bpo-37261: Fix support.catch_unraisable_exception() (GH-14052)

The __exit__() method of test.support.catch_unraisable_exception
context manager now ignores unraisable exception raised when clearing
self.unraisable attribute.

Doc/library/test.rst

@@ -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:

Lib/test/support/__init__.py

@@ -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:
@@ -3058,6 +3070,8 @@ class catch_unraisable_exception:
         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):
@@ -3066,6 +3080,10 @@ class catch_unraisable_exception:
         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

Misc/NEWS.d/next/Tests/2019-06-13-12-19-56.bpo-37261.NuKFVo.rst

@@ -0,0 +1,3 @@
+Fix :func:`test.support.catch_unraisable_exception`: its __exit__() method
+now ignores unraisable exception raised when clearing its ``unraisable``
+attribute.