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

Lib/test/test_io/test_bufferedio.py

@@ -10,7 +10,7 @@
 from collections import deque, UserList
 from itertools import cycle, count
 from test import support
-from test.support import os_helper, threading_helper
+from test.support import check_sanitizer, os_helper, threading_helper
 from .utils import byteslike, CTestCase, PyTestCase
 
 
@@ -623,6 +623,25 @@ def test_bad_readinto_type(self):
  bufio.readline()
  self.assertIsInstance(cm.exception.__cause__, TypeError)
 
+ @unittest.skipUnless(sys.maxsize > 2**32, 'requires 64bit platform')
+ @unittest.skipIf(check_sanitizer(thread=True),
+ 'ThreadSanitizer aborts on huge allocations (exit code 66).')
+ def test_read1_error_does_not_cause_reentrant_failure(self):
+ self.addCleanup(os_helper.unlink, os_helper.TESTFN)
+ with self.open(os_helper.TESTFN, "wb") as f:
+ f.write(b"hello")
+
+ with self.open(os_helper.TESTFN, "rb", buffering=0) as raw:
+ bufio = self.tp(raw, buffer_size=8)
+ # To request a size that is far too huge to ever be satisfied,
+ # so that the internal buffer allocation reliably fails with MemoryError.
+ huge = sys.maxsize // 2 + 1
+ with self.assertRaises(MemoryError):
+ bufio.read1(huge)
+
+ # Used to crash before gh-143689:
+ self.assertEqual(bufio.read1(1), b"h")
+
 
 class PyBufferedReaderTest(BufferedReaderTest, PyTestCase):
  tp = pyio.BufferedReader

Misc/NEWS.d/next/Library/2026-01-11-14-14-19.gh-issue-143689.fzHJ2W.rst

@@ -0,0 +1 @@
+Fix :meth:`io.BufferedReader.read1` state cleanup on buffer allocation failure.

Modules/_io/bufferedio.c

@@ -1073,6 +1073,7 @@ _io__Buffered_read1_impl(buffered *self, Py_ssize_t n)
 
  PyBytesWriter *writer = PyBytesWriter_Create(n);
  if (writer == NULL){
+ LEAVE_BUFFERED(self)
  return NULL;
  }