Is it possible to liberate source code through double-licensing changes?

Question

Let me describe the situation: there's an old piece of software (a game) with source code publically available. Source code was (officially, legally) released with following non-free license:

The source code to GAME NAME is copyright (c) YEAR-YEAR NAME and is provided as-is with no warranty for its suitability for use. You may not use this source code in full or in part for commercial purposes. Any use must include a clearly visible credit to NAME as the creators and owners, and reiteration of this license.

Since the initial release, several people/teams updated it, modified (substantially, e.g. by porting to SDL2) - but every fork eventually fizzled out and died (it's playable on modern Linux systems, but I haven't tried it myself yet).

I am thinking about picking up the fork and maintaining it, but I don't feel like working on non-free software (it would be pointless, IMHO). I was planning to dual-license all of my future changes under original license and GPLv3, and clearly indicate this in Git repo - e.g. by marking every change that modifies code (so every change sans formatting) as GPL-compatible. This way I was planning to gradually liberate the code, so eventually, when all lines would be covered by GPL, I could release it with GPLv3 being primary license. Maybe some older maintainers would agree to re-license their changes on GPL as well (that would speed up the process).

Is it legally defensible? Has there been any precedence for liberating source code this way? Is there some better license than GPLv3 to handle such a transition? (LGPL maybe?)

[edit] Clarifying question: what if the gradual rewrite was not line-by-line (commit-by-commit), but instead module-by-module (maybe even using completely different language linking to the original source code compiled as shared object). LGPL seems to be ideally suited for this purpose.

Answer 1

The most obvious problem with this plan is that by including GPL-licensed code alongside code under a GPL-incompatible license, you might make it impossible for anyone else to distribute the software. As far as my legal knowledge extend, downstream recipients may only distribute a work that includes your GPL-licensed code if they distribute the overall work as a whole under the terms of the GPL (per GPLv3 sect. 5(c)). You could avoid this problem by employing a GPL exception. Supposing your game was called, say, Foos Versus Bar, you might draft an exception like

Additional permission under GNU GPL version 3 section 7

If you modify this Program, or any covered work, by linking or combining it with Foos Versus Bar (or a modified version of that program), containing parts covered by the terms of Foos Versus Bar, the licensors of this Program grant you additional permission to convey the resulting work. Corresponding Source for a non-source form of such a combination shall include the source code for the parts of Foos Versus Bar used as well as that of the covered work.

In this way, downstream distributors have permission to distribute the work notwithstanding the inability to distribute the whole work under GPL terms.

With that problem out of the way, there still remains these problems:

• It isn't perfectly clear that this license allows modification at all (the only permission granted is a vague "use" which doesn't clearly correspond to any particular set of copyright rights). It clearly indicates what you may not do, and requirements for use, but does not make explicit what use is allowed.

• Considering that developers doing internal modifications to files have definitely seen the original code, their code is very likely a derivative of the original code. Per the above issue, this license doesn't clearly license the right to prepare derivatives or, if it is allowed, under what terms they may be prepared. This means the derivative modifications may be disallowed entirety or may have to be made under the original non-commercial terms. If either of those cases applied, then even an all-at-once wholesale rewrite of the entire program could not be licensed under the GPL, unless you applied clean-room practices.

Answer 2

No, your tactic is not legally defensible.

First of all, the GPL license requires that all code of the project/game is available under a GPL-compatible open-source license and the original game code isn't. This means that you can't use the GPL or a similar copyleft license.

Secondly, even if you would use a compatible license, you will run into Thesius' paradox. If you replace the existing code, it is unclear what would happen to the copyright of the original authors. The current safe assumption is that they can still claim copyright and must agree to a license change.