We use the lazylogger for something similar to this. You can specify the path to a custom script, and it will be run for each midas file that gets written:
https://midas.triumf.ca/MidasWiki/index.php/Lazylogger#Using_a_script
This means that you don't have to wait until the end of the run to start processing.

If the ROOT conversion is going to be slow, but you have a batch system available, you could use the lazylogger script to submit a job to the batch system for each file.

> 
> > Hi,
> > 
> > I was wondering if there is a "mechanism" to run an executable
> > file after each subrun is closed...
> > 
> > I need to convert .mid.lz4 subrun files to ROOT (TTree) files;
> > 
> > Thanks,
> > Gennaro