Fixed #27368 -- Modifed BaseEmailBackend.__enter__() to close the connection if an exception occurs.

Fixes unclosed socket ResourceWarning in mail test.

Thanks Claude Paroz for the review.

django/core/mail/backends/base.py

@@ -40,7 +40,11 @@ class BaseEmailBackend(object):
         pass
 
     def __enter__(self):
+        try:
             self.open()
+        except Exception:
+            self.close()
+            raise
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):

tests/mail/tests.py

@@ -1289,11 +1289,9 @@ class SMTPBackendTests(BaseEmailBackendTests, SMTPBackendTestsBase):
         """
         backend = smtp.EmailBackend(
             username='not empty username', password='not empty password')
-        try:
         with self.assertRaisesMessage(SMTPException, 'SMTP AUTH extension not supported by server.'):
-                backend.open()
-        finally:
-            backend.close()
+            with backend:
+                pass
 
     def test_server_open(self):
         """
@@ -1315,7 +1313,8 @@ class SMTPBackendTests(BaseEmailBackendTests, SMTPBackendTestsBase):
 
         backend = CustomEmailBackend(username='username', password='password')
         with self.assertRaises(SMTPAuthenticationError):
-            backend.open()
+            with backend:
+                pass
 
     @override_settings(EMAIL_USE_TLS=True)
     def test_email_tls_use_settings(self):
@@ -1377,21 +1376,17 @@ class SMTPBackendTests(BaseEmailBackendTests, SMTPBackendTestsBase):
     def test_email_tls_attempts_starttls(self):
         backend = smtp.EmailBackend()
         self.assertTrue(backend.use_tls)
-        try:
         with self.assertRaisesMessage(SMTPException, 'STARTTLS extension not supported by server.'):
-                backend.open()
-        finally:
-            backend.close()
+            with backend:
+                pass
 
     @override_settings(EMAIL_USE_SSL=True)
     def test_email_ssl_attempts_ssl_connection(self):
         backend = smtp.EmailBackend()
         self.assertTrue(backend.use_ssl)
-        try:
         with self.assertRaises(SSLError):
-                backend.open()
-        finally:
-            backend.close()
+            with backend:
+                pass
 
     def test_connection_timeout_default(self):
         """Test that the connection's timeout value is None by default."""