Skipping work in a processor

[ClearlyClaire]

Is your feature request related to a problem? Please describe.

We've been using Paperclip for a while in Mastodon, and we're chaining processors. Sometimes, a processor can decide it doesn't need to perform any change to the file. In this case, we just re-open the file to pass it to the next processor:

class LazyThumbnail < Paperclip::Thumbnail
  def make
    return File.open(@file.path) unless needs_convert?

    # … actual code
  end
end

This worked up until Paperclip 6.1, which caused the files to be closed after each processor in thoughtbot@90f9121, resulting in Errno::ENOENT errors.

This behavior remains unchanged in kt-paperclip so I was wondering how to best implement this “lazy” processor.

Looking at https://github.com/kreeti/kt-paperclip/blob/master/lib/paperclip/attachment.rb#L523-L530, there seem to be no clear way to handle that case.

Describe the solution you'd like

kt-paperclip should maybe unlink the original tempfile only once it has been fully processed. Pushing original into intermediate_files instead of immediately unlinking it should do the trick.

Describe alternatives you've considered

Our LazyThumbnail could create a copy of the file, but it seems wasteful.