Issue #20317: Don't create a reference loop in ExitStack

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index f8e026b..f878285 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -231,11 +231,19 @@ class ExitStack(object):
         # we were actually nesting multiple with statements
         frame_exc = sys.exc_info()[1]
         def _fix_exception_context(new_exc, old_exc):
+            # Context isn't what we want, so find the end of the chain
             while 1:
                 exc_context = new_exc.__context__
-                if exc_context in (None, frame_exc):
+                if exc_context is old_exc:
+                    # Context is already set correctly (see issue 20317)
+                    return
+                if exc_context is None or exc_context is frame_exc:
                     break
+                details = id(new_exc), id(old_exc), id(exc_context)
+                raise Exception(str(details))
                 new_exc = exc_context
+            # Change the end of the chain to point to the exception
+            # we expect it to reference
             new_exc.__context__ = old_exc
 
         # Callbacks are invoked in LIFO order to match the behaviour of