bpo-36829: Add sys.unraisablehook() (GH-13187)

Add new sys.unraisablehook() function which can be overridden to
control how "unraisable exceptions" are handled. It is called when an
exception has occurred but there is no way for Python to handle it.
For example, when a destructor raises an exception or during garbage
collection (gc.collect()).

Changes:

* Add an internal UnraisableHookArgs type used to pass arguments to
  sys.unraisablehook.
* Add _PyErr_WriteUnraisableDefaultHook().
* The default hook now ignores exception on writing the traceback.
* test_sys now uses unittest.main() to automatically discover tests:
  remove test_main().
* Add _PyErr_Init().
* Fix PyErr_WriteUnraisable(): hold a strong reference to sys.stderr
  while using it

1 files changed, 10 insertions, 0 deletions

diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 5f8208d5bf..63b8200a15 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -481,6 +481,16 @@ and manipulating normal distributions of a random variable.
     [7.672102882379219, 12.000027119750287, 4.647488369766392]
 
 
+sys
+---
+
+Add new :func:`sys.unraisablehook` function which can be overridden to control
+how "unraisable exceptions" are handled. It is called when an exception has
+occurred but there is no way for Python to handle it. For example, when a
+destructor raises an exception or during garbage collection
+(:func:`gc.collect`).
+
+
 tarfile
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