django.db.utils.InterfaceError: connection already closed

[michelematera]

Hello!
Stack:

• Ubuntu (20.04 LTS)
• Nginx
• Postgresql (v13.3)

An AWS load balancer sends traffic to the Ubuntu instance(k8s cluster), which is handled by Nginx, which forwards on to Django (4.0.3) running in gunicorn (19.9.0). Django connects to the database using psycopg2 (2.8.6).
The issue I have is that the database connection seems to shut down randomly. Django reports errors like this:

InterfaceError: connection already closed
  File "django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "contextlib.py", line 79, in inner
    return func(*args, **kwds)
  File "django/views/generic/base.py", line 84, in view
    return self.dispatch(request, *args, **kwargs)
  File "django/utils/decorators.py", line 46, in _wrapper
    return bound_method(*args, **kwargs)
  File "django/views/decorators/cache.py", line 62, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "django/views/generic/base.py", line 119, in dispatch
    return handler(request, *args, **kwargs)
  File "django/views/generic/detail.py", line 108, in get
    self.object = self.get_object()
  File "django/views/generic/detail.py", line 53, in get_object
    obj = queryset.get()
  File "django/db/models/query.py", line 492, in get
    num = len(clone)
  File "django/db/models/query.py", line 302, in __len__
    self._fetch_all()
  File "django/db/models/query.py", line 1507, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "django/db/models/query.py", line 57, in __iter__
    results = compiler.execute_sql(
  File "django/db/models/sql/compiler.py", line 1359, in execute_sql
    cursor = self.connection.cursor()
  File "django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
  File "django/db/backends/base/base.py", line 284, in cursor
    return self._cursor()
  File "django/db/backends/base/base.py", line 262, in _cursor
    return self._prepare_cursor(self.create_cursor(name))
  File "django/db/utils.py", line 91, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "django/db/backends/base/base.py", line 262, in _cursor
    return self._prepare_cursor(self.create_cursor(name))
  File "django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
  File "django/db/backends/postgresql/base.py", line 256, in create_cursor
    cursor = self.connection.cursor()

Postgresql does not report any major errors: I can only assume that the connection was closed correctly and Django was not expecting it.
This happens quite rarely, but enough to be a concern: once every 3 days.
Any suggestions of how to investigate this further would be gratefully accepted. Thanks in advance!

[Mariusz Felisiak]

Duplicate of #33085.