Fail gracefully when transcoder is missing.

If there is no transcoder for the default content type, sending a
response would fail with a "500 Internal Server Error" due to an
*unhandled* KeyError.  This commit catches the KeyError and produces
an explicit "500 Internal Server Error" with an appropriate error
message being logged.

docs/history.rst

@@ -7,6 +7,7 @@ Version History
 - Return a "406 Not Acceptable" if the :http:header:`Accept` header values cannot be matched
   and there is no default content type configured
 - Deprecate not having a default content type configured
+- Fail gracefully when a transcoder does not exist for the default content type
 
 :compare:`3.0.4 <3.0.3...3.0.4>` (2 Nov 2020)
 ---------------------------------------------

sprockets/mixins/mediatype/content.py

@@ -414,9 +414,17 @@ class ContentMixin(web.RequestHandler):
             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)
-            self.add_header('Vary', 'Accept')
-        self.write(data_bytes)
+        try:
+            handler = settings[response_type]
+        except KeyError:
+            self._logger.error(
+                'no transcoder for the selected response content type %s, '
+                'is the default content type %r correct?', response_type,
+                settings.default_content_type)
+            raise web.HTTPError(500)
+        else:
+            content_type, data_bytes = handler.to_bytes(body)
+            if set_content_type:
+                self.set_header('Content-Type', content_type)
+                self.add_header('Vary', 'Accept')
+            self.write(data_bytes)

tests.py

@@ -150,6 +150,15 @@ class SendResponseTests(testing.AsyncHTTPTestCase):
                               })
         self.assertEqual(response.code, 406)
 
+    def test_misconfigured_default_content_type(self):
+        settings = content.get_settings(self.application, force_instance=True)
+        settings.default_content_type = 'application/xml'
+        response = self.fetch('/',
+                              method='POST',
+                              body='{}',
+                              headers={'Content-Type': 'application/json'})
+        self.assertEqual(response.code, 500)
+
 
 class GetRequestBodyTests(testing.AsyncHTTPTestCase):
     def setUp(self):