Closing SMTP backend raises AttributeError when already closed

[Sebastian Noack]

>>> from django.core.mail import get_connection
>>> smtp = get_connection('django.core.mail.backends.smtp.EmailBackend')
>>> smtp.close()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/sebastian/p24/src/placement24/placement24/django/core/mail/backends/smtp.py", line 66, in close
    self.connection.quit()
AttributeError: 'NoneType' object has no attribute 'quit'

The SMTP backend is the only mail backend that can't be closed without opening it. In fact the filebased backend contains code to explicitly handle that case. So the smtp backend should handle it as well.

If that is not reason enough, note that it is not always possible to know whether the connection is open and must be closed or not. For example, when open() was called and an SMTP connection was established, but the login failed, close() must be called even though open() raised an exception. However when the exception was raised while establishing the connection, close() would raise the AttributeError, shown above.

Therefore the close() method must just return in case the connection isn't open.

[Claude Paroz]

Tests are needed.

[Claude Paroz]

This is very close to #19134. There, I've given a clue about how to test it. Might be even worth fixing both tickets together.

[Claude Paroz <claude@…>]

In 5b8c0d22cc07ca727c032edf3309fab9cb39f21d:

[1.5.x] Fixed #19382 -- Stopped smtp backend raising exception when already closed

Thanks Sebastian Noack for the report and the initial patch.
Backport of ffa50ca35 from master.