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I'm thinking about releasing a plugin under the AGPL license.

Would this force people to make public the source code of other unrelated parts of their WordPress installations? (I've tried to read the AGPL license but I'm not sure.)

For example, if they install my AGPL plugin, would they have to make available the source code for any other plugins and themes they've installed? (Which are probably licensed under the GPL.)

Would it have any legal effects if I publicly stated that I, the copyright holder, don't consider other plugins and themes and WordPress being part of a combined work? (If other pugins etcetera were not part of a combined work, other people would not have to publish source code for their other plugins etcetera.)

(PS. Since most people aren't lawyers, I'd take any advice with a grain of salt :-) )

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Since we are not lawyers, as you have already noted, this is not the right place to get an answer based on facts and expertise. –  toscho Jun 15 '12 at 20:56
    
@toscho There are some other questions related to licensing, and even a <license> tag — why close this question but allow the other ones? You could search for the [license] tag? –  KajMagnus Jun 15 '12 at 21:06
    
(I didn't suggest that you close any other licensing question — I think they're interesting and useful. On SO they allow licensing related questions by the way) –  KajMagnus Jun 15 '12 at 21:13
    
We had a very similar question already – Am I allowed to license my Wordpress theme under the aGPL – which should answer at least one of the three questions you asked here. I don’t see much room for additional information. You may narrow down the scope of your question and flag it for moderator attention to reopen it. I’ll leave this then for another mod to decide. –  toscho Jun 15 '12 at 21:17
    
@toscho Okay, thanks for the feedback. I removed the very general question about any other drawbacks with using the AGPL. Now I think there are no similarities between my question and the one you linked, so I'll flag the question for attention ... –  KajMagnus Jun 15 '12 at 21:29
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2 Answers

I did cast a close-vote, since this question is a duplicate of the indicated question. (A Theme and a Plugin are functionally interchangeable as extensions of Wordpress; therefore, any licensing implications of one would apply equally to the other.) That said, I will offer some thoughts.

Caveats:

  1. The official WordPress Foundation stance is that PHP code in WordPress extensions is inherently derivative, and therefore must be distributed under GPL.
  2. I do not personally agree with the official stance, and have documented why I do not believe the official stance has any legal precedent.
  3. But I will not debate the official policy, or the matter of license derivation. While I do not believe licensing debates bear anything beneficial within the community, I state my bias, merely as a matter of full disclosure.
  4. I choose to release all my WordPress-related code under GPL, for many reasons. Whether or not you agree with the official stance, if you want to be welcomed within the WordPress community, you should likewise release your WordPress-related code under GPL.

Now, on to your questions:

Would this force people to make public the source code of other unrelated parts of their WordPress installations? (I've tried to read the AGPL license but I'm not sure.)

As far as I know, GPL and AGPL are written explicitly to coexist peacefully. One does not "infect" the other, and clear boundaries exist between works conveyed in a combination/compilation.

Let's just use some common-sense tests:

  1. If your work is a derivative of another work, for which you are not the copyright holder, and which requires that any derivative works be distributed under a specific license, do you have a legal right to distribute that work under a different license?
  2. Do you have a legal right to dictate the terms under which a work for which you are not the copyright holder is distributed?

I believe the answer to both of these questions is a resounding "no".

Let's set aside question #1, as it probably requires no further discussion.

Regarding question #2: let's say a user has both your Plugin and the Akismet Plugin installed in their WordPress installation. In what conceivable way is Akismet a derivative work of your Plugin?

Let's take it a step farther: in what conceivable way is WordPress itself a derivative work of your Plugin?

Would it have any legal effects if I publicly stated that I, the copyright holder, don't consider other plugins and themes and WordPress being part of a combined work?

No. The only thing that matters is the actual text of the license under which you distribute your work. How you personally define "derivative work" doesn't matter; all that matters is how copyright law and precedent case law define "derivative work".

Note that, IMHO, GNU fails in this regard also. With the GPL, they define "derivative work" in at least three different ways - but regardless of how GNU defines "derivative work", all that really matters is how copyright law defines "derivative work".

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Conserning your common sence test no. 1: I think the answer is actually Yes, in the particular case of GPL and AGPL (but No in "all" other cases I suppose) — WordPress is licensed under GPL v2 or any later version. GPL v3 has a specific clause about using GPL v3 with AGPL v3. It says that one may "link or combine any [GPL v3] covered work with a work licensed under version 3 of the GNU AGPL into a single combined work". –  KajMagnus Jun 15 '12 at 23:19
    
Concerning common sence test no. 2: Actually I think it is not of relevance, when you're mixing AGPL v3 and GPL v3 code. Because both AGPL v3 and GPL v3 says that AGPL code and GPL code can be combined into one single work — and that this does not affect the licensing of the individual parts. For example, if my plugin is under the AGPL, then Akismet and WordPress would still be under the GPL. –  KajMagnus Jun 15 '12 at 23:21
    
Here is a relevant snippet from the AGPL license, section 13: "you have permission to link or combine any covered work with a work licensed under version 3 of the GNU GPL into a single combined work" ... "but the work with which it is combined will remain governed by version 3 of the GNU GPL" –  KajMagnus Jun 15 '12 at 23:22
    
+1 because it seems reasonable that, as you write, how I personally define "derivative work" doesn't matter. That all that matters is how copyright law and precedent cases. And this is probably reasons enough for me to abandon the AGPL. (Since it wouldn't be reasonable that other people were forced to open source their WP installations, in order to use the AGPL plugin.) –  KajMagnus Jun 15 '12 at 23:24
    
Concerning GPL v2 and defining "derivative work" in 3 different ways — GPL v3 doesn't mention "derivative work" at all, it mentions "combined work" instead. I feel confused :-) Well perhaps I should forget everything and use GPL instead. (That's what I'll do :-) ) –  KajMagnus Jun 15 '12 at 23:27
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I'd say: Perhaps no, not necessarily. (But I'm not a lawyer.)

Since one is (perhaps — I think the license text is ambiguous, see below) allowed to add additional permissions to the GPL and AGPL license, I could grant permissions to people in a way that they would not have to open source their whole WordPress installation.

Details: In both the AGPL and the GPL, there is a section 7 about Additional Terms. Those additional terms "supplement the terms of the AGPL/GPL license, by making exceptions from one or more of its conditions".

It's not okay to add further restrictions to the license, except in a few specific cases detailed in section 7 (e.g. regarding the disclaimer and author attributions).

It is, however, okay to add additional permissions (as far as I've understood, but I'm no lawyer). From section 7:

You may place additional permissions on material, added by you to a covered work, for which you have or can give appropriate copyright permission.

— I wonder what "which" in the last sentence refers back to. Does it refer to the material I add, or to the "covered work"?

If it refers to the material I add, to which I have copyright, then, if I give the appropriate permissions, I suppose people won't have to open source their whole WP installation.

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