Having just re-read the GNU GPL v2 and currently thinking about how an ‘app store’ for Discourse would work, it occurred to me that it is arguable that the use of Discourse plugins for which the source code is not publicly available (e.g. in a private repository) breaches the terms of the GPL v2.
Taken together, these parts of the GPL v2 seem to require that the source code of plugins be publicly available:
…a "work based on the Program"
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
2. b) You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.
[ All of section 3]
For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
There are further parts I could cite, but this would get too long…
Maybe that is not the way people have understood the terms of the GPL v2 in the past, but it is seems eminently arguable from a contractual perspective.
Disclaimer: This is an impression based on a brief reading of the documents. This is not a formal legal opinion.
Doing some very preliminary research it seems that my impression may be shared by others, and by some courts.
Interestingly, it seems that the folks pursuing cases over the availability of GPL v2 source code are typically not the copyright holders.