post replacements: allow replacing post with identical file.

app/models/post_replacement.rb

@@ -19,7 +19,14 @@ class PostReplacement < ApplicationRecord
 
   def process!
     transaction do
-      upload = Upload.create!(file: replacement_file, source: replacement_url, rating: post.rating, tag_string: self.tags)
+      upload = Upload.create!(
+        file: replacement_file,
+        source: replacement_url,
+        rating: post.rating,
+        tag_string: self.tags,
+        replaced_post: post,
+      )
+
       upload.process_upload
       upload.update(status: "completed", post_id: post.id)
 

app/models/upload.rb

@@ -7,7 +7,7 @@ class Upload < ApplicationRecord
   attr_accessor :file, :image_width, :image_height, :file_ext, :md5, 
     :file_size, :as_pending, :artist_commentary_title, 
     :artist_commentary_desc, :include_artist_commentary,
-    :referer_url, :downloaded_source
+    :referer_url, :downloaded_source, :replaced_post
   belongs_to :uploader, :class_name => "User"
   belongs_to :post
   before_validation :initialize_uploader, :on => :create
@@ -22,7 +22,7 @@ class Upload < ApplicationRecord
     :tag_string, :status, :backtrace, :post_id, :md5_confirmation, 
     :parent_id, :server, :artist_commentary_title,
     :artist_commentary_desc, :include_artist_commentary,
-    :referer_url
+    :referer_url, :replaced_post
 
   module ValidationMethods
     def uploader_is_not_limited
@@ -46,7 +46,10 @@ class Upload < ApplicationRecord
     # Because uploads are processed serially, there's no race condition here.
     def validate_md5_uniqueness
       md5_post = Post.find_by_md5(md5)
-      if md5_post
+
+      if md5_post && replaced_post
+        raise "duplicate: #{md5_post.id}" if replaced_post != md5_post
+      elsif md5_post
         raise "duplicate: #{md5_post.id}"
       end
     end