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Hi,
During the development of Python 3.4, I tried to emit warnings if a file is destroyed without being explicitly closed. The warnings were not emited in threads during Python finalization. Here are related changes:
- Issue #19421: "FileIO destructor imports indirectly the io module at exit"
- Issue #19424: "_warnings: patch to avoid conversions from/to UTF-8"
- Issue #19442: "Python crashes when a warning is emitted during shutdown"
- Change in Python shutdown: issue #19466 "Clear state of threads earlier in Python shutdown"
- Regression => issue #20526
The change #19466 had to be reverted a few days before the release of Python 3.4.0 because it caused the regression #20526.
I'm still not convinced that #20526 was a new bug. IMO the bug still exists, but it is just less likely without the change #19466.
There is still something wrong in Python finalization, so I open this issue to rework it.
The goal is to get warnings in test_4_daemon_threads() of test_threading. I attached the test as a Python script.

Benjamin changed ThreadState_DeleteCurrent() to fix an issue in this part of the code, but then reverted his change:
"revert tstate_delete_common, since it's pretty much wrong"
http://hg.python.org/cpython/rev/3ce746735b99
(Yeah this part of the code is really tricky, likely to deadlock.)

Charles-François Natali wrote a general comment about daemon threads:
http://bugs.python.org/issue19466#msg206028 

This issue was more a reminder for myself (TODO). I'm no more interesting to work on it, so I close it.
The Python finalization is very fragile. I prefer to not touch it again :-p