_io.BufferedReader.read1() leaves object in reentrant state after MemoryError

[hyongtao-code]

Bug report

Bug description:

BufferedReader.read1() can leave the object in a broken (reentrant) state after an allocation failure.

Reproducer

import io
import sys

def repro():
 br = io.BufferedReader(io.BytesIO(b"hello world"), buffer_size=8)

 huge = 10**18
 try:
 br.read1(huge)
 except Exception as e:
 print("first call exception:", type(e).__name__, e)

 try:
 x = br.read1(1)
 print("second call ok:", x)
 except Exception as e:
 print("second call exception:", type(e).__name__, e)

 br2 = io.BufferedReader(io.BytesIO(b"hello world"), buffer_size=8)
 print("fresh object read1:", br2.read1(1))

if __name__ == "__main__":
 print("python:", sys.version)
 repro()

Wrong behavior

D:\MyCode\cpython\PCbuild\amd64>python_d.exe py_iobuffer.py
python: 3.15.0a3+ (heads/main-dirty:78e868fa28, Jan 11 2026, 12:40:52) [MSC v.1950 64 bit (AMD64)]
first call exception: MemoryError
second call exception: RuntimeError reentrant call inside <_io.BufferedReader>
fresh object read1: b'h'
Exception ignored while finalizing file <_io.BufferedReader>:
Traceback (most recent call last):
 File "D:\MyCode\cpython\PCbuild\amd64\py_iobuffer.py", line 24, in <module>
 repro()
 ~~~~~^^
RuntimeError: reentrant call inside <_io.BufferedReader>

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-143689: Fix BufferedReader.read1 leaving object in reentrant state on error #143690