add new model for post replacements, add undo functionality

app/models/post_replacement.rb

@@ -0,0 +1,64 @@
+class PostReplacement < ActiveRecord::Base
+  DELETION_GRACE_PERIOD = 30.days
+
+  belongs_to :post
+  belongs_to :creator, class_name: "User"
+  before_validation :initialize_fields
+  attr_accessible :replacement_url
+
+  def initialize_fields
+    self.creator = CurrentUser.user
+    self.original_url = post.source
+  end
+
+  def undo!
+    undo_replacement = post.replacements.create(replacement_url: original_url)
+    undo_replacement.process!
+  end
+
+  def process!
+    # TODO for posts with notes we need to rescale the notes if the dimensions change.
+    if post.notes.any?
+      raise NotImplementedError.new("Replacing images with notes not yet supported.")
+    end
+
+    # TODO for ugoiras we need to replace the frame data.
+    if post.is_ugoira?
+      raise NotImplementedError.new("Replacing ugoira images not yet supported.")
+    end
+
+    # TODO images hosted on s3 need to be deleted from s3 instead of the local filesystem.
+    if Danbooru.config.use_s3_proxy?(post)
+      raise NotImplementedError.new("Replacing S3 hosted images not yet supported.")
+    end
+
+    transaction do
+      upload = Upload.create!(source: replacement_url, rating: post.rating, tag_string: post.tag_string)
+      upload.process_upload
+      upload.update(status: "completed", post_id: post.id)
+
+      # queue the deletion *before* updating the post so that we use the old
+      # md5/file_ext to delete the old files. if saving the post fails,
+      # this is rolled back so the job won't run.
+      Post.delay(queue: "default", run_at: Time.now + DELETION_GRACE_PERIOD).delete_files(post.id, post.file_path, post.large_file_path, post.preview_file_path)
+
+      post.md5 = upload.md5
+      post.file_ext = upload.file_ext
+      post.image_width = upload.image_width
+      post.image_height = upload.image_height
+      post.file_size = upload.file_size
+      post.source = upload.source
+      post.tag_string = upload.tag_string
+
+      post.comments.create!({creator: User.system, body: post.presenter.comment_replacement_message(creator), do_not_bump_post: true}, without_protection: true)
+      ModAction.log(post.presenter.modaction_replacement_message)
+
+      post.save!
+    end
+
+    # point of no return: these things can't be rolled back, so we do them
+    # only after the transaction successfully commits.
+    post.distribute_files
+    post.update_iqdb_async
+  end
+end