test_io leaks references on AMD64 Fedora Rawhide Refleaks 3.8:https://buildbot.python.org/all/#/builders/229/builds/10test_io leaked [23208, 23204, 23208] references, sum=69620test_io leaked [7657, 7655, 7657] memory blocks, sum=22969The issue has been introduced by my change:commitc15a682603a47f5aef5025f6a2e3babb699273d6Author: Victor Stinner <vstinner@redhat.com>Date:   Thu Jun 13 00:23:49 2019 +0200bpo-37223: test_io: silence destructor errors (GH-14031)        *bpo-18748: Fix _pyio.IOBase destructor (closed case) (GH-13952)        _pyio.IOBase destructor now does nothing if getting the closed    attribute fails to better mimick _io.IOBase finalizer.        (cherry picked from commit4f6f7c5a611905fb6b81671547f268c226bc646a)        *bpo-37223: test_io: silence destructor errors (GH-13954)        Implement also MockNonBlockWriterIO.seek() method.        (cherry picked from commitb589cef9c4dada2fb84ce0fae5040ecf16d9d5ef)        *bpo-37223, test_io: silence last 'Exception ignored in:' (GH-14029)        Use catch_unraisable_exception() to ignore 'Exception ignored in:'    error when the internal BufferedWriter of the BufferedRWPair is    destroyed. The C implementation doesn't give access to the    internal BufferedWriter, so just ignore the warning instead.        (cherry picked from commit913fa1c8245d1cde6edb4254f4fb965cc91786ef)It seems like the root issue is the usage of catch_unraisable_exception() in test_io.class catch_unraisable_exception:    def __init__(self):        self.unraisable = None        self._old_hook = None    def _hook(self, unraisable):        self.unraisable = unraisable    def __enter__(self):        self._old_hook = sys.unraisablehook        sys.unraisablehook = self._hook        return self    def __exit__(self, *exc_info):        # Clear the unraisable exception to explicitly break a reference cycle        del self.unraisable        sys.unraisablehook = self._old_hook*Sometimes* "del self.unraisable" of __exit__() triggers a new unraisable exception which calls again the _hook() method which sets again the unraisable attribute.catch_unraisable_exception resurects _io.BufferedWriter objects, and then "del self.unraisable" calls again their finalizer: iobase_finalize() is called again, and iobase_finalize() calls PyErr_WriteUnraisable() on close() failure.

