Here's the scenario:

DB::beginTransaction();
DB::insert(); // QueryException thrown

This causes the following to happen

1. Connection::tryAgainIfCausedByLostConnection() is called inside Connection::run()
2. Connection::reconnect() is called inside tryAgainIfCausedByLostConnection()
3. DatabaseManager::reconnect() is called
4. DatabaseManager::disconnect() is called inside DatabaseManager::reconnect()
5. Connection::disconnect() is called inside DatabaseManager::disconnect()
6. Connection::setPdo(null) is called inside Connection::disconnect()

Inside setPDO():

if ($this->transactions >= 1) {
    throw new RuntimeException("Can't swap PDO instance while within transaction.");
}

So while there's a transaction open a retry can never happen, the result of this behaviour is that a RuntimeException is thrown hiding the QueryException which contains useful information about the problem.

This PR checks if there's a transaction open when a QueryException is thrown, if so it just throws the exception again for developers to handle it properly and never retries the query because it'll fail anyway.