gh-159739: Handle overlapping buffers in BytesIO.readinto (#159777)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: jelle-openai

Lib/test/test_io/test_memoryio.py

@@ -962,6 +962,12 @@ class CBytesIOTest(PyBytesIOTest):
     ioclass = io.BytesIO
     UnsupportedOperation = io.UnsupportedOperation
 
+    def test_readinto_overlapping_buffer(self):
+        with self.ioclass(b'abcd') as stream:
+            with stream.getbuffer()[1:] as target:
+                self.assertEqual(stream.readinto(target), 3)
+            self.assertEqual(stream.getvalue(), b'aabc')
+
     def test_getstate(self):
         memio = self.ioclass()
         state = memio.__getstate__()

Misc/NEWS.d/next/Library/2026-10-19-20-38-17.gh-issue-159739.a81270.rst

@@ -0,0 +1 @@
+Fix :class:`io.BytesIO` reads into overlapping views of the same buffer.

Modules/_io/bytesio.c

@@ -714,7 +714,7 @@ _io_BytesIO_readinto_impl(bytesio *self, Py_buffer *buffer)
 
     assert(self->pos + len <= PY_SSIZE_T_MAX);
     assert(len >= 0);
-    memcpy(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
+    memmove(buffer->buf, PyBytes_AS_STRING(self->buf) + self->pos, len);
     self->pos += len;
 
     return PyLong_FromSsize_t(len);