gevent.hub.LoopExit: This operation would block forever

After running the program normally for a period of time, an error suddenly occurs：
```
Traceback (most recent call last):
File "/opt/calamari/venv/bin/cthulhu-manager", line 9, in
load_entry_point('calamari-cthulhu==0.1', 'console_scripts', 'cthulhu-manager')()
File "/opt/calamari/venv/lib/python2.7/site-packages/calamari_cthulhu-0.1-py2.7.egg/cthulhu/manager/manager.py", line 1156, in main
complete.wait(timeout=1)
File "/opt/calamari/venv/lib/python2.7/site-packages/gevent/event.py", line 77, in wait
result = self.hub.switch()
File "/opt/calamari/venv/lib/python2.7/site-packages/gevent/hub.py", line 338, in switch
return greenlet.switch(self)
gevent.hub.LoopExit: This operation would block forever
```
Has anyone encountered the same problem before? How to solve it?
```[tasklist]
### Tasks
```

Start at cthulhu/manager/manager.py line 1156, where complete.wait(timeout=1) appears in the traceback, then trace the call through gevent.event.Event.wait and gevent.hub.LoopExit. Reproduce the failure after the manager has run for a period and determine the condition that leaves the event loop with no runnable work. Done means the cause and a verified resolution are documented.

Written by the indexing model from the issue text.