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d587252ddb
A SIGTERM can show up at any point in the ioloop, even in places which
are not prepared to handle it. This results in the process ignoring
the `sys.exit` which the SIGTERM handler calls, with an uncaught
SystemExit exception:
```
2021-11-09 15:37:49.368 ERR [tornado.application:9803] Uncaught exception
Traceback (most recent call last):
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/http1connection.py", line 238, in _read_message
delegate.finish()
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/httpserver.py", line 314, in finish
self.delegate.finish()
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/routing.py", line 251, in finish
self.delegate.finish()
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 2097, in finish
self.execute()
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 2130, in execute
**self.path_kwargs)
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/gen.py", line 307, in wrapper
yielded = next(result)
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 1510, in _execute
result = method(*self.path_args, **self.path_kwargs)
File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/handlers.py", line 150, in get
request = self.convert_tornado_request_to_django_request()
File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/handlers.py", line 113, in convert_tornado_request_to_django_request
request = WSGIRequest(environ)
File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/django/core/handlers/wsgi.py", line 66, in __init__
script_name = get_script_name(environ)
File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/event_queue.py", line 611, in <lambda>
signal.signal(signal.SIGTERM, lambda signum, stack: sys.exit(1))
SystemExit: 1
```
Supervisor then terminates the process with a SIGKILL, which results
in dropping data held in the tornado process, as it does not dump its
queue.
The only command which is safe to run in the signal handler is
`ioloop.add_callback_from_signal`, which schedules the callback to run
during the course of the normal ioloop. This callbacks does an
orderly shutdown of the server and the ioloop before exiting.
(cherry picked from commit bc5539d871)