Hide exceptions that occur during session.close() (#4891)

* Hide exceptions that occur during session.close()

This fixes #4840. Exceptions that are raised out of __del__ methods are caught
and printed to stderr. By catching any exceptions that occur, we now prevent
this from happening.

Alternative solutions to this would have been either not calling
session.close() at all or adding a close() method to acme.client.ClientNetwork,
acme.client.Client, and certbot.client.Client and using certbot.client.Client
in a context manager to ensure close() is called. The former means that users
of the ACME library never properly close their connections until their program
exits and the latter adds a lot of complexity and nesting of client code for
little benefit.

* Only catch Exceptions

acme/acme/client.py

@@ -519,7 +519,12 @@ class ClientNetwork(object):  # pylint: disable=too-many-instance-attributes
         self._default_timeout = timeout
 
     def __del__(self):
-        self.session.close()
+        # Try to close the session, but don't show exceptions to the
+        # user if the call to close() fails. See #4840.
+        try:
+            self.session.close()
+        except Exception:  # pylint: disable=broad-except
+            pass
 
     def _wrap_in_jws(self, obj, nonce):
         """Wrap `JSONDeSerializable` object in JWS.

acme/acme/client_test.py

@@ -600,12 +600,19 @@ class ClientNetworkTest(unittest.TestCase):
             mock.ANY, mock.ANY, verify=mock.ANY, headers=mock.ANY,
             timeout=45)
 
-    def test_del(self):
+    def test_del(self, close_exception=None):
         sess = mock.MagicMock()
+
+        if close_exception is not None:
+            sess.close.side_effect = close_exception
+
         self.net.session = sess
         del self.net
         sess.close.assert_called_once_with()
 
+    def test_del_error(self):
+        self.test_del(ReferenceError)
+
     @mock.patch('acme.client.requests')
     def test_requests_error_passthrough(self, mock_requests):
         mock_requests.exceptions = requests.exceptions