory at all?  That's a bug IMO.
> No application should do that.  I understand that a change in the inode
> number points to the fact that the directory has been replaced
> underfoot, but why should tar be concerned that a directory has changed
> its size while it's reading files from it?  I mean, even during a tar
> backup, there's no reason to expect that files are *not* added or
> deleted to a directory by other applications, and these actions may
> naturally change the size of the directory.

When tar is trying to capture the entire contents of the directory, any
modification to that directory (including adding a file to the directory
which changes the st_size of the directory) means that tar missed
something, so tar emits a warning to tell you about that fact.

> 
> Having said that, I don't think it's correct to change Cygwin here.
> It's just a bug in tar.

I'm not sure we've established that point yet, but I'll raise the
question upstream.  But, assuming tar is doing the right thing...

>  The fact that the directory size changes even
> if the content hasn't changed is just a side effect of the OS MO.  It
> doesn't matter if the directory has actually changed or not.  It's not
> in the hand of a single application.

then cygwin's behavior DOES matter to tar - no other system changes the
st_size of a directory without ALSO adding or deleting files within that
directory and also updating the st_mtim of the directory; at which point
tar is entirely within its rights to complain (the directory changed
while trying to read it, so the tar file may be incomplete).  Locking
st_size at 0 avoids the issue entirely, whether or not upstream tar
agrees that checking changes in st_mtim but ignoring changes in st_size
would be a more appropriate behavior.

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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