log failure properly, add test

src/allmydata/frontends/magic_folder.py

@@ -681,7 +681,7 @@ class Uploader(QueueMixin):
             return self._real_notify(opaque, path, events_mask)
         except Exception as e:
             self._log(u"error calling _real_notify: {}".format(e))
-            return Failure()
+            twlog.err(Failure())
 
     def _real_notify(self, opaque, path, events_mask):
         self._log("inotify event %r, %r, %r\n" % (opaque, path, ', '.join(self._inotify.humanReadableMask(events_mask))))

src/allmydata/test/test_magic_folder.py

@@ -1,6 +1,7 @@
 
 import os, sys, time
 import shutil, json
+import mock
 from os.path import join, exists
 
 from twisted.trial import unittest
@@ -1553,6 +1554,35 @@ class SingleMagicFolderTestMixin(MagicFolderCLITestMixin, ShouldFailMixin, Reall
         for item0, item1 in zip(upstatus0, upstatus1):
             self.assertEqual(item0, item1)
 
+    @defer.inlineCallbacks
+    def test_real_notify_failure(self):
+
+        def bad_stuff(*args, **kw):
+            # the function we replaced would trigger this hook, so we
+            # have to or things will stall
+            self.magicfolder.uploader._call_hook(u"foo", "inotify")
+            raise RuntimeError("the bad stuff")
+
+        patch_notify = mock.patch.object(
+            self.magicfolder.uploader,
+            '_real_notify',
+            mock.Mock(side_effect=bad_stuff),
+        )
+        with patch_notify:
+            path0 = os.path.join(self.local_dir, u'foo')
+            yield self.fileops.write(path0, 'foo\n')
+
+        # do a reactor turn; this is necessary because our "bad_stuff"
+        # method calls the hook (so the above 'yield' resumes) right
+        # *before* it raises the exception; thus, we ensure all the
+        # pending callbacks including the exception are processed
+        # before we flush the errors.
+        yield task.deferLater(reactor, 0, lambda: None)
+        errors = self.flushLoggedErrors()
+
+        self.assertEqual(1, len(errors))
+        self.assertIn("the bad stuff", str(errors[0]))
+
     @defer.inlineCallbacks
     def test_delete_and_restore(self):
         # setup: create a file