Refactor checksum code: extract hexdigest from Upload and use in LfsObject and Gitlab::Ci::Trace

Summary

Follow up https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14477#note_188552032:

This is sprinkled all over the codebase. Digest::SHA256.file(file_path).hexdigest

I like the idea that each model knows how to calculate its checksum. We can introduce a new concern 'Checksummable' with some shared code?

Improvements

https://en.wikipedia.org/wiki/Don%27t_repeat_yourself

For this iteration, the scope is limited to extracting hexdigest out of Upload and reusing it in only LfsObject and Gitlab::Ci::Trace. It looks like we'll need to add some tests too.

module Checksummable
  extend ActiveSupport::Concern

  class_methods do
    def hexdigest(path)
      Digest::SHA256.file(path).hexdigest
    end
  end
end

class Upload
  include Checksummable

  # remove this method
  def self.hexdigest(path)
    Digest::SHA256.file(path).hexdigest
  end

  # modify this method to be like
  def calculate_checksum!
    self.checksum = nil
    return unless checksummable?

    self.checksum = self.class.hexdigest(absolute_path)
  end
end

Risks

Low risk. Just extracting shared code.

Involved components

Optional: Intended side effects

Optional: Missing test coverage