MiniMagick get_pixels gives "unable to write file"

[jeffcovey]

This has been working for us for years as a quick-and-dirty way to check for duplicate images being uploaded to our Ruby on Rails site:

file.blob.open do |temp_file|
  file_data = MiniMagick::Image.open(temp_file.path)
  file_data.resize '1024x768'
  @md5 = Digest::MD5.hexdigest(file_data.get_pixels.join)
  # etc.
end

After recent Rails upgrades (I believe it started around 6.0.4; we're gradually moving to 7.0), this started throwing errors like:

/home/jeff/.rvm/gems/ruby-3.1.2/gems/mini_magick-4.11.0/lib/mini_magick/shell.rb:17:in `run': `convert /tmp/mini_magick20221120-1647726-dit0yb.png -depth 8 RGB:-` failed with error: (MiniMagick::Error) convert-im6.q16: unable to write file `-' @ error/constitute.c/WriteImage/1341.

I tracked the problem down to get_pixels. Everything works fine until there, including resizing the image in the previous line. Why do we get this error when calling get_pixels? Why is it trying to write a file when getting the pixels matrix?

Thanks for your help!
Jeff

[jeffcovey]

I asked this on StackOverflow, too, and someone confirmed the problem:

There is probably something wrong with get_pixels. I just installed MiniMagick, opened an image and applied get_pixels and my console was left hanging ... (Rails 7, Ruby 3.1.0)

Can anyone help?

[jeffcovey]

Is no one there?