[sword-devel] GPL restrictions (was Re: using a zText module)

[Chris Little]

On 08/12/2012 01:11 PM, Greg Hellings wrote:
> On Sun, Aug 12, 2012 at 2:55 PM, Chris Little <chrislit at crosswire.org> wrote:
>> I'm not sure I see the distinction. For GPLv2 software A to be used by or
>> incorporated in some other software B, GPLv2 must be compatible with
>> software B's license. The set of licenses that GPLv2 is compatible with
>> consists of exactly and only GPLv2. (This is distinct from the set of
>> licenses that are compatible with GPLv2, which includes GPLv2, BSD, and many
>> others.) GPLv2 and GPLv3 are mutually incompatible.
>
> I think you're understanding "compatible" more narrowly than it
> actually means. Here is the quote from the email exchange with the
> FSF, asking specifically about public domain:
>
>> The crucial words here are "or a GPL-compatible license".
>> Public domain is compatible with the GPL.
>> http://www.gnu.org/licenses/license-list.html#PublicDomain
>>
>> Therefore this is ok and the application code can be public domain,
>> while the library is GPL.
>
> You might not understand, but this is how the FSF explains it. An
> application may be non-GPL, so long as its license is compatible with
> the GPL. That is, the license has to be one which would permit the
> application to utilize GPL libraries. Any of the licenses on the
> compatability list are OK for the application.

I would guess that the FSF's respondent either misunderstood the 
question that was posed, answered for the reverse (incorporating 
PD-licensed software into GPL-licensed software), or simply didn't know 
what he was talking about. Given the final sentence, I would lean 
towards the last option. I'm not sure what "or a GPL-compatible license" 
quotes from, as that is not a phrase present within GPLv2.

The FSF's staff aren't the judicial branch or anything, so they don't 
get to re-interpret what was meant by the GPL after the fact. The GPL 
licenses have enumerated rights & requirements, those sets of rights & 
requirements are not identical for the two licenses, and both licenses 
prohibit any addition or removal of rights from the licensed work or its 
derivatives---hence they are inherently mutually incompatible licenses.

The page cited by FSF's respondent actually explicitly states that GPLv2 
is not GPLv3-compatible and GPLv3 is not GPLv2-compatible (unless it's 
the "version 2 or later" version of licensing). So, in addition to the 
links I posted previously, there's the FSF's explicit statement on GPL 
inter-version compatibility, which is the specific case in question.

Crucially, license compatibility is directional. PD and BSD are 
GPL-compatible, which means PD and BSD software may be incorporated into 
GPL software. GPL is not PD- or BSD-compatible, and that would be 
requisite to incorporation of GPL software into PD or BSD software.

It's permissible to release code that makes use of the SWORD API or 
other GPLv2 software available under another license _in addition to_ 
GPLv2, but that code must also be GPLv2-licensed and any binaries must 
be licensed GPLv2-only. (Section 2b and the paragraph following section 
2c of GPLv2 make this explicit.)

If it were permissible to license software that links against GPLv2 
software using any 'GPL-compatible' license, then one could trivially 
write a library that simply replicates the SWORD API and passes calls on 
to the SWORD library itself. Then one could have a BSD-, GPLv3-, LGPL-, 
or PD-licensed SWORD API via this intermediary, and by the same method, 
anyone could switch the license of any GPLv2 or v3 library to some other 
'GPL-compatible' license. I hope it's clear that that is not the case. 
The whole concept of license virality would fall apart under this 
assumption, and literally any piece of GPL software could be converted 
to public domain via this means. (Conversely, it is permissible to write 
such libraries that replicate BSD-, LGPL-, or PD-licensed APIs and 
license those replicating libraries as GPL, though obviously it makes 
more sense to simply re-license under the more restrictive GPL if that's 
your aim. We effectively re-license any time we link SWORD against 
BSD/MIT libraries.)

> A modification of SWORD itself (including of the bindings included
> with the library) would need GPLv2 licensing to be released. THAT is
> the restrictiveness of GPLv2. But as long as I'm just binding or
> linking against the library, I don't have to be limited to GPLv2.
> GPLv2 compatible is OK.

What you're describing is roughly LGPL. Actual LGPL doesn't have any 
restriction on GPL-compatibility of software employing the LGPL library, 
but that's the nearest thing to what you describe.

--Chris