CSVSink throws if `m_directory` not provided

What

Currently, CSVSink does not appear to absolutely require that the field "directory" be defined in the datasinks configuration:

void CSVSink::setConfiguration(const nlohmann::json& config)
{
  for (const auto &kv : config.items())
    {
      if(kv.key()=="directory")
        {
          m_directory=kv.value();
        }
    }
}

And m_directory is initialized as the empty string ("").

Later on, in CSVSink::startMeasurement the output filename is formed as:

 std::string fileName = m_directory+"/"+m_measurement+".csv";

This will fail if the "directory" field is not provided in the "datasinks" configuration since only the root user has access to /. That is, the default filename is /foo.csv which cannot be accessed by a non-root user.

Suggested Action

1. Make "directory" a required field

By making the "directory" field required, then in CSVSink::setConfiguration we can check that the directory is readable/writeable and valid. If the "directory" field is not provided in the configuration, then the code should throw an exception letting the user know that the CSVSink configuration does not respect the expected schema.

2. Make m_directory default to a well-known valid directory

If the "directory" field in the JSON configuration for the CSVSink class is going to not be required, then it should be safe to not provide it. By defining a "default" directory that is guaranteed to always be readable/writeable by the user this will work. For example, by setting m_directory = "./" by default then things should be OK and the output files will by default be written to in the caller's current working directory.