gh-60107: avoid io.RawIOBase.read from reading into a temporary bytearray

Misc/NEWS.d/next/Library/2025-09-07-11-30-53.gh-issue-60107.TFrtr-.rst

@@ -0,0 +1,2 @@
+Avoid :meth:`io.RawIOBase.read` from reading into a temporary :class:`bytearray`
+when calling its own :meth:`io.RawIOBase.readinto` internally.

Modules/_io/iobase.c

@@ -921,19 +921,25 @@ static PyObject *
 _io__RawIOBase_read_impl(PyObject *self, Py_ssize_t n)
 /*[clinic end generated code: output=6cdeb731e3c9f13c input=b6d0dcf6417d1374]*/
 {
-    PyObject *b, *res;
+    PyObject *b, *res, *mv;
 
     if (n < 0) {
         return PyObject_CallMethodNoArgs(self, &_Py_ID(readall));
     }
 
-    /* TODO: allocate a bytes object directly instead and manually