From: Jeff King <peff@peff.net>
Date: Tue, 23 Sep 2025 17:48:25 GMT
Subject: Re: gitoxide-compatible licensing of Git's Rust code, was Re: [PATCH 6/7] xdiff: conditionally use Rust's implementation of xxhash
Message-ID: <20250923174825.GB1136654@coredump.intra.peff.net>
In-Reply-To: <9818dc92-3569-3e6f-0252-245c2bf0bf84@gmx.de>

On Tue, Sep 23, 2025 at 11:57:18AM +0200, Johannes Schindelin wrote:

> It would not even take more than something as simple as GPLv2+exception.
> We do have prior art for that: The Git project itself suggests in its very
> own `COPYING` file to use the following license in new files:
> 
>         This file is licensed under the GPL v2, or a later version
>         at the discretion of Linus.
> 
> Note the exception? For new Rust code (and of course excluding code that
> has been ported verbatim from GPLv2-licensed code), GPL v2 could be used
> with an exception along these lines: This file is licensed under the GPL
> v2, with the exception that it can be freely used in the Gitoxide project.

I think this "and of course" parenthetical might be a sticking point.
Obviously taking the code verbatim and re-licensing it is not allowed.
But I think even reading the C code and then writing substantially
similar Rust code may be legally questionable. The Rust code under the
more permissive license has to either be clean-room, or have permission
for re-licensing from the original authors (which is getting to be all
but impossible over time as code ends up being touched by many people).

I think this is the same issue that libgit2 ran into. If it were just a
matter of porting over and re-writing new features, more of it would
have been done. But for code to come under the new license it can't just
be a port, but has to be an independent work.

So I wonder if this just creates the same awkward silo between Git's
Rust code and its C code (that we already have between Git and libgit2).

> I am not a lawyer (which everybody but laywers are nowadays required to
> say), therefore this likely needs some tweaking.

Me either. I do like the goal you're trying to accomplish, but I worry
that it will end up causing headaches down the line. Even if we, the
developers, are a bit permissive about what constitutes "porting" and
don't require a clean-room implementation, this kind of thing scares off
the legal teams that approve using the Rust modules in bigger projects.
IIRC Microsoft put in a big effort into vetting libgit2's provenance
before agreeing to use it in Visual Studio.

-Peff