Ensure test threads share a DB connection

This ensures multiple threads inside a transactional test to see consistent
database state.

When a system test starts Puma spins up one thread and Capybara spins up
another thread. Because of this when tests are run the database cannot
see what was inserted into the database on teardown. This is because
there are two threads using two different connections.

This change uses the statement cache to lock the threads to using a
single connection ID instead of each not being able to see each other.
This code only runs in the fixture setup and teardown so it does not
affect real production databases.

When a transaction is opened we set `lock_thread` to `Thread.current` so
we can keep track of which connection the thread is using. When we
rollback the transaction we unlock the thread and then there will be no
left-over data in the database because the transaction will roll back
the correct connections.

[ Eileen M. Uchitelle, Matthew Draper ]

3 files changed, 20 insertions, 0 deletions

diff --git a/activerecord/test/cases/fixtures_test.rb b/activerecord/test/cases/fixtures_test.rb
index 61e596e208..afe761cb55 100644
--- a/activerecord/test/cases/fixtures_test.rb
+++ b/activerecord/test/cases/fixtures_test.rb
@@ -640,6 +640,8 @@ class TransactionalFixturesOnConnectionNotification < ActiveRecord::TestCase
   def test_transaction_created_on_connection_notification
     connection = stub(transaction_open?: false)
     connection.expects(:begin_transaction).with(joinable: false)
+    pool = connection.stubs(:pool).returns(ActiveRecord::ConnectionAdapters::ConnectionPool.new(ActiveRecord::Base.connection_pool.spec))
+    pool.stubs(:lock_thread=).with(false)
     fire_connection_notification(connection)
   end
 
@@ -647,12 +649,16 @@ class TransactionalFixturesOnConnectionNotification < ActiveRecord::TestCase
     # Mocha is not thread-safe so define our own stub to test
     connection = Class.new do
       attr_accessor :rollback_transaction_called
+      attr_accessor :pool
       def transaction_open?; true; end
       def begin_transaction(*args); end
       def rollback_transaction(*args)
         @rollback_transaction_called = true
       end
     end.new
+    connection.pool = Class.new do
+      def lock_thread=(lock_thread); false; end
+    end.new
     fire_connection_notification(connection)
     teardown_fixtures
     assert(connection.rollback_transaction_called, "Expected <mock connection>#rollback_transaction to be called but was not")
diff --git a/activerecord/test/cases/query_cache_test.rb b/activerecord/test/cases/query_cache_test.rb
index d8cf235000..494663eb04 100644
--- a/activerecord/test/cases/query_cache_test.rb
+++ b/activerecord/test/cases/query_cache_test.rb
@@ -532,4 +532,16 @@ class QueryCacheExpiryTest < ActiveRecord::TestCase
       end
     end
   end
+
+  test "threads use the same connection" do
+    @connection_1 = ActiveRecord::Base.connection.object_id
+
+    thread_a = Thread.new do
+      @connection_2 = ActiveRecord::Base.connection.object_id
+    end
+
+    thread_a.join
+
+    assert_equal @connection_1, @connection_2
+  end
 end
diff --git a/activerecord/test/cases/scoping/default_scoping_test.rb b/activerecord/test/cases/scoping/default_scoping_test.rb
index 3a04f4bf7d..14fb2fbbfa 100644
--- a/activerecord/test/cases/scoping/default_scoping_test.rb
+++ b/activerecord/test/cases/scoping/default_scoping_test.rb
@@ -10,6 +10,8 @@ require "concurrent/atomic/cyclic_barrier"
 class DefaultScopingTest < ActiveRecord::TestCase
   fixtures :developers, :posts, :comments
 
+  self.use_transactional_tests = false
+
   def test_default_scope
     expected = Developer.all.merge!(order: "salary DESC").to_a.collect(&:salary)
     received = DeveloperOrderedBySalary.all.collect(&:salary)