@rpearce I think the safePrefix thing in your PR can be achieved just by doing validDerivationName "${safePrefix}-${string}"

Would adding tests in something like https://github.com/NixOS/nixpkgs/blob/master/lib/tests/misc.nix be useful? I must confess that I don't know how to run them, though

👍 works perfectly for me

From the title of this PR I foolishly expected a string -> bool. May I suggest sanitizeDerivationName? It's accurate and it verbs.

Changed the name to the suggested one and added some tests. Ideally the tests would generate random strings, put them through the function, then try to create a derivation with it. But our test infrastructure isn't really set up for that (yet?). So for now there's just a bunch of hardcoded ones.

Some test improvements:

• test the allowed characters by invoking derivation
• test a string that is too long
• test a string that is too long by itself, but not after bad chars are filtered out

@roberth Now did that, and found 2 bugs in the function while doing so :D

• One bug was that I used a regex like "[^+-.]", thinking it would mean exclude any of "+", "-" and ".", but really it meant exclude the ascii range from "+" to ".". So now I moved the "-" to the end of the brackets, so it's interpreted not as a range.
• The other bug was regarding the derivation length: Nix also counts the ".drv" as being part of the name, so the name itself needs to be trimmed to 207 instead of 211 characters

@Profpatsch Also applied your suggestion

One question to discuss: If the name is too long, should it be trimmed from the left or the right? Currently the function trims from the left, which would be better when names represent file names, where the left-most chars are the least specific. But trimming from the right would make more sense when it's a "${name}-${version}" like string, where the ${name} shouldn't be removed.