Prevent deleting processing image while attachment is processing

[cgunther]

On destroying a model, Paperclip will delete any styles for which exists? returns true. exists? checked the path, which would point to the processing image while the attachment was processing when processing_image_url is set. This leads to Paperclip deleting the
processing image, thinking it is the style for the attachment.

Given:

class Photos < ActiveRecord::Base
  has_attached_file :image, styles: { thumbnail: '100x' }, only_process: [:original]
  process_in_background :image, only_process: [:thumbnail], processing_image_url: '/images/:class/processing/:attachment/:style.gif'
end

When deleting a photo that was not yet processed, you'd see this in your logs as Paperclip deleted both the original (correctly) and the thumbnail processing image (incorrectly):

[paperclip] deleting /Users/chrisgunther/Sites/sample/public/system/photos/images/000/000/185/original/brick20150303-42943-1bubzim.jpeg
[paperclip] deleting /Users/chrisgunther/Sites/sample/public/images/photos/processing/images/thumbnail.gif

This patch overrides Paperclip's exists? method so that if the given style is processing, exists? will always return false, thus preventing Paperclip from trying to delete the processing image. This is not backwards compatible as exists? previously returned true even if the
style was not yet processed.

Previously:

photo = Photo.create(image: File.open('/path/to/image')
photo.image.exists?(:original) # => true
photo.image.exists?(:thumbnail) # => true

Now:

photo = Photo.create(image: File.open('/path/to/image')
photo.image.exists?(:original) # => true
photo.image.exists?(:thumbnail) # => false
# Process styles
photo.image.exists?(:thumbnail) # => true

This may still leave some orphaned files as if the model is destroyed while the processing job is currently running, no styles will be deleted, but some styles may have already been written to disk as the job runs.

While it's a small window to delete the model in before the background job finishes running, it's a pretty big issue that the processing images that you think are safe in your app may be deleted and will now 404 on future requests to them until you deploy again, adding the processing images, only for them to be deleted again when someone deletes the model before the background job finishes.

[ScotterC]

hey @cgunther this is really useful! Would you mind taking a look at the rails 3.2 failures? This is telling me maybe we should stop supporting Rails < 4 but if it's an easy fix it'd be welcome