system_update.py keeps file handles of stdout/stderr open after exit of group_system_updates.py

[mayrstefan]

We use group_system_updates.py to apply updates to our systems. This iterates through all systems of a group and calls system_update.py to schedule updates for each system.
After the update is scheduled group_system_updates.py exits but system_update.py is still running with file handles for stdout/stderr open. But system_update.py is no more a child process of the shell that called group_system_update.py. This is a situation that is not easy to handle. It should be one of the two options:

1. Real async: system_update should not share file handles with group_system_updates
2. Sync: group_system_updates should wait until all system_update process have closed their handles

For the async case it would great to have an alternative way to find when all scheduled updates are finished

[mayrstefan]

Our current workaround is the following: we have a sleep loop that checks if any process is sharing stdout with our current shell. When no more processes are left using the same file descriptor we know the updates have finished.