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

Lib/test/test_io/test_bufferedio.py

@@ -623,6 +623,28 @@ def test_bad_readinto_type(self):
             bufio.readline()
         self.assertIsInstance(cm.exception.__cause__, TypeError)
 
+    def test_read1_error_does_not_cause_reentrant_failure(self):
+        # 32-bit builds (e.g. win32) can raise OverflowError
+        # converting huge Python int to Py_ssize_t.
+        if sys.maxsize <= 2**32:
+            self.skipTest("requires 64-bit build")
+        # Under TSan, the process may abort on huge allocation
+        # attempts (exit code 66).
+        if support.check_sanitizer(thread=True):
+            self.skipTest("TSan aborts on huge allocations")
+
+        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)
+            huge = 10**18
+            with self.assertRaises(MemoryError):
+                bufio.read1(huge)
+
+            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 @@
+Fixed BufferedReader.read1() leaving the object in a reentrant state after an error.

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;
     }