On Thursday, 30. August 2007, Jeff Eaton wrote:
For quite some time, it has been commonly understood in the Drupal community that non-GPL software (like a third-party PHP message board system) can be integrated into Drupal legally by using an intermediary 'bridge' module. After some in-depth emails with the Free Software foundation's license gurus, it's become clear that this is NOT the case.
Before going any further, I want to make clear that I'm not expressing approval or disapproval of this: I'm just relaying the conclusions that were reached after several days of discussion and questioning with the FSF.
Why do these modules violate the GPL?
1) Under the FSF's accepted interpretation of the GPL, if a module is integrating Drupal and another PHP script, by calling one's APIs when triggered by the other for example, its purpose is to make a single unit of software out of those parts.
My understanding was that non-GPL software may not be a derivative work of GPL code, but that it can go the other way round. As David Strauss mentioned, it is indeed possible to write GPL software by using the Win32 or Cocoa API. Following this reasoning, it would mean the following for Drupal modules: - Drupal modules are based on Drupal, so they are GPL in any case. - Thus, modules may not incorporate code that make the integrated software derive (call functions) from Drupal or the module itself. - Whereas it should be possible to call functions from the integrated software from the module. For example, that would mean no authentication in the integrated software that is based on Drupal users and passwords. Or displaying any information that comes from Drupal. But it should be possible to manage and display stuff from the integrated software from Drupal. Like is done with Apache, for example (which uses the GPLv2 incompatible Apache license). I'm sure there are lots of other good examples. Would this make sense?
2) If multiple programs are operating together and functioning as one unit, all the pieces must be GPL'd.
There are a lot of angles to approach the question from, but that's what it boils down to. From a developer's perspective, if debug_backtrace() can ever include functions from both Drupal and an external program, you've turned them into a single program.
David's argument that the GPL only covers distribution, not usage, is a good counter argument to this point of view. Of course, this is somewhat of a difficult issue, quite comparable to the kernel module blobs for the ATI and nVidia graphics card drivers which are widely considered a legal gray area. Also, you could extend the issue not only to PHP code but also to XML-RPC calls, where you do not even know if the software that you communicate with is GPL or not. That is, I'm a big fan of the FSF and their views, but I think their opinion on this specific issue is a tad too radical, and not legally binding as well. Regards, Jakob