Open Source Licenses like GPL in Multiple Licenses models

Question

How can an open source license be compatible with a multi-licensing scheme?

According to a similar question, the author of a library could potentially change its license. But problems arise when the project as already received contributions from the community, because the contributors kind of 'own' the software it self.

Is this right?

I don't understand how such reasonable restriction, apply when a double license is applied to a product. E.g. I release a software with a double license:

• GPLv2|v3 for open source projects (users must release their code if using the given library publicly distributing the software using it)
• Commercial license that under certain conditions (e.g. payment), allows the user to not distribute their application's source code (e.g. they can pay the author and release a commercial closed source application)

I have several questions on this topic:

How this could be possible? Does the changes/contributions provided by the community itself be used under the commercial license once received? Isn't this something similar to change the license (see the linked question): you accept changes by the community and release them under another license? If this is legal, are there any difference/compatibility between major open source license such as GPLv2|3, lGPL, Mit, Bsd?

Answer 1

If the contributors of changes agree to a Contributor License Agreement (CLA), then the project owner holds all of the copyright. In essence, contributors can be asked to give up their rights to own the contributions. They may continue to receive credit for their contributions, but depending on the wording of the agreement, may cause them to assign copyright to the project owner or give permission for the product owner to license under terms suitable for the project.

If there's an open source license (like GPL), this doesn't stop me from making a fork and releasing it under the GPL (or other appropriate, compatible license) and also putting my changes there. I now have a project fork that's GPL licensed. I can offer a patch, for example, between my fork and the main project.

Also note that there are, at a very high level, two types of CLA. One gives rights to the project maintainer to use your contributions in other ways, such as releasing them under a dual-license model. The other gives up your copyright on the contributions.