pythongh-157048: check for buffer errors before mutating in BytesIO.__init__ (pythonGH-157049)

(cherry picked from commit d125f00)

Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>

Author: ngoldbaum

Lib/test/test_memoryio.py

@@ -923,6 +923,18 @@ def test_cow_mutable(self):
         memio = self.ioclass(ba)
         self.assertEqual(sys.getrefcount(ba), old_rc)
 
+    def test_write_with_export(self):
+        memio = self.ioclass(b"abcd")
+        memio.seek(2)
+        with memio.getbuffer() as view:
+            self.assertRaises(BufferError, memio.__init__, b"replacement")
+            self.assertEqual(memio.tell(), 2)
+            self.assertEqual(memio.getvalue(), b"abcd")
+            self.assertEqual(bytes(view), b"abcd")
+        memio.write(b"X")
+        self.assertEqual(memio.getvalue(), b"abXd")
+
+
 class CStringIOTest(PyStringIOTest):
     ioclass = io.StringIO
     UnsupportedOperation = io.UnsupportedOperation

Misc/NEWS.d/next/Library/2026-09-06-15-23-24.gh-issue-157048.UtFyiJ.rst

@@ -0,0 +1,2 @@
+Preserve the position and contents of :class:`io.BytesIO` when
+:meth:`!BytesIO.__init__` fails because a buffer is exported.

Modules/_io/bytesio.c

@@ -1078,15 +1078,16 @@ static int
 _io_BytesIO___init___impl(bytesio *self, PyObject *initvalue)
 /*[clinic end generated code: output=65c0c51e24c5b621 input=3da5a74ee4c4f1ac]*/
 {
-    /* In case, __init__ is called multiple times. */
-    self->string_size = 0;
-    self->pos = 0;
-
     if (FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) > 0) {
         PyErr_SetString(PyExc_BufferError,
                         "Existing exports of data: object cannot be re-sized");
         return -1;
     }
+
+    /* In case, __init__ is called multiple times. */
+    self->string_size = 0;
+    self->pos = 0;
+
     if (initvalue && initvalue != Py_None) {
         if (PyBytes_CheckExact(initvalue)) {
             Py_XSETREF(self->buf, Py_NewRef(initvalue));