Merge #20317 from 3.3

diff --cc Lib/contextlib.py
Simple merge

diff --cc Lib/test/test_contextlib.py
Simple merge

diff --cc Misc/NEWS
index 56c06ab36fe58ead66f5bbbfb0f828a80ba06836,a89762d85087775e74dea5b5607819d6d5ee2589..72ae4fa01d500faba002af142f2b4ba2437ef693
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -32,11 -32,30 +32,17 @@@ Core and Builtin
  Library
  -------
  
 +- Issue #20311: selector.PollSelector.select() 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.
 +
+ - 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.