gh-141311: Avoid assertion in BytesIO readinto

Lib/test/test_io/test_memoryio.py

@@ -12,6 +12,7 @@
 import pickle
 import sys
 import weakref
+from test.support.import_helper import import_module
 
 class IntLike:
     def __init__(self, num):
@@ -552,6 +553,17 @@ def test_relative_seek(self):
         memio.seek(1, 1)
         self.assertEqual(memio.read(), buf[1:])
 
+    def test_issue141311(self):
+        _testcapi = import_module("_testcapi")
+
+        memio = self.ioclass()
+        # Seek allows PY_SSIZE_T_MAX, read handle that.
+        # Past end of buffer read should always return 0 (EOF).
+        self.assertEqual(_testcapi.PY_SSIZE_T_MAX,
+                         memio.seek(_testcapi.PY_SSIZE_T_MAX))
+        buf = bytearray(2)
+        self.assertEqual(0, memio.readinto(buf))
+
     def test_unicode(self):
         memio = self.ioclass()
 

Misc/NEWS.d/next/Library/2025-11-09-18-55-13.gh-issue-141311.qZ3swc.rst

@@ -0,0 +1,3 @@
+Fix assertion failure in :class:`io.BytesIO` implementation of
+:func:`~io.BufferedIOBase.readinto` when the current offset is at the max
+offset and readinto is called.

Modules/_io/bytesio.c

@@ -609,8 +609,9 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
     n = self->string_size - self->pos;
     if (len > n) {
         len = n;
-        if (len < 0)
-            len = 0;
+        if (len < 0) {
+            return PyLong_FromSsize_t(0);
+        }
     }
 
     assert(self->pos + len < PY_SSIZE_T_MAX);