fix(tag): prevent dupl. tags on concurrent inserts (mastodon#35792)

Co-authored-by: Christian Oelschlegel <oelschle@sciphy.de>
Co-authored-by: Claire <claire.github-309c@sitedethib.com>

Author: 3 people

app/models/tag.rb

@@ -90,8 +90,17 @@ def history
 
   class << self
     def find_or_create_by_names(name_or_names)
-      Array(name_or_names).map(&method(:normalize)).uniq { |str| str.mb_chars.downcase.to_s }.map do |normalized_name|
-        tag = matching_name(normalized_name).first || create(name: normalized_name)
+      names = Array(name_or_names).map { |str| [normalize(str), str] }.uniq(&:first)
+
+      names.map do |(normalized_name, display_name)|
+        tag = begin
+          matching_name(normalized_name).first || create!(
+            name: normalized_name,
+            display_name: display_name.gsub(HASHTAG_INVALID_CHARS_RE, '')
+          )
+        rescue ActiveRecord::RecordNotUnique
+          find_normalized(normalized_name)
+        end
 
         yield tag if block_given?
 

spec/models/tag_spec.rb

@@ -134,6 +134,27 @@
     end
   end
 
+  describe '.find_or_create_by_names_race_condition' do
+    it 'handles simultaneous inserts of the same tag in different cases without error' do
+      tag_name_upper = 'Rails'
+      tag_name_lower = 'rails'
+
+      threads = []
+
+      2.times do |i|
+        threads << Thread.new do
+          described_class.find_or_create_by_names(i.zero? ? tag_name_upper : tag_name_lower)
+        end
+      end
+
+      threads.each(&:join)
+
+      tags = described_class.where('lower(name) = ?', tag_name_lower.downcase)
+      expect(tags.count).to eq(1)
+      expect(tags.first.name.downcase).to eq(tag_name_lower.downcase)
+    end
+  end
+
   describe '.search_for' do
     it 'finds tag records with matching names' do
       tag = Fabricate(:tag, name: "match")