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[3.8] bpo-37261: Fix support.catch_unraisable_exception() (GH-14052) #14056

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Merged
miss-islington merged 1 commit into from
Jun 13, 2019
Merged

[3.8] bpo-37261: Fix support.catch_unraisable_exception() (GH-14052) #14056

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12 changes: 12 additions & 0 deletions Doc/library/test.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1086,6 +1086,18 @@ The :mod:`test.support` module defines the following functions:
Context manager catching unraisable exception using
:func:`sys.unraisablehook`.

If the *object* attribute of the unraisable hook is set and the object is
being finalized, the object is resurrected because the context manager
stores a strong reference to it (``cm.unraisable.object``).

Storing the exception value (``cm.unraisable.exc_value``) creates a
reference cycle. The reference cycle is broken explicitly when the context
manager exits.

Exiting the context manager clears the stored unraisable exception. It can
trigger a new unraisable exception (ex: the resurrected object is finalized
again and raises the same exception): it is silently ignored in this case.

Usage::

with support.catch_unraisable_exception() as cm:
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22 changes: 20 additions & 2 deletions Lib/test/support/__init__.py
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Expand Up @@ -3040,6 +3040,18 @@ class catch_unraisable_exception:
"""
Context manager catching unraisable exception using sys.unraisablehook.

If the *object* attribute of the unraisable hook is set and the object is
being finalized, the object is resurrected because the context manager
stores a strong reference to it (cm.unraisable.object).

Storing the exception value (cm.unraisable.exc_value) creates a reference
cycle. The reference cycle is broken explicitly when the context manager
exits.

Exiting the context manager clears the stored unraisable exception. It can
trigger a new unraisable exception (ex: the resurrected object is finalized
again and raises the same exception): it is silently ignored in this case.

Usage:

with support.catch_unraisable_exception() as cm:
Expand All @@ -3058,6 +3070,8 @@ def __init__(self):
self._old_hook = None

def _hook(self, unraisable):
# Storing unraisable.object can resurrect an object which is being
# finalized. Storing unraisable.exc_value creates a reference cycle.
self.unraisable = unraisable

def __enter__(self):
Expand All @@ -3066,6 +3080,10 @@ def __enter__(self):
return self

def __exit__(self, *exc_info):
# Clear the unraisable exception to explicitly break a reference cycle
del self.unraisable
# Clear the unraisable exception to explicitly break a reference cycle.
# It can call _hook() again: ignore the new unraisable exception in
# this case.
self.unraisable = None

sys.unraisablehook = self._old_hook
del self.unraisable
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Tests/2019-06-13-12-19-56.bpo-37261.NuKFVo.rst
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@@ -0,0 +1,3 @@
Fix :func:`test.support.catch_unraisable_exception`: its __exit__() method
now ignores unraisable exception raised when clearing its ``unraisable``
attribute.