Issue #20317: Don't create a reference loop in ExitStack
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 @@
# 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
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index 9e45f70..e5365f7 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -600,6 +600,29 @@
else:
self.fail("Expected KeyError, but no exception was raised")
+ def test_exit_exception_with_correct_context(self):
+ # http://bugs.python.org/issue20317
+ @contextmanager
+ def gets_the_context_right():
+ try:
+ yield 6
+ finally:
+ 1 / 0
+
+ # The contextmanager already fixes the context, so prior to the
+ # fix, ExitStack would try to fix it *again* and get into an
+ # infinite self-referential loop
+ try:
+ with ExitStack() as stack:
+ stack.enter_context(gets_the_context_right())
+ stack.enter_context(gets_the_context_right())
+ stack.enter_context(gets_the_context_right())
+ except ZeroDivisionError as exc:
+ self.assertIsInstance(exc.__context__, ZeroDivisionError)
+ self.assertIsInstance(exc.__context__.__context__, ZeroDivisionError)
+ self.assertIsNone(exc.__context__.__context__.__context__)
+
+
def test_body_exception_suppress(self):
def suppress_exc(*exc_details):
return True
diff --git a/Misc/NEWS b/Misc/NEWS
index 66f5220..a89762d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -50,6 +50,12 @@
Library
-------
+- Issue #20317: ExitStack.__exit__ could create a self-referential loop if an
+ exception raised by a cleanup operation already had its context set
+ correctly (for example, by the @contextmanager decorator). The infinite
+ loop this caused is now avoided by checking if the expected context is
+ already set before trying to fix it.
+
- Issue #20311: select.epoll.poll() now rounds the timeout away from zero,
instead of rounding towards zero. For example, a timeout of one microsecond
is now rounded to one millisecond, instead of being rounded to zero.