GC of unclosed io.BuffererdRWPair after `.readinto` results in unraisable exception

[cmaloney]

Bug report

Bug description:

In BufferedRWPairTest.test_readinto a BufferedRWPair is created but not explicitly closed which results in it being Garbage Collected (GC). During GC the destruction order often results in an unraisable exception. This can be reliably reproduced by modifying the test to end with support.gc_collect(). The BufferedRWPair should cleanup reliably in this case.

cpython/Lib/test/test_io/test_general.py

Lines 2281 to 2288 in 6bc65c3

	def test_readinto(self):
	for method in ("readinto", "readinto1"):
	with self.subTest(method):
	pair = self.tp(self.BytesIO(b"abcdef"), self.MockRawIO())
	data = byteslike(b'\0' * 5)
	self.assertEqual(getattr(pair, method)(data), 5)
	self.assertEqual(bytes(data), b"abcde")

Warning -- Unraisable exception
Exception ignored while finalizing file <_io.BufferedRWPair object at 0x7fd9d7577950>:
Traceback (most recent call last):
 File "<checkout_dir>/cpython/Lib/test/support/__init__.py", line 837, in gc_collect
 gc.collect()
 ~~~~~~~~~~^^
ValueError: flush of closed file
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.BufferedWriter>:
Traceback (most recent call last):
 File "<checkout_dir>/cpython/Lib/test/support/__init__.py", line 837, in gc_collect
 gc.collect()
 ~~~~~~~~~~^^
ValueError: flush of closed file

Found while working on GH-138475

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-138720: Make Buffered closed check match flush #138724
• gh-138013: Remove load_tests in test_io.test_general #138771