Confusing dual licensing from the plan9port project

Question

I intend to include some source codes from the plan9port project. The page states that the license is "original Bell Labs MIT-like or Lucent Public License". You can see that the MIT-like license is very permissive. I am sure it is compatible with most (if not all) other licenses. The problem is the Lucent License. According to wiki, this license is also an open source license, but not GPL compatible.

As I understand, dual licensing is usually consisting of an open-source copy-left license and a proprietary license. What is the intention of licensing with two open-source ones, especially when one license is much more permissive than the other? Can I include Plan 9 source codes in any GPL'ed projects? Can I use these source codes in a proprietary/closed-source project? Thanks in advance.

EDIT: I have changed the title as the old one is not correct.

Answer 1

I should start with the obligatory disclaimer - I am not a lawyer and anything below is not a legal advice. :-)

Dual licensing means that the source code can be licensed under one of two separate licenses, not that it's simultaneously licensed under both. In the usual case of GPL and proprietary, the default license is the GPL, but if you contact the source authors, they can license their source code under proprietary one, though they would usually ask for money in exchange.

There's nothing that prevents source code also being licensed under two separate OSI approved licenses. Usually, this is the case where the source code was initially licensed under a non-GPL-compatible license, so the authors later also offered it under second license to enable use in GPLed products, though in some cases the second license was added to cover some specific modules that fall under export limitations and such.

In any case, in most such cases, the actual license that applies is left as a choice to the licensee; though I've seen couple of specific cases, where there were specific requirements by the author that outlined which of the two licenses would apply to what scenario (mainly had to do with specific hardware). Of course, that choice is valid only for parts that are covered by both licenses; anything that's explicitly covered under only one of the two licenses has to be used under that license, and it might actually the use of the whole product under that same license (in the example above for the hardware-specific license there was a driver that was licensed under specific license and was forcing the whole product under it).

If you want to include Plan9 source code in GPL product, you will need to take the source under the Bell Labs MIT-like license, since Lucent is not GPL-compatible. The Bell Labs one looks like the Expat and X11 licenses, which are both listed by FSF as GPL-compatible; still I'd actually send a quick email to FSF to see what their position on this is.

If you want to include Plan9 source code in commercial product, you might be able to license it under either one, but Bell Labs might still be the more permissive one, so you might choose to stick with it.