On Tue, Nov 6, 2012 at 2:09 PM, Josef Wolf <j...@raven.inka.de> wrote:
> On Tue, Nov 06, 2012 at 09:41:29PM +0000, John McKown wrote:
> > Josef Wolf <jw <at> raven.inka.de> writes:
> > > Just for curiosity: what would happen if such a collision would occur 
> > > within
> > > one repository?
> > In a sense, this cannot happen.
> In the scenario you described, contents of this version of file "b" are lost
> and replaced by the contents of file "a". So file "b" is broken.
> What happens when files "a" and "b" are added into different repositories?
> File "a" is added to repos "A", and file "b" is added to repos "B". Now it
> depends from which repository you fetch the collided blob first. If you fetch
> it from "A", file "b" will be broken. If you fetch first from "B", your "a"
> will be broken.
> It becomes even more interesting, if some commit or tree object would have
> the same SHA1 as some other object. I guess, in such a case the repository
> would be completely hosed?

When exchanging objects over the network, Git compares byte-for-byte
any object that one side sent that the other side already has, and
complains loudly when there is a collision detected. This only works
if the sender includes the "wrong" content for the named object. Git
also does assume the SHA-1 is unique and that it is not always
necessary to transmit the object. In these cases you would not be able
to detect the collision, because there isn't one. Your repository
would simply be using the wrong content for a file. Presumably one
would notice your build doesn't work anymore and investigate why.
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