gdb traceback on "del self.unraisable":(gdb) where#0  _PyErr_WriteUnraisableDefaultHook (    args=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <traceback at remote 0x7fffe9cfb5f0>, None, <_io.BufferedWriter at remote 0x7fffe9bcb890>)) atPython/errors.c:1294#1  0x00000000005726ec in sys_unraisablehook (module=<module at remote 0x7fffeac10710>,     unraisable=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <traceback at remote 0x7fffe9cfb5f0>, None, <_io.BufferedWriter at remote 0x7fffe9bcb890>)) at ./Python/sysmodule.c:702#2  0x0000000000435741 in _PyMethodDef_RawFastCallKeywords (method=0x7bb2e0 <sys_methods+1216>, self=<module at remote 0x7fffeac10710>, args=0x7ffffffd1d00,     nargs=1, kwnames=0x0) atObjects/call.c:650#3  0x00000000004359f4 in _PyCFunction_Vectorcall (func=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00,     nargsf=1, kwnames=0x0) atObjects/call.c:737#4  0x0000000000534666 in _PyObject_Vectorcall (callable=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00,     nargsf=1, kwnames=0x0) at ./Include/cpython/abstract.h:127#5  0x00000000005346b1 in _PyObject_FastCall (func=<built-in method unraisablehook of module object at remote 0x7fffeac10710>, args=0x7ffffffd1d00, nargs=1)    at ./Include/cpython/abstract.h:147#6  0x0000000000536da4 in _PyErr_WriteUnraisableMsg (err_msg_str=0x0, obj=<_io.BufferedWriter at remote 0x7fffe9bcb890>) atPython/errors.c:1379#7  0x0000000000536ee6 in PyErr_WriteUnraisable (obj=<_io.BufferedWriter at remote 0x7fffe9bcb890>) atPython/errors.c:1425#8  0x00000000005eb97d in iobase_finalize (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at ./Modules/_io/iobase.c:299#9  0x0000000000479fd2 in PyObject_CallFinalizer (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) atObjects/object.c:307#10 0x000000000047a044 in PyObject_CallFinalizerFromDealloc (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) atObjects/object.c:325#11 0x00000000005eb9e0 in _PyIOBase_finalize (self=<_io.BufferedWriter at remote 0x7fffe9bcb890>) at ./Modules/_io/iobase.c:320#12 0x00000000005f27d3 in buffered_dealloc (self=0x7fffe9bcb890) at ./Modules/_io/bufferedio.c:387#13 0x000000000047ebff in _Py_Dealloc (op=<_io.BufferedWriter at remote 0x7fffe9bcb890>) atObjects/object.c:2213#14 0x00000000005f210a in _Py_DECREF (filename=0x71eef3 "./Modules/_io/bufferedio.c", lineno=2138, op=<_io.BufferedWriter at remote 0x7fffe9bcb890>)    at ./Include/object.h:478#15 0x00000000005f716a in bufferedrwpair_dealloc (self=0x7fffe9a90d70) at ./Modules/_io/bufferedio.c:2138#16 0x000000000047ebff in _Py_Dealloc (op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>) atObjects/object.c:2213#17 0x000000000048d1b6 in _Py_DECREF (filename=0x69fe40 "./Include/object.h", lineno=541, op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>)    at ./Include/object.h:478#18 0x000000000048d1e2 in _Py_XDECREF (op=<_io.BufferedRWPair at remote 0x7fffe9a90d70>) at ./Include/object.h:541# del UnraisableHookArgs#19 0x000000000048d569 in structseq_dealloc (obj=0x7fffe9ad1360) atObjects/structseq.c:85#20 0x000000000047ebff in _Py_Dealloc (    op=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>))    atObjects/object.c:2213#21 0x0000000000463be6 in _Py_DECREF (filename=0x696dc9 "Objects/dictobject.c", lineno=1585,     op=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>))    at ./Include/object.h:478#22 0x0000000000468428 in delitem_common (mp=0x7fffe9b17950, hash=-822296730167156087, ix=0,     old_value=(<type at remote 0x7a9fa0>, ValueError('flush of closed file',), <unknown at remote 0x7fffe9a8ad70>, None, <_io.BufferedRWPair at remote 0x7fffe9a90d70>)) atObjects/dictobject.c:1585#23 0x0000000000468663 in _PyDict_DelItem_KnownHash (op={'_old_hook': <built-in method unraisablehook of module object at remote 0x7fffeac10710>},     key='unraisable', hash=-822296730167156087) atObjects/dictobject.c:1637#24 0x00000000004684e5 in PyDict_DelItem (op={'_old_hook': <built-in method unraisablehook of module object at remote 0x7fffeac10710>}, key='unraisable')    atObjects/dictobject.c:1603# del self.unraisable#25 0x000000000046ef21 in _PyObjectDict_SetItem (tp=0xa58830, dictptr=0x7fffe9a8ad30, key='unraisable', value=0x0) atObjects/dictobject.c:4614

New changeset6d22cc8e90ccb1e1965b1a4bc79456e2cc1e5a3e by Victor Stinner in branch 'master':bpo-37261: Fix support.catch_unraisable_exception() (GH-14052)https://github.com/python/cpython/commit/6d22cc8e90ccb1e1965b1a4bc79456e2cc1e5a3e

New changesetb4f5b212535e75503fc33513676837089037bb48 by Miss Islington (bot) in branch '3.8':bpo-37261: Fix support.catch_unraisable_exception() (GH-14052)https://github.com/python/cpython/commit/b4f5b212535e75503fc33513676837089037bb48

I tested manually "./python -m test test_io -m CBufferedRWPairTest -R 3:3": it failed before the fix, it now pass on 3.8 and master with the fix. I close the issue.Enjoy resurrected objects...

New changeset212646cae6b7c4ddc8d98c8b9b6d39a5f259e864 by Victor Stinner in branch 'master':bpo-37261: Document sys.unraisablehook corner cases (GH-14059)https://github.com/python/cpython/commit/212646cae6b7c4ddc8d98c8b9b6d39a5f259e864

New changeset3b976d19c8c09e83eec63a5b62daf4d55bfd6aeb by Miss Islington (bot) in branch '3.8':bpo-37261: Document sys.unraisablehook corner cases (GH-14059)https://github.com/python/cpython/commit/3b976d19c8c09e83eec63a5b62daf4d55bfd6aeb