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If we decide to use a piece of open sourced software that is currently released under a license that allows inclusion in an internal closed source system, what protection are we offered if the owners of the open source software decide to change the license? Typically I'm thinking of where the new license does not allow inclusion in closed source software (I think the PDF generation api iText did this back in 2009).

Can the new license be applied retrospectively to the old versions? I'm guessing it can, but often isn't. We are just looking at worst case scenario, would we be in a position where we could just continue using an old version for as long as it suited and then make a decision about the future - or would we be in a position where we'd have to decide to either pay the commercial fee that is often available or find a different "free" alternative.

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Does the license mention if it can be revoked or replaced by another license? –  Jaap Jun 7 '11 at 11:10
    
I have never heard of an example of somebody trying to change an open source license already released, although different versions may have different licenses. I suppose one thing that might come up is if the licensor was legally unable to issue the previous license, like a BSD project with substantial GPL content. –  David Thornley Jun 7 '11 at 17:44

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This question is discussed in some detail in chapter 4 of http://rosenlaw.com/oslbook.htm.

The short answer is that promissory estoppel probably protects you. But there hasn't been significant litigation on this point, and so you aren't guaranteed that this legal theory will win in court. (And even if it was followed in the USA, you wouldn't be guaranteed that it would be followed in the UK. Though the legal systems are close enough that they are likely to do the same thing.)

If you go to a country that doesn't follow British Common Law, then the legal situation could be very different again. I know nothing about that.

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OK, thanks for the link to the book - looks well worth a read and the second link for the short answer. –  Paul Hadfield Jun 7 '11 at 8:46
    
The book is good, but it's also about seven years old in a legal field that's developing rapidly. In the meantime, the GPL has been found valid in at least some courts, and I remember reading about litigation concerning whether it was a license or a contract. Don't rely too much on the book. –  David Thornley Jun 7 '11 at 17:37
    
@David Thornley: I've been following legal developments. I'm pretty sure that none directly relate to the question at hand. As for litigation, you may be thinking of en.wikipedia.org/wiki/Jacobsen_v._Katzer where Katzer tried to make the argument that the Artistic license was a contract, and no valid contract was formed, so it was null and void. This got up to the United States Court of Appeals for the Federal Circuit which ruled that the Artistic license is an enforceable copyright license, and no contract needs to be formed to enforce it. –  btilly Jun 7 '11 at 20:25

IANAL but I doubt the terms of a license can be changed to include previous implementations like that.
They can (and iText did this) of course change the license for new downloads of the old versions, but they're not going to get away with changing the license on versions already deployed with users.
Were that possible, it'd be easy for people to do a bait and switch that grants them ownership of any piece of software ever created with any version of their library simply by changing the license of a new version to read that. Or change to GPL and suddenly thousands of products are forced under GPL that weren't in the past. Richard Stallman would love that...
So common sense tells you're safe, but should probably consider changing existing systems to not use a library once its license changes in a way that makes it incompatible with your acceptable standards (I did this myself, cut iText out of a product for that very reason just to be on the safe side as well as because it's a kludge to use in comparison to FOP).

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The FSF does not claim it can force other people's software to be released under the GPL. It does claim that it can get injunctions and perhaps damages for GPLed code in a non-GPLed work. Releasing an entire work under GPL, when this does not violate other obligations, is of course a quick way out of a GPL lawsuit, but it's hardly mandatory. –  David Thornley Jun 7 '11 at 17:39
    
no, the GPL requires you release your code under GPL if you use anything that's released under GPL in it (either in binary or source form). If the FSF doesn't try to enforce that it's because they don't believe they can make it stick in front of a judge, not because the GPL doesn't state it. –  jwenting Jun 8 '11 at 5:23
    
@jwenting: If somebody uses GPLed code on a proprietary application and distributes it, they're violating the license. The FSF's stand on this is that they're distributing without a license. The GPL says "you must" license the whole under the GPL, not that a derived work is licensed under the GPL, and the FSF doesn't believe somebody can be compelled to execute that by such a license. (IANAL, of course, but I think I'm expressing the FSF and SFLC's positions accurately.) –  David Thornley Jun 8 '11 at 13:42
    
the GPL states that anything that links to anything that's released under GPL must be released under GPL as well. That means that strictly if you use Linux systems libraries you should release your application under GPL, as those libraries are released under GPL. And that's probably why the FSF doesn't try to enforce this clearly ludicrous clause, despite its very intention being to force eventually all software under GPL whether the authors want it to be there or not. –  jwenting Jun 8 '11 at 14:33
    
@jwenting: There's the GPL system libraries exemption, and a lot of additional permissions on various projects. Last I looked, you could use Bison to create a parser to go into a proprietary compiler (Bison copies a lot of GPLed code to go with the parse tables), since there was a specific exemption. The reason the FSF gives for not being able to force GPLing of arbitrary code is that that's not a legally compellable remedy for a copyright violation, and probably not breach of contract. The FSF and SFLC do enforce that clause. –  David Thornley Jun 8 '11 at 15:27

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