Return 406 Not Acceptable appropriately.

If there is no default content type and nothing matches in the Accept
header, previous versions would fail with an Internal Server Error
because of an unhandled exception.  Instead we should be returning
a "406 Not Acceptable".

docs/history.rst

@@ -4,6 +4,8 @@ Version History
 :compare:`Next <3.0.4...master>`
 --------------------------------
 - Add type annotations (see :ref:`type-info`)
+- Return a "406 Not Acceptable" if the :http:header:`Accept` header values cannot be matched
+  and there is no default content type configured
 
 :compare:`3.0.4 <3.0.3...3.0.4>` (2 Nov 2020)
 ---------------------------------------------

sprockets/mixins/mediatype/content.py

@@ -391,8 +391,14 @@ class ContentMixin(web.RequestHandler):
 
         """
         settings = get_settings(self.application, force_instance=True)
-        # TODO -- account for get_response_type returning None
-        handler = settings[self.get_response_content_type()]  # type: ignore
+        response_type = self.get_response_content_type()
+        if response_type is None:
+            self._logger.error('failed to find a suitable response '
+                               'content type for request')
+            self._logger.error('please set a default content type')
+            raise web.HTTPError(406)
+
+        handler = settings[response_type]
         content_type, data_bytes = handler.to_bytes(body)
         if set_content_type:
             self.set_header('Content-Type', content_type)

tests.py

@@ -61,8 +61,13 @@ def pack_bytes(payload):
 
 
 class SendResponseTests(testing.AsyncHTTPTestCase):
+    def setUp(self):
+        self.application = None
+        super().setUp()
+
     def get_app(self):
-        return examples.make_application()
+        self.application = examples.make_application()
+        return self.application
 
     def test_that_content_type_default_works(self):
         response = self.fetch('/',
@@ -129,6 +134,22 @@ class SendResponseTests(testing.AsyncHTTPTestCase):
         self.assertEqual(response.code, 200)
         self.assertEqual(response.headers['Content-Type'], 'expected/content')
 
+    def test_that_no_default_content_type_will_406(self):
+        # NB if the Accept header is omitted, then a default of `*/*` will
+        # be used which results in a match against any registered handler.
+        # Using an accept header forces the "no match" case.
+        settings = content.get_settings(self.application, force_instance=True)
+        settings.default_content_type = None
+        settings.default_encoding = None
+        response = self.fetch('/',
+                              method='POST',
+                              body='{}',
+                              headers={
+                                  'Accept': 'application/xml',
+                                  'Content-Type': 'application/json',
+                              })
+        self.assertEqual(response.code, 406)
+
 
 class GetRequestBodyTests(testing.AsyncHTTPTestCase):
     def setUp(self):