Gracefully handle transcoder encoding failures.

Since the form encoder refuses to handle nested sequences, the content
mixin explicitly handles this case instead of letting the unhandled
exception bubble up.

docs/history.rst

@@ -9,6 +9,8 @@ Version History
   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
+- Fail gracefully when a transcoder raises a :exc:`TypeError` or :exc:`ValueError` when encoding
+  the response
 
 .. _application/x-www-formurlencoded: https://url.spec.whatwg.org/#application/x-www-form-urlencoded
 

sprockets/mixins/mediatype/content.py

@@ -423,8 +423,15 @@ class ContentMixin(web.RequestHandler):
                 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)
+            try:
+                content_type, data_bytes = handler.to_bytes(body)
+            except (TypeError, ValueError) as e:
+                self._logger.error(
+                    'selected transcoder (%s) failed to encode response '
+                    'body: %s', handler.__class__.__name__, e)
+                raise web.HTTPError(500, reason='Response Encoding Failure')
+            else:
+                if set_content_type:
+                    self.set_header('Content-Type', content_type)
+                    self.add_header('Vary', 'Accept')
+                self.write(data_bytes)

tests.py

@@ -177,6 +177,35 @@ class SendResponseTests(testing.AsyncHTTPTestCase):
         self.assertEqual('application/foo+json',
                          response.headers.get('Content-Type'))
 
+    def test_that_transcoder_failures_result_in_500(self):
+        class FailingTranscoder:
+            content_type = 'application/vnd.com.example.bad'
+
+            def __init__(self):
+                self.exc_class = TypeError
+
+            def to_bytes(self, inst_data, encoding=None):
+                raise self.exc_class('I always fail at this')
+
+            def from_bytes(self, data_bytes, encoding=None):
+                return {}
+
+        transcoder = FailingTranscoder()
+        content.add_transcoder(self.application, transcoder)
+        for _ in range(2):
+            response = self.fetch(
+                '/',
+                method='POST',
+                body=b'{}',
+                headers={
+                    'Accept': 'application/vnd.com.example.bad',
+                    'Content-Type': 'application/json',
+                },
+            )
+            self.assertEqual(500, response.code)
+            self.assertEqual('Response Encoding Failure', response.reason)
+            transcoder.exc_class = ValueError
+
 
 class GetRequestBodyTests(testing.AsyncHTTPTestCase):
     def setUp(self):