#431

backend/app/services/shared_resource_id_allocator.rb

@@ -4,7 +4,14 @@ require 'digest'
 class SharedResourceIdAllocator
   SPACES = {
       'tags' => ['tags', 'external_tags'],
-      'wiki' => ['tag_descriptions', 'tag_groups'] }.freeze
+      'wiki' => ['tag_descriptions', 'tag_groups'],
+      'wiki_history' => ['tag_description_versions', 'tag_group_versions'] }.freeze
+  HISTORY_IDS = {
+      'tags' => [
+          ['tag_versions', 'tag_id'],
+          ['nico_tag_versions', 'tag_id'],
+          ['tag_description_versions', 'tag_id']],
+      'wiki' => [['tag_group_versions', 'tag_group_id']] }.freeze
   LOCK_TIMEOUT = 10
 
   def self.assign! record
@@ -13,7 +20,9 @@ class SharedResourceIdAllocator
 
     connection = record.class.connection
     transaction = record.class.current_transaction
-    raise ActiveRecord::ActiveRecordError, 'Shared ID requires a transaction' unless transaction.open?
+    unless transaction.open?
+      raise ActiveRecord::ActiveRecordError, 'Shared ID requires a transaction'
+    end
 
     database = connection.pool.db_config.database
     name = "btrc:ids:#{ Digest::SHA256.hexdigest(database)[0, 16] }:#{ space }"
@@ -41,16 +50,45 @@ class SharedResourceIdAllocator
       raise
     end
 
-    return if record.id.present?
-
-    # Locking reads see committed maxima even after earlier repeatable reads.
-    maxima = record.class.uncached do
-      tables.map do |table|
+    next_id = record.class.uncached do
+      historical_max = HISTORY_IDS.fetch(space, []).map do |table, column|
         quoted_table = connection.quote_table_name(table)
+        quoted_column = connection.quote_column_name(column)
         connection.select_value(
-            "SELECT id FROM #{ quoted_table } ORDER BY id DESC LIMIT 1 FOR UPDATE").to_i
-      end
+            "SELECT MAX(#{ quoted_column }) FROM #{ quoted_table }").to_i
+      end.max.to_i
+      [next_ids(connection, tables).max, historical_max + 1].max
     end
-    record.id = maxima.max + 1
+    if record.id.present?
+      if record.id < next_id
+        raise ActiveRecord::RecordNotUnique, 'Shared ID is below the reserved position'
+      end
+    else
+      record.id = next_id
+    end
+    # The INSERT advances InnoDB's persistent counter, including explicit IDs.
   end
+
+  def self.next_ids connection, tables
+    expiry = connection.select_value('SELECT @@SESSION.information_schema_stats_expiry').to_i
+    begin
+      connection.execute('SET SESSION information_schema_stats_expiry = 0')
+      quoted_tables = tables.map { connection.quote(_1) }.join(', ')
+      rows = connection.select_all(<<~SQL).to_a
+        SELECT TABLE_NAME, ENGINE, AUTO_INCREMENT
+        FROM information_schema.tables
+        WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME IN (#{ quoted_tables })
+      SQL
+      persistent = rows.all? do |row|
+        row['ENGINE'] == 'InnoDB' && row['AUTO_INCREMENT'].to_i.positive?
+      end
+      unless rows.size == tables.size && persistent
+        raise ActiveRecord::ActiveRecordError, 'Persistent InnoDB counters are required'
+      end
+      rows.map { _1.fetch('AUTO_INCREMENT').to_i }
+    ensure
+      connection.execute("SET SESSION information_schema_stats_expiry = #{ expiry }")
+    end
+  end
+  private_class_method :next_ids
 end