bpo-17852: Maintain a list of BufferedWriter objects. Flush them on exit. (#1908)

* Maintain a list of BufferedWriter objects.  Flush them on exit.

In Python 3, the buffer and the underlying file object are separate
and so the order in which objects are finalized matters.  This is
unlike Python 2 where the file and buffer were a single object and
finalization was done for both at the same time.  In Python 3, if
the file is finalized and closed before the buffer then the data in
the buffer is lost.

This change adds a doubly linked list of open file buffers.  An atexit
hook ensures they are flushed before proceeding with interpreter
shutdown.  This is addition does not remove the need to properly close
files as there are other reasons why buffered data could get lost during
finalization.

Initial patch by Armin Rigo.

* Use weakref.WeakSet instead of WeakKeyDictionary.

* Simplify buffered double-linked list types.

* In _flush_all_writers(), suppress errors from flush().

* Remove NEWS entry, use blurb.

1 files changed, 2 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-04-12-46-25.bpo-17852.OxAtCg.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-04-12-46-25.bpo-17852.OxAtCg.rst
new file mode 100644
index 0000000..185664c
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-04-12-46-25.bpo-17852.OxAtCg.rst
@@ -0,0 +1,2 @@
+Maintain a list of open buffered files, flush them before exiting the
+interpreter.  Based on a patch from Armin Rigo.