fix: zstd compression level handling

[domwst]

(Hopefully) fixes #2127

[NicholasBHubbard]

Thanks for the PR! In this comment on issue #2127 I reproduced the problem of higher compression levels leading to larger packages. This PR does fix that problem:

$ git log -1 --oneline
8150c59 (HEAD -> domwst/main) Fix zstd compression level handling
$ bundle exec bin/fpm -s python -t deb --deb-compression=zst --deb-compression-level=3 django
Created package {path: "python-django_5.2.8_all.deb"}
$ stat -c %s python-django_5.2.8_all.deb
6991954
$ rm python-django_5.2.8_all.deb
$ bundle exec bin/fpm -s python -t deb --deb-compression=zst --deb-compression-level=9 django
Created package {path: "python-django_5.2.8_all.deb"}
$ stat -c %s python-django_5.2.8_all.deb
5992540

Issue #2127 mentions that values 0-19 are valid zstd compression levels, but fpm only allows values 0-9 for the --deb-compression flag. This problem is not addressed in this PR.

[domwst]

Yeah, it's kind of fixed by the environment variable ZSTD_CLEVEL which allows to pass an arbitrary compression level. I can try to find out how to broaden --deb-compression range for zstd compression, but it will take some time.

[NicholasBHubbard]

Ah yes, I see that the ZSTD_CLEVEL env var could be used to pass an arbitrary compression level.

Something like the following (UNTESTED) update could be used to broaden the --deb-compression flag:

  option "--compression-level", "[0-9]", "Select a compression level. 0 is none or minimal. 9 is max compression.",
    # Specify which compression level to use on the compressor backend, when building a package
    :default => nil do |value|
    valint = value.to_i
    max_compression = 9
    if self.attributes[:deb_compression] == "zst"
      max_compression = 19
    end
    unless value =~ /^\d$/ && valint >= 0 && valint <= max_compression
      raise "Invalid compression level '#{value}'. Valid values are integers between 0 and #{max_compression} inclusive."
    end
    valint
  end

Do any of the other available compression types allow levels outside the range 0-9?