lint: Require access_message to access Message objects.

tools/lint-all

@@ -344,6 +344,11 @@ def build_custom_checkers(by_lang):
          '''Do not call self.client directly for put/patch/post/get.
     See WRAPPER_COMMENT in test_helpers.py for details.
     '''},
+        # Directly fetching Message objects in e.g. views code is often a security bug.
+        {'pattern': '[^r][M]essage.objects.get',
+         'exclude': set(["zerver/tests", "zerver/worker/queue_processors.py"]),
+         'description': 'Please use access_message() to fetch Message objects',
+        },
         # This rule might give false positives in virtualenv setup files which should be excluded,
         # and comments which should be rewritten to avoid use of "python2", "python3", etc.
         {'pattern': 'python[23]',