#424

backend/app/services/shared_resource_id_allocator.rb

@@ -0,0 +1,56 @@
+require 'digest'
+
+
+class SharedResourceIdAllocator
+  SPACES = {
+      'tags' => ['tags', 'external_tags'],
+      'wiki' => ['tag_descriptions', 'tag_groups'] }.freeze
+  LOCK_TIMEOUT = 10
+
+  def self.assign! record
+    space, tables = SPACES.find { |_name, names| names.include?(record.class.table_name) }
+    raise ArgumentError, 'Unknown shared ID resource' unless tables
+
+    connection = record.class.connection
+    transaction = record.class.current_transaction
+    raise ActiveRecord::ActiveRecordError, 'Shared ID requires a transaction' unless transaction.open?
+
+    database = connection.pool.db_config.database
+    name = "btrc:ids:#{ Digest::SHA256.hexdigest(database)[0, 16] }:#{ space }"
+    quoted_name = connection.quote(name)
+    acquired = record.class.uncached do
+      connection.select_value("SELECT GET_LOCK(#{ quoted_name }, #{ LOCK_TIMEOUT })")
+    end
+    unless acquired.to_i == 1
+      raise ActiveRecord::LockWaitTimeout, "Shared ID lock unavailable: #{ space }"
+    end
+
+    release = lambda do
+      result = record.class.uncached do
+        connection.select_value("SELECT RELEASE_LOCK(#{ quoted_name })")
+      end
+      unless result.to_i == 1
+        raise ActiveRecord::ActiveRecordError, "Shared ID lock release failed: #{ space }"
+      end
+    end
+    begin
+      transaction.after_commit(&release)
+      transaction.after_rollback(&release)
+    rescue StandardError
+      release.call
+      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|
+        quoted_table = connection.quote_table_name(table)
+        connection.select_value(
+            "SELECT id FROM #{ quoted_table } ORDER BY id DESC LIMIT 1 FOR UPDATE").to_i
+      end
+    end
+    record.id = maxima.max + 1
+  end
+end