Re: Git commit hash clash prevention

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Oct 2, 2008, 10:07 UTC
Message-ID: <alpine.DEB.1.00.0810021202420.22125@pacific.mpi-cbg.de.mpi-cbg.de>
In-Reply-To: <20081002085358.GA5342@lapse.rw.madduck.net>

Hi,

On Thu, 2 Oct 2008, martin f krafft wrote:

Show 7 quoted lines

> the other day during a workshop on Git, one of the attendants asked 
> about the scenario when two developers, Jane and David, both working on 
> the same project, both create a commit and the two just so happen to 
> have the same SHA-1. I realise that the likelihood of this happening is 
> about as high as the chance of <insert witty joke here>, but it *is* 
> possible, isn't it? Even though this is thus somewhat academic, I am 
> still very curious about it.

It _is_ academic. Did you already discuss the chance that your wife gives birth to a mouse? I haven't done the maths yet, but I am pretty certain that this would be more likely than an unintended SHA-1 collision.

> What happens when David now pulls from Jane? How does Git deal with 
> this?

Basically, the commit that David has will not be overwritten. So every commit referring to Jane's commit would point to David's in his repository.

But the more likely case (well, as likely goes) would be that either Jane's or David's object is actually a blob. And Git would complain about a type mismatch then.

Ciao, Dscho