post replacements: don't queue deletion when file is unchanged (#3235).

When replacing a post with the same file, don't queue a deletion and
don't leave a comment or modaction. There's no need to do these things
when we're restoring a missing or corrupted image with the original file.

app/models/post_replacement.rb

@@ -29,6 +29,7 @@ class PostReplacement < ApplicationRecord
 
       upload.process_upload
       upload.update(status: "completed", post_id: post.id)
+      md5_changed = (upload.md5 != post.md5)
 
       if replacement_file.present?
         update(replacement_url: "file://#{replacement_file.original_filename}")
@@ -39,7 +40,9 @@ class PostReplacement < ApplicationRecord
       # 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)
+      if md5_changed
+        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)
+      end
 
       post.md5 = upload.md5
       post.file_ext = upload.file_ext
@@ -51,8 +54,10 @@ class PostReplacement < ApplicationRecord
       rescale_notes
       update_ugoira_frame_data(upload)
 
-      post.comments.create!({creator: User.system, body: comment_replacement_message, do_not_bump_post: true}, without_protection: true)
-      ModAction.log(modaction_replacement_message)
+      if md5_changed
+        post.comments.create!({creator: User.system, body: comment_replacement_message, do_not_bump_post: true}, without_protection: true)
+        ModAction.log(modaction_replacement_message)
+      end
 
       post.save!
     end

test/unit/post_replacement_test.rb

@@ -261,6 +261,14 @@ class PostReplacementTest < ActiveSupport::TestCase
           end
         end
       end
+
+      should "not queue a deletion or log a comment" do
+        upload_file("#{Rails.root}/test/files/test.jpg", "test.jpg") do |file|
+          assert_no_difference(["@post.comments.count", "ModAction.count"]) do
+            @post.replace!(replacement_file: file, replacement_url: "")
+          end
+        end
+      end
     end
   end
 end