gh-60107: Remove a copy from RawIOBase.read

If the underlying I/O class keeps a reference to the memory raise BufferError.

Author: cmaloney

Lib/test/test_io/test_general.py

@@ -724,6 +724,26 @@ def test_RawIOBase_readall(self):
         rawio = self.MockRawIOWithoutRead((b"abc", b"d", b"efg"))
         self.assertEqual(rawio.readall(), b"abcdefg")
 
+        # gh-60107: Ensure a "Raw I/O" which keeps a reference to the
+        # mutable memory doesn't allow making a mutable bytes.
+        class RawIOKeepsReference(self.MockRawIOWithoutRead):
+            def __init__(self, *args, **kwargs):
+                self.buf = None
+                super().__init__(*args, **kwargs)
+
+            def readinto(self, buf):
+                # buf is the bytearray so keeping a reference to it doesn't keep
+                # the memory alive; a memoryview does.
+                self.buf = memoryview(buf)
+                buf[0:4] = self._read_stack.pop()
+                return 3
+
+        with self.assertRaises(BufferError):
+            rawio = RawIOKeepsReference([b"1234"])
+            rawio.read(4)
+
+
+
     def test_BufferedIOBase_readinto(self):
         # Exercise the default BufferedIOBase.readinto() and readinto1()
         # implementations (which call read() or read1() internally).

Misc/NEWS.d/next/Library/2025-11-13-13-11-02.gh-issue-60107.LZq3QF.rst

@@ -0,0 +1,2 @@
+Remove a copy from :meth:`io.RawIOBase.read`. If the underlying I/O class
+keeps a reference to the mutable memory raise a :exc:`BufferError`.

Modules/_io/iobase.c

@@ -953,7 +953,18 @@ _io__RawIOBase_read_impl(PyObject *self, Py_ssize_t n)
         return NULL;
     }
 
-    res = PyBytes_FromStringAndSize(PyByteArray_AsString(b), bytes_filled);
+    res = PyObject_CallMethod(b, "resize", "i", bytes_filled);
+    if (res != Py_None) {
+        Py_DECREF(b);
+        if (res != NULL) {
+            PyErr_Format(PyExc_ValueError,
+                         "resize returned unexpected value %R",
+                        res);
+            Py_DECREF(res);
+        }
+        return res;
+    }
+    res = PyObject_CallMethod(b, "take_bytes", NULL);
     Py_DECREF(b);
     return res;
 }