For Cactus, it is very important that the licence is not standard GPL, as this would make it impossible to use standard Cactus thorns with arbitrary other licences. Cactus needs to be open, and the GPL places too tight a restriction on codes that use Cactus. Of course, this hold only for the standard Cactus thorns (the thorns in arrangements with names starting with "Cactus"); any thorns developed by others can of course be released (or not) under any licence they choose.

The original formulation of the Cactus thorns' licence was somewhat misleading; I believe it used the standard GPL text, and then added a clause that using such a thorn in an application was also allowed. We later noticed that this is essentially the content of the LGPL, and thus changed all Cactus thorns to be released under the LGPL. This policy still stands. If we missed the licence text in a thorn, it should be corrected. If a new thorn was added with GPL as licence, then this needs to be discussed, and either the licence changed or the thorn be moved to an auxiliary arrangement (i.e. not be part of standard Cactus any more).

Hi Erik,

Thanks for the explanation.  Since this comes up every so often, I have added some information, including a quote of your email, to the wiki at https://docs.einsteintoolkit.org/et-docs/Licences.  Assuming that this is not controversial (hah!), we can change the quote into normal text and this can serve as a reference for the policy and the reasons behind it.

I would make the text a bit more precise: the use of GPL vs LGPL doesn't make it  "impossible to use standard Cactus thorns with arbitrary other licences" in general.  You can of course download the thorns, combine them with other thorns which are licensed in any other way, and use the executable yourself.  One thing it prohibits is *distributing* such an executable containing both GPL and non-GPL thorns, without distributing also the source, which the LGPL doesn't.  I'm not sure of the extent to which the GPL and LGPL differ if you don't care about redistributing binaries.  i.e. if I package together GPL source with non-GPL source, can I distribute the combined source tree?

I think the idea of using LGPL was that some company may want to create and distribute a non-free executable based on Cactus, without distributing the source code of their contribution, and the Cactus authors did not want to prohibit this, with the idea that Cactus may eventually be used in a commercial setting where such things are desired.