diff -r 1ff29b57628e Lib/contextlib.py
--- a/Lib/contextlib.py	Wed Dec 02 14:37:35 2015 +0100
+++ b/Lib/contextlib.py	Wed Dec 02 14:59:08 2015 -0500
@@ -320,6 +320,8 @@
         frame_exc = sys.exc_info()[1]
         def _fix_exception_context(new_exc, old_exc):
             # Context may not be correct, so find the end of the chain
+            if new_exc is old_exc:
+                return
             while 1:
                 exc_context = new_exc.__context__
                 if exc_context is old_exc:
diff -r 1ff29b57628e Lib/test/test_contextlib.py
--- a/Lib/test/test_contextlib.py	Wed Dec 02 14:37:35 2015 +0100
+++ b/Lib/test/test_contextlib.py	Wed Dec 02 14:59:08 2015 -0500
@@ -762,6 +762,21 @@
         stack.push(cm)
         self.assertIs(stack._exit_callbacks[-1], cm)
 
+    def test_context_cycle(self):
+        # Fixes behaviour discovered in issue #25779.
+        @contextmanager
+        def generator_context_manager():
+            yield None
+
+        def main():
+            es = ExitStack()
+            with es:
+                es.enter_context(generator_context_manager())
+                raise RuntimeError
+
+        with self.assertRaises(RuntimeError):
+            main()
+
 
 class TestRedirectStream: