exc_info on a successfully finished greenlet shouldn't raise. Fixes #821

src/gevent/greenlet.py

@@ -309,10 +309,16 @@ class Greenlet(greenlet):
 
     @property
     def exc_info(self):
-        """Holds the exc_info three-tuple raised by the function if the greenlet finished with an error.
-        Otherwise a false value."""
+        """
+        Holds the exc_info three-tuple raised by the function if the
+        greenlet finished with an error. Otherwise a false value.
+
+        .. note:: This is a provisional API and may change.
+
+        .. versionadded:: 1.1
+        """
         e = self._exc_info
-        if e:
+        if e and e[0] is not None:
             return (e[0], e[1], load_traceback(e[2]))
 
     def throw(self, *args):

src/greentest/test__greenlet.py

@@ -650,6 +650,14 @@ class TestBasic(greentest.TestCase):
     def test_kill_running_noblock(self):
         self._test_kill_running(block=False)
 
+    def test_exc_info_no_error(self):
+        # Before running
+        self.assertFalse(greenlet.Greenlet().exc_info)
+        g = greenlet.Greenlet(gevent.sleep)
+        g.start()
+        g.join()
+        self.assertFalse(g.exc_info)
+
 
 class TestStart(greentest.TestCase):