gh-158316: Avoid second buffer fetch on bytearray_decode() (#158485)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: johng

Lib/test/test_bytes.py

@@ -18,6 +18,7 @@
 import textwrap
 import threading
 import unittest
+from _codecs import _unregister_error as _codecs_unregister_error
 
 import test.support
 from test import support
@@ -2195,6 +2196,30 @@ def delslice():
         self.assertRaises(BufferError, delslice)
         self.assertEqual(b, orig)
 
+    def test_decode_resize_forbidden(self):
+        # The storage is pinned while it is decoded, so an error handler
+        # cannot resize the bytearray.
+        b = bytearray(b'ab\xffcd')
+        errors = 'test.bytearray_decode_resize'
+        def handler(exc):
+            self.assertRaises(BufferError, b.clear)
+            self.assertRaises(BufferError, b.append, 0)
+            return ('?', exc.end)
+        self.addCleanup(_codecs_unregister_error, errors)
+        codecs.register_error(errors, handler)
+        for encoding in 'utf-8', 'utf-8-sig':
+            with self.subTest(encoding=encoding):
+                self.assertEqual(b.decode(encoding, errors), 'ab?cd')
+        self.assertEqual(b, b'ab\xffcd')
+
+    def test_decode_subclass_buffer(self):
+        # decode() decodes the buffer that the object exports.
+        class B(bytearray):
+            def __buffer__(self, flags):
+                return memoryview(b'other')
+        self.assertEqual(B(b'mine').decode(), 'other')
+        self.assertEqual(B(b'mine').decode('latin-1'), 'other')
+
     @test.support.cpython_only
     def test_obsolete_write_lock(self):
         _testcapi = import_helper.import_module('_testcapi')

Misc/NEWS.d/next/Core_and_Builtins/2026-09-30-10-12-41.gh-issue-158316.Yv3kQe.rst

@@ -0,0 +1,2 @@
+On Free Threading, speed up :meth:`bytearray.decode` by decoding the underlying
+storage directly instead of going through the buffer protocol.

Objects/bytearrayobject.c

@@ -2599,7 +2599,19 @@ bytearray_decode_impl(PyByteArrayObject *self, const char *encoding,
 {
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();
-    return PyUnicode_FromEncodedObject((PyObject*)self, encoding, errors);
+    if (Py_TYPE(self)->tp_as_buffer->bf_getbuffer != bytearray_getbuffer) {
+        /* A subclass may export a different buffer. */
+        return PyUnicode_FromEncodedObject((PyObject*)self, encoding, errors);
+    }
+
+    /* Decode the storage directly instead of exporting a buffer, which
+       would re-acquire the critical section we already hold.  Increase
+       exports to prevent the storage from changing during the decode. */
+    self->ob_exports++;
+    PyObject *res = PyUnicode_Decode(PyByteArray_AS_STRING(self),
+                                     Py_SIZE(self), encoding, errors);
+    self->ob_exports--;
+    return res;
 }
 
 PyDoc_STRVAR(alloc_doc,