Re: [PATCH 09/17] midx: do not require packs to be sorted in lexicographic order

From: Taylor Blau <me@ttaylorr.com>
Date: Dec 9, 2025, 02:07 UTC
Message-ID: <aTeEZX4036A9YecX@nand.local>
In-Reply-To: <aTcYbRt-aUIcym77@pks.im>

On Mon, Dec 08, 2025 at 07:26:53PM +0100, Patrick Steinhardt wrote:

Show 10 quoted lines

> On Sat, Dec 06, 2025 at 03:31:25PM -0500, Taylor Blau wrote:
> > Note that this produces MIDXs which may be incompatible with earlier
> > versions of Git that have stricter requirements on the layout of packs
> > within a MIDX. This patch does *not* modify the version number of the
> > MIDX format, since existing versions of Git already know to gracefully
> > ignore a MIDX with packs that appear out-of-order.
>
> Interesting. Did you verify how other implementations of Git behave if
> we start to relax this requirement? It seems like a somewhat dangerous
> assumption to me that this will just continue to work.

That's a great point. It looks like current libgit2 assumes[1] that the list is sorted and complains loudly if it is not. Presumably other implementations behave similarly.

I think that is a compelling enough argument to swing us towards bumping the version number to avoid compatibility issues.

> Also, is there a reason why you prefer this over bumping the version
> number?

I was trying to avoid having all existing Git clients be unable to read v2 MIDXs for such a seemingly minor change, but I think the above compels us to.

> I assume that it cannot happen that we append to the array of MIDX'd
> packs after we have sorted. It would mean that the MIDX somehow changed
> its representation or was amended to, which isn't possible.

That's right.

Thanks, Taylor

[1]: https://github.com/libgit2/libgit2/blob/v1.9.2/src/libgit2/midx.c#L75-L76