Re: [eigen] MPL2 is really compatible with GPL/LGPL

[Daniel Berlin <dannyb@xxxxxxxxxx>]

On Thu, Jun 28, 2012 at 5:05 PM, Christoph Hertzberg
<chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> On 28.06.2012 22:44, Benoit Jacob wrote:
>>
>> 2012/6/28 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>>>
>>> Hi,
>>>
>>> I had a look too, and the FAQ really confuses me, especially the
>>> requirements 2 and 3 of this entry
>>> http://www.mozilla.org/MPL/2.0/FAQ.html#mpl-and-lgpl:
>>>
>>> 2 - The Larger Work must be "a combination of Covered Software with a
>>> work governed by one or more Secondary Licenses." So you can't just
>>> say "I really prefer (L)GPL" - you must have a need to combine with
>>> another, existing GPL work. (This is different from a traditional
>>> dual-license, which does not require you to combine, and instead
>>> allows you to simply say "I've decided to be GPL-only.")
>>>
>>> 3 - You must "additionally distribute" under (L)GPL. In other words,
>>> you must make the MPL-licensed source code available to your
>>> recipients under both MPL and (L)GPL. Someone downstream from your
>>> recipients can then take under (L)GPL-only or MPL-only. This is
>>> different from a traditional dual-license, which never requires
>>> publication under both licenses, and so always gives you the option of
>>> releasing incompatibly-licensed code.
>>>
>>> Does that means if someone combines MPL2 code and LGPL code to build
>>> an app, then the app can be either MPL2 or LGPL?? This does not make
>>> sense to me.
>>
>>
>> No. That's the whole point here: the MPL2 is trying to strike a
>> balance between GPL compatibility, and fixing the loophole whereby
>> people can take MPL code, improve it, and only release the
>> improvements under GPL-only. This is a compromise between two really
>> hard-to-reconcile opposite things, which is why it's complicated.
>>
>> So here's how it works, in my understanding. If someone combines
>> MPL2-licensed Eigen with their (L)GPL code, then they MUST offer the
>> result under MPL, but they are also allowed to dual-license it with
>> one of the Secondary Licenses. So for example, they can dual-license
>> MPL/GPL. A third-party can then take it and drop either license, but
>> they can't directly do it themselves. So the loophole still exists (if
>> they can find a complacent/fake "third-party" to drop the MPL license
>> for them), but it's a lot more inconvenient/dangerous to exploit.
>
>
> Wouldn't that logic also allow you to "combine" current (L)GPL Eigen with
> some MPL2 code and wait for some "complacent/fake 'third-party'" to
> redistribute that "combination" under MPL only?
Well, no, for starters, the LGPLv3 section Eigen ostensibly falls
under (section 3) only applies to the license of the produced *object
code*, not the original source code.
Besides that, in the normal case, LGPLv3 requires the LGPLv3 portion
stay LGPLv3 (4a: "Give prominent notice with each copy of the Combined
Work that the Library is used in it and that the Library and its use
are covered by this License.")