As of early 2020, RT is under GPLv2. GPLv3 is incompatible with
GPLv2. So you might ask the question: Why and how is rt-liberation
under GPLv3?

It's because I emailed Best Practical (the people behind RT) and got
their excplicit permission to do so. Included below is the complete
email correspondence:

----------------------------------------------------------------------

From: "Jim Brandt via RT" <sales@bestpractical.com>
Subject: [bestpractical.com #225843] licensing question
To: yoni@rabkins.net
Date: Mon, 16 Mar 2020 08:52:08 -0400 (11 hours, 9 minutes, 18 seconds ago)
Reply-To: sales@bestpractical.com

On Tue, Mar 10, 2020 3:21:16 PM, yoni@rabkins.net wrote:
> 
> Hello,
> 
> I'm the author of rt-liberation, which is an interface to RT from
> GNU/Emacs. It is available at: http://www.nongnu.org/rtliber/
> 
> rt-liberation uses the REST API to communicate with RT.
> 
> rt-liberation is licensed under GPLv2 in order to be compatible with
> the license of RT.
> 
> However, I've been asked to add rt-liberation to Emacs proper (via the
> GNU ELPA). This is great news, since it will make using RT a built-in
> property of GNU/Emacs.
> 
> But GNU/Emacs is licensed under GPLv3, which is mutually incompatible
> with GPLv2. I would need to upgrade my work, rt-liberation, to GPLv3 in
> order to be accepted into Emacs.
> 
> My question is therefore: may I upgrade rt-liberation to GPLv3? Are you
> OK with a GPLv3-licensed work communicating with RT via the REST API?
> Another way of putting it: do you consider communication via the REST
> API to RT to be creating a derivative work under the terms of GPLv2.
> 
> Thank you for your consideration.

Hi Yoni,

I agree it would be really cool to have an RT interface built into
emacs, thanks for your work.

Regarding the license, if you wrote the code that interfaces with RT
only via the REST API, you can change the license for your code as you
see fit. Using the REST API doesn't require pulling in any of RT's
code and i don't think we would seek to release any of your code as
part of RT. There are many different systems, open source and
proprietary, that interface with RT via the REST API and each of those
systems retains its own license.

There are various projects that provide wrappers on the RT REST API to
make it easier to use from different languages (Python, Ruby, etc.)
and if you used an existing project or library to access the REST API,
you may need to consider the license for that project. But it looks
like you wrote the REST code yourself when you converted from using
the RT CLI, so it should be an issue.

Thanks and good luck,
Jim

----

From: "Jim Brandt via RT" <sales@bestpractical.com>
Subject: [bestpractical.com #225843] licensing question
To: yoni@rabkins.net
Date: Mon, 16 Mar 2020 13:55:49 -0400 (6 hours, 5 minutes, 44 seconds ago)
Reply-To: sales@bestpractical.com

> > But it looks
> > like you wrote the REST code yourself when you converted from using
> > the RT CLI, so it should be an issue.
> 
> Yes, I wrote all of it myself so that it is self-contained. I'm going to
> assume this is a typo and you meant to write "shouldn't".

Correct, that should have said "so it shouldn't be an issue". Sorry about that!

Thanks,
Jim