ActiveStorage accepts broken images and throws exceptions when creating variants

[Signum]

TL;DR There is no way to detect broken image files. So exceptions are thrown later when ActiveStorage tries to render variants.

I originally submitted this issue to rails/rails#40493 but was directed here.

Steps to reproduce

Create a controller action that receives uploaded image files. A part from my Rails application:

def upload_receive
  params[:file].each do |img|
    new_screenshot = @package.screenshots.new(image: img)
    
    if new_screenshot.valid?  
      new_screenshot.save
    end
  end
end

My model:

class Screenshot < ApplicationRecord
  has_one_attached :image
  validates :image, attached: true, content_type: [:png]

  def medium_image
    if self.image.attached?
      self.image.variant(
        resize_to_limit: [670, 600]
      ).processed
    end
  end
end

(The validation uses the 'active_storage_validations' gem but the problem is most certainly not coming from there.)

This works for intact PNG files. A deliberately broken PNG file will be accepted and saved though. My test file is 5 KB of zero bytes that I named "broken.png" and uploaded to Rails.

Expected behavior

One of…

1. ActiveStorage detect broken image files by trying to open them with Minimagick/Imagemagick as a validation.
2. ActiveStorage offers a common validation to detect broken images. (I tried to write my own custom validator to run the file through Minimagick but it I could not find a way to access the blob's data in the model before a ".save".)
3. Creating variants does not just throw an exception.

Actual behavior

The file gets saved by ActiveStorage with content_type='image/png'. I suspect that 'marcel' and 'mimemagick' (which I believe are used for the content type detection) can't figure out what this file is and just look at the '.png' suffix.

Once a template tries to render a variant using the Screenshot.medium_image method it will call Minimagick and fail:

convert /tmp/ActiveStorage-21670-… failed with error:
convert-im6.q16: improper image header `/tmp/ActiveStorage-21670-…' @ error/png.c/ReadPNGImage/4092.
convert-im6.q16: no images defined `/tmp/image_processing….png' @ error/convert.c/ConvertImageCommand/3258.

System configuration

Rails version: 6.0.3.4

Ruby version: 2.6.5p114

[samiggapps]

I did find this answer on SO which helped fix my issue
https://stackoverflow.com/a/60084096/7913423

[Mth0158]

For your information, this issue was solved in October 2022 by @kukicola as mentioned in the thread.
The solution is to add the processable_image validator as shown below. It will validate that the image can be processed by MiniMagick or Vips.

class Screenshot < ApplicationRecord
  has_one_attached :image
  validates :image, attached: true, content_type: [:png], processable_image: true

  def medium_image
    if self.image.attached?
      self.image.variant(
        resize_to_limit: [670, 600]
      ).processed
    end
  end
end