Handle huge single message email notifications

fedmsg_meta does some insane things in its msg2long_form[0]. Add a guard
to make sure any crazy msg2long_form implementation can't produce a
multi-{MB,GB,TB...ZB} message.

Signed-off-by: Jeremy Cline <jeremy@jcline.org>

CHANGELOG.rst

@@ -17,6 +17,9 @@ Bug fixes
 * Set digest content limits to 1k messages and 500k content length
   (`#280 <https://github.com/fedora-infra/fmn/pull/280>`_).
 
+* Limit single message emails to 500K characters to ensure multi-MB emails
+  don't get stuck in the message queue
+  (`#288 <https://github.com/fedora-infra/fmn/pull/288>`_).
 
 v2.0.2
 ======

fmn/formatters.py

@@ -412,6 +412,12 @@ def email(message, recipient):
     except Exception:
         _log.exception('fedmsg.meta.msg2link failed to handle %r', message)
 
+    if len(content) >= 500000:
+        # This email is enormous, too large to be sent. This happens if fedmsg_meta
+        # did something silly in its msg2long_form implementation.
+        content = (u'This message was too large to be sent!\n'
+                   u'The message ID was: {}\n\n').format(message['msg_id'])
+
     # Since we do simple text email, adding the footer to the content
     # before setting the payload.
     footer = config.app_conf.get('fmn.email.footer', u'')

fmn/tests/test_formatters.py

@@ -379,6 +379,29 @@ def test_email(self):
         actual = formatters.email(self.message, self.recipient)
         self.assertEqual(expected, actual)
 
+    @mock.patch('fmn.formatters.fedmsg.meta.msg2long_form', mock.Mock(return_value='a'*500000))
+    def test_email_too_big(self):
+        """Assert huge single message emails are handled gracefully."""
+        expected = (
+            'Precedence: Bulk\n'
+            'Auto-Submitted: auto-generated\n'
+            'From: notifications@fedoraproject.org\n'
+            'To: jeremy@jcline.org\n'
+            'X-Fedmsg-Topic: org.fedoraproject.dev.fmn.filter.update\n'
+            'X-Fedmsg-Category: fmn\n'
+            'X-Fedmsg-Username: jcline\n'
+            'X-Fedmsg-Num-Packages: 0\n'
+            'Subject: jcline updated the rules on a fmn email filter\n'
+            'MIME-Version: 1.0\n'
+            'Content-Type: text/plain; charset="utf-8"\n'
+            'Content-Transfer-Encoding: base64\n\n'
+            'VGhpcyBtZXNzYWdlIHdhcyB0b28gbGFyZ2UgdG8gYmUgc2VudCEKVGhlIG1lc3NhZ2UgSUQgd2Fz\n'
+            'OiAyMDE3LTZhYTcxZDViLWZiZTQtNDllNy1hZmRkLWFmY2YwZDIyODAyYgoK\n'
+        )
+
+        actual = formatters.email(self.message, self.recipient)
+        self.assertEqual(expected, actual)
+
     @mock.patch.dict('fmn.formatters.config.app_conf', {'fmn.email.subject_prefix': 'PREFIX: '})
     def test_subject_prefix(self):
         """Assert the subject prefix is added if configured."""