## Reproducer
Importing the `XRootD.client` after `ROOT` and opening a file causes XRootD to deadlock when the `python` exits:
```python
import ROOT
import XRootD.client
f = ROOT.TFile.Open("root://eospublic.cern.ch//eos/opendata/lhcb/AntimatterMatters2017/data/B2HHH_MagnetDown.root")
tree = f.Get("DecayTree")
```
## Cause
The cause is the `XRootD` `atexit` handler stopping the job manager:
https://github.com/xrootd/xrootd/blob/246ae390e877f5d40fa04c88b87c96f28b698001/bindings/python/libs/client/finalize.py#L51
Meanwhile ROOT's `atexit` handler which [deletes all objects and implicitly closes any open files]9https://github.com/root-project/root/blob/8844e28d7792900932b826c37f755504cb75067f/bindings/pyroot/pythonizations/python/ROOT/__init__.py#L106).
This means that if `ROOT` is imported before `XRootD.client` the `ROOT` cleanup will run after the job manager has been destroyed and the `kXR_close` message will never be picked up (`atexit` handlers are ran in the reverse registeration order):
```log
[2021-11-08 10:08:53.333384 +0100][Debug ][JobMgr ] Stopping the job manager...
[2021-11-08 10:08:53.333414 +0100][Dump ][JobMgr ] Stopping worker #0...
[2021-11-08 10:08:53.333619 +0100][Dump ][JobMgr ] Worker #0 stopped
[2021-11-08 10:08:53.333631 +0100][Dump ][JobMgr ] Stopping worker #1...
[2021-11-08 10:08:53.333784 +0100][Dump ][JobMgr ] Worker #1 stopped
[2021-11-08 10:08:53.333796 +0100][Dump ][JobMgr ] Stopping worker #2...
[2021-11-08 10:08:53.333875 +0100][Dump ][JobMgr ] Worker #2 stopped
[2021-11-08 10:08:53.333880 +0100][Debug ][JobMgr ] Job manager stopped
[2021-11-08 10:08:53.333893 +0100][Debug ][TaskMgr ] Stopping the task manager...
[2021-11-08 10:08:53.333977 +0100][Debug ][TaskMgr ] Task manager stopped
[2021-11-08 10:08:53.333982 +0100][Debug ][Poller ] Stopping the poller...
[2021-11-08 10:08:53.334189 +0100][Debug ][File ] [0x7a2ebca0@root://eospublic.cern.ch:1094//eos/opendata/lhcb/AntimatterMatters2017/data/B2HHH_MagnetDown.root?xrdcl.requuid=a4583384-30b5-4e69-98a0-792e29621a65] Sending a close command for handle 0x0 to st-096-ff761117.cern.ch:1095
[2021-11-08 10:08:53.334206 +0100][Dump ][XRootD ] [st-096-ff761117.cern.ch:1095] Sending message kXR_close (handle: 0x00000000)
[2021-11-08 10:08:53.334216 +0100][Debug ][ExDbgMsg ] [st-096-ff761117.cern.ch:1095] MsgHandler created: 0x7ae787e0 (message: kXR_close (handle: 0x00000000) ).
[2021-11-08 10:08:53.334236 +0100][Dump ][PostMaster ] [st-096-ff761117.cern.ch:1095] Sending message kXR_close (handle: 0x00000000) (0x7ae61610) through substream 0 expecting answer at 0
# The process is now deadlocked:
```
To see this more easily you can run this in an interactive (i)python REPL:
```python
# Add printouts each time a atexit handler is registered
import atexit
orig_register = atexit.register
def my_register(func, *args, **kwargs):
print("Registering atexit handler from", getattr(func, "__module__", "Unknown"), ":", func, args, kwargs)
return orig_register(func, *args, **kwargs)
atexit.register = my_register
# Open a ROOT file but don't clos it
import ROOT
import XRootD.client
f = ROOT.TFile.Open("root://eospublic.cern.ch//eos/opendata/lhcb/AntimatterMatters2017/data/B2HHH_MagnetDown.root")
tree = f.Get("DecayTree")
# Calling f.Close() here would prevent the deadlock
# Run the relvant parts of the atexit handlers by hand to make the issue clearer
from pyxrootd import client
import libROOTPythonizations
# Would be ran as part of XRootD.client's exit handler
client.__XrdCl_Stop_Threads()
# Would be ran as part of ROOT's exit handler
# This line will deadlock
libROOTPythonizations.ClearProxiedObjects()
```
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