Convert license to array of identifiers

[voxik]

RubyGems currently complain during gem build:

WARNING:  license value 'MIT AND (BSD-2-Clause OR GPL-2.0-or-later)' is invalid.
Use a license identifier from http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
Did you mean 'AGPL-1.0-or-later', 'AGPL-3.0-or-later', 'BSD-2-Clause-Patent', 'GPL-2.0-or-later', 'LGPL-2.0-or-later'?

Nothing else then plain list of license identifers is supported ATM.

Relates #309

Types of Changes

• Bug fix.

Contribution

• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Does this indicate that we are all 3 licenses? I assume we can't represent OR with this list, only AND. If that's the case, we should choose BSD as we aren't releasing the combined total under the GPL.

[voxik]

I don't think it really is defined what the content of this field means, if that is just list or if there is any relation between the items. Therefore I have opened rubygems/rubygems#7355. However, there is other option:

spec.license = "LicenseRef-license.md"

But I am not sure if that is any better :/

Nevertheless, the intention of this PR is just to merely list all the licenses, which is good for practical purposes. I.e. if somebody installs this package, they knows there those licenses applies. Where and how, they would need to look around. But that is not different to the "MIT AND (BSD-2-Clause OR GPL-2.0-or-later)" string IMHO.

[ioquatix]

Okay, makes sense. Let's see the conclusion of rubygems/rubygems#7355 and then move forward here.

[tarcieri]

Perhaps for simplicity's sake you could relicense the whole thing as BSD, with permission from the original authors.

[ioquatix]

I think it would be hard work: https://github.com/socketry/nio4r/blob/main/license.md

[ioquatix]

I think it's okay to specify spec.licenses = ["MIT", "BSD-2-Clause"].

In our case, we have license.md which clarifies the situation, and additionally, if we were adopting the GPL from libev our own code would also need to be GPL, therefore, we couldn't also release under the MIT license. Thus, I think we can simplify this and assume that spec.licenses is a union of all licenses, in this case, the position we are taking is:

• nio4r code is licensed under the MIT license.
• libev code is licensed under the BSD-2-Clause license.

If anyone has a problem with this interpretation, please let me know. The goal here is to make it easier for automated tools to analyse the license of this code. I think this is a reasonable middle ground, with the full details being outlined in license.md.