CBuffered*Test.test_flush_error_on_close test failure investigation #3826

Issue

Reason for test: test_flush_error_on_close in CBufferedReaderTest, CBufferedWriterTest, CBufferedRandomTest

Finding

Buffered*.closed need to acquire lock to access raw

which makes b.closed(L1156) failed to execute because lock already acquired on b.close(L1160)

In cpython, it doesn't acquire lock in b.closed which makes them passing this test.
https://github.com/python/cpython/blob/774ef28814d0d9d57ec813cb31b0a7af6c476127/Modules/_io/bufferedio.c#L449-L461

IMO, accessing raw should acquire a lock (then cpython is wrong?), and the test scenario doesn't seem so common in real world use case. Maybe we shouldn't care about this failure test that much. But since I've already spent several night digging on this issue, I would like to post a issue describing this issue in case someone else also falling down on this rabbit hole.