Fix #4969: Tag changes made by replacements wipe out edits done at the same time.

Lock the post during replacement to ensure we have the latest version of
the tags and to ensure nobody else can modify the post until after the
replacement is finished.

app/logical/post_replacement_processor.rb

@@ -34,13 +34,14 @@ class PostReplacementProcessor
     replacement.image_width = media_asset.image_width
     replacement.md5 = media_asset.md5
 
+    post.lock!
     post.md5 = media_asset.md5
     post.file_ext = media_asset.file_ext
     post.image_width = media_asset.image_width
     post.image_height = media_asset.image_height
     post.file_size = media_asset.file_size
     post.source = replacement.final_source.presence || replacement.replacement_url
-    post.tag_string = replacement.tags
+    post.tag_string = "#{post.tag_string} #{replacement.tags}"
 
     rescale_notes(post)
     post.save!

app/models/post_replacement.rb

@@ -12,8 +12,6 @@ class PostReplacement < ApplicationRecord
 
   def initialize_fields
     self.original_url = post.source
-    self.tags = "#{post.tag_string} #{tags}"
-
     self.old_file_ext = post.file_ext
     self.old_file_size = post.file_size
     self.old_image_width = post.image_width

test/functional/post_replacements_controller_test.rb

@@ -6,13 +6,14 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
       context "replacing a post from a source url" do
         should "replace the post" do
           assert_difference("PostReplacement.count") do
-            @post = create(:post)
+            @post = create(:post, tag_string: "image_sample")
 
             post_auth post_replacements_path, create(:moderator_user), params: {
               format: :json,
               post_id: @post.id,
               post_replacement: {
                 replacement_url: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg",
+                tags: "replaced -image_sample"
               }
             }
 
@@ -31,6 +32,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(@post.file_size, @replacement.old_file_size)
           assert_equal(@post.file_ext, @replacement.old_file_ext)
           assert_equal(@post.md5, @replacement.old_md5)
+          assert_equal(@post.tag_string, "image_sample")
 
           @post.reload
           assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", @post.md5)
@@ -40,6 +42,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(650, @post.image_height)
           assert_equal(127_238, @post.file_size)
           assert_equal("jpg", @post.file_ext)
+          assert_equal("replaced", @post.tag_string)
         end
       end