Connection leak if CONN_MAX_AGE == None

[cryptogun]

PostgreSQL default max connection is 100.
if CONN_MAX_AGE == None in setting.py:
Every time I authenticate() a user, a new connection is opened but not closed.
user = authenticate(request=request, username='admin', password='123456')

if CONN_MAX_AGE == 0:
The connection will be closed properly.

Traceback (most recent call last):
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\utils\autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\core\management\commands\runserver.py", line 120, in inner_run
    self.check_migrations()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\core\management\base.py", line 442, in check_migrations
    executor = MigrationExecutor(connections[DEFAULT_DB_ALIAS])
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\migrations\executor.py", line 18, in __init__
    self.loader = MigrationLoader(self.connection)
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\migrations\loader.py", line 49, in __init__
    self.build_graph()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\migrations\loader.py", line 212, in build_graph
    self.applied_migrations = recorder.applied_migrations()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\migrations\recorder.py", line 61, in applied_migrations
    if self.has_table():
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\migrations\recorder.py", line 44, in has_table
    return self.Migration._meta.db_table in self.connection.introspection.table_names(self.connection.cursor())
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\base\base.py", line 256, in cursor
    return self._cursor()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\base\base.py", line 233, in _cursor
    self.ensure_connection()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\base\base.py", line 217, in ensure_connection
    self.connect()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\base\base.py", line 217, in ensure_connection
    self.connect()
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\base\base.py", line 194, in connect
    self.connection = self.get_new_connection(conn_params)
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\django\db\backends\postgresql\base.py", line 178, in get_new_connection
    connection = Database.connect(**conn_params)
  File "C:\Users\me\AppData\Local\Programs\Python\Python36\lib\site-packages\psycopg2\__init__.py", line 126, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
django.db.utils.OperationalError: FATAL:  remaining connection slots are reserved for non-replication superuser connections

Bug reproduce (with demo project attached):

1. set CONN_MAX_AGE to None:

   # settings.py
   DATABASES = {
       'default': {
           'ENGINE': 'django.db.backends.sqlite3',
           'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
           'CONN_MAX_AGE': None,
       }
   }
   

2. Print and trace db connection:

Modify site-packages\django\db\backends\base\base.py

# site-packages\django\db\backends\base\base.py
# Add a print line:
    def ensure_connection(self):
        """Guarantee that a connection to the database is established."""
        if self.connection is None:
            with self.wrap_database_errors:
                print('open connection +++++++++++++++++++')
                self.connect()

# Add a print line:
    def close(self):
        """Close the connection to the database."""
        self.validate_thread_sharing()
        self.run_on_commit = []

        # Don't call validate_no_atomic_block() to avoid making it difficult
        # to get rid of a connection in an invalid state. The next connect()
        # will reset the transaction state anyway.
        if self.closed_in_transaction or self.connection is None:
            return
        try:
            print('close connection ......................')
            self._close()
        finally:
            if self.in_atomic_block:
                self.closed_in_transaction = True
                self.needs_rollback = True
            else:
                self.connection = None

3. Open a new incognito browser Ctrl + Shift + n.
4. Goto landing page.
5. Check Django log output.

open connection +++++++++++++++++++
No close log printed.

6. Goto 3. and try as many times as you want.
7. Switch to PostgreSQL backend, refresh 100 times, and you will get the above OperationalError.

8. Set CONN_MAX_AGE to 0 in settings.py.
9. Retry 3~5

Now the close was executed.

open connection +++++++++++++++++++
[04/May/2019 16:09:53] "GET / HTTP/1.1" 200 2
close connection ......................

[cryptogun]

Demo project.