return 400 on bad mf2-json input, not 500

https://console.cloud.google.com/errors/COyl7MTulffpuAE

app.py

@@ -156,8 +156,11 @@ def fetch_mf2_func(url):
         _, doc = self._fetch(url)
         return mf2py.parse(doc=doc, url=url)
 
-      actor = microformats2.find_author(mf2, fetch_mf2_func=fetch_mf2_func)
-      title = microformats2.get_title(mf2)
+      try:
+        actor = microformats2.find_author(mf2, fetch_mf2_func=fetch_mf2_func)
+        title = microformats2.get_title(mf2)
+      except (KeyError, ValueError) as e:
+        raise exc.HTTPBadRequest('Could not parse %s as %s: %s' % (url, input, e))
 
     if input in ('as1', 'activitystreams'):
       activities = body_items

granary/microformats2.py

@@ -914,7 +914,7 @@ def find_author(parsed, **kwargs):
 
 
 def get_title(mf2):
-  """Returns the author of a page as a ActivityStreams actor dict.
+  """Returns an mf2 object's title, ie its name.
 
   Args:
     mf2: dict, parsed mf2 object (ie return value from mf2py.parse())

test_app.py

@@ -575,3 +575,27 @@ def test_hub(self):
     headers = resp.headers.getall('Link')
     self.assertIn('<http://a/hub>; rel="hub"', headers)
     self.assertIn('<%s>; rel="self"' % self_url, headers)
+
+
+  def test_bad_mf2_json_input_400s(self):
+    """If a user sends JSON Feed input, but claims it's mf2 JSON, return 400.
+
+    https://console.cloud.google.com/errors/COyl7MTulffpuAE
+    """
+    self.expect_requests_get('http://some/jf2', {
+      'data': {
+        'type': 'feed',
+        'items': [{
+          'type': 'entry',
+          'published': '2018-05-24T08:58:44+02:00',
+          'url': 'https://realize.be/notes/1463',
+          'content': {
+            'text': 'foo',
+            'html': '<p>bar</p>',
+        },
+      }],
+    }})
+    self.mox.ReplayAll()
+    resp = app.application.get_response('/url?url=http://some/jf2&input=mf2-json')
+    self.assert_equals(400, resp.status_int)
+