Issue #3405976 by alexpott, jrglasgow, fago, catch, mondrake, solideogloria,...

    $reflectionProperty = new \ReflectionProperty(TransactionManagerBase::class, 'connectionTransactionState');

    $reflectionProperty->setValue($manager, ClientConnectionTransactionState::Active);

    $this->expectException(\AssertionError::class);

    $this->expectExceptionMessageMatches("/^Transaction .stack was not empty\\. Active stack: bar\\\\qux/");

    // Ensure that __destruct() results in an assertion error. Note that this

    // will normally be called by PHP during the object's destruction but Drupal

    // will commit all transactions when a database is closed thereby making

    // this impossible to test with calling it directly.

    // this impossible to test unless it is called directly.

    try {

      $manager->__destruct();

      $this->fail("Expected AssertionError error not thrown");

    }

    catch (\AssertionError $e) {

      $this->assertStringStartsWith('Transaction $stack was not empty. Active stack: bar\\qux', $e->getMessage());

    }

    // Clean up.

    $reflectionProperty = new \ReflectionProperty(TransactionManagerBase::class, 'stack');

    $reflectionProperty->setValue($manager, []);

    unset($testConnection);

    Database::closeConnection('test_fail');

  }