This was found when writing the session module for tlctl.
If we in the VSM server call specify that we want to verify the sessions but our filtering doesn't return any sessions (e.g. no sessions running or no matching username), VSM server will hang since we never call xmlrpc's handle_response().
We should add some handling for this corner case.
This can actually happen in practice with tlwebadm:
* Open the details for a user, getting the URL https://example.com:1010/status/sessions?details=user1
* Kill the session for that user
* Refresh the page
This just stalls as you end up with the request in this here bug.
I found one more scenario in webadmin where this hang can happen in the wild:
1. Open the details for a user (same way as in comment 4)
2. Kill the session for the user, e.g. using tlctl.
3. Click terminate session with the confirmation checkbox ticked.
4. Notice the page being hanged.