Lazily generate a UUID for AR transactions

Author: fxn

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/digest"
+
 module ActiveRecord
   module ConnectionAdapters
     # = Active Record Connection Adapters Transaction State
@@ -119,6 +121,7 @@ def materialized?; false; end
       def before_commit; yield; end
       def after_commit; yield; end
       def after_rollback; end # noop
+      def uuid; Digest::UUID.nil_uuid; end
     end
 
     class Transaction < ActiveRecord::Transaction # :nodoc:

activerecord/lib/active_record/transaction.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/digest"
+
 module ActiveRecord
   class Transaction
     class Callback # :nodoc:
@@ -23,6 +25,7 @@ def after_rollback
 
     def initialize # :nodoc:
       @callbacks = nil
+      @uuid = nil
     end
 
     # Registers a block to be called before the current transaction is fully committed.
@@ -60,6 +63,11 @@ def after_rollback(&block)
       (@callbacks ||= []) << Callback.new(:after_rollback, block)
     end
 
+    # Returns a UUID for this transaction.
+    def uuid
+      @uuid ||= Digest::UUID.uuid_v4
+    end
+
     protected
       def append_callbacks(callbacks)
         (@callbacks ||= []).concat(callbacks)

activerecord/test/cases/transactions_test.rb

@@ -1650,6 +1650,29 @@ def test_no_automatic_savepoint_for_inner_transaction
   end
 end if Topic.lease_connection.supports_savepoints?
 
+class TransactionUUIDTest < ActiveRecord::TestCase
+  def test_the_uuid_is_lazily_computed
+    Topic.transaction do
+      transaction = Topic.connection.current_transaction
+      assert_nil transaction.instance_variable_get(:@uuid)
+    end
+  end
+
+  def test_the_uuid_for_regular_transactions_is_generated_and_memoized
+    Topic.transaction do
+      transaction = Topic.connection.current_transaction
+      uuid = transaction.uuid
+      assert_match(/\A[[:xdigit:]]{8}-(?:[[:xdigit:]]{4}-){3}[[:xdigit:]]{12}\z/, uuid)
+      assert_equal uuid, transaction.uuid
+    end
+  end
+
+  def test_the_uuid_for_null_transactions_is_the_nil_uuid
+    null_transaction = ActiveRecord::ConnectionAdapters::TransactionManager::NULL_TRANSACTION
+    assert_equal Digest::UUID.nil_uuid, null_transaction.uuid
+  end
+end
+
 class ConcurrentTransactionTest < ActiveRecord::TestCase
   if ActiveRecord::Base.lease_connection.supports_transaction_isolation? && !current_adapter?(:SQLite3Adapter)
     self.use_transactional_tests = false