Re: upgrading GIT from version 1.7.0.4 to 1.7.6.1.

[Jeff King <peff@peff.net>]

On Mon, Sep 19, 2011 at 02:49:53PM +0200, Andreas Ericsson wrote:

> On 09/19/2011 02:30 PM, Mikaël wrote:
> > 
> > 1- Is it possible to have two GIT installations pointing on the same 
> > repositories?
> > 
> 
> The core structure and layout of a git repository hasn't changed
> since may 2005, so it should work just fine provided you use any
> git version that actually has a version number.
> 
> We've upgraded in a hodge-podge fashion at $dayjob. One of our
> servers is still running 1.4.something. We've never even come close
> to anything resembling a problem. It's actually a bit weird, since
> we started using git in late 2005 and it's so far been the most
> reliable and backwards-compatible piece of software we have in the
> company pretty much ever since.

This is not completely true. Any two versions should be able to
interoperate over the network using the git:// protocol. However, the
disk format has changed slightly over time.

Since v1.6.0 (August 2008), git defaults to using delta base offsets in
packfiles and version 2 of the pack index format. These features are not
understood by versions before v1.5.2 (May 2007) or v1.4.4.5 (July 2008).
A very old git accessing those repositories directly on disk (or by a
dumb protocol like rsync or non-smart http) would have problems[1].

So it has happened[2], but it's important to note that:

  1. We waited a year to flip the default for the code supporting it to
     become more pervasive.

  2. The switch happened on a major version boundary (1.5 -> 1.6),
     was already supported by versions in the prior major series (1.5),
     and we released a maintenance version for the series before that
     (1.4.4.x).

  3. The change, along with the affected versions, was mentioned
     prominently in the 1.6.0 release notes.

So no, I wouldn't expect any disk format issues moving from v1.7.0.4 to
v1.7.6.1. But it never hurts to read the release notes if you're unsure.

-Peff

[1] I do very occasionally run into this while bisecting some ancient
    code on a modern repository.

[2] I suspect a similar thing happened with turning on packed refs
    (around the v1.4.4 era?), but I didn't dig around for details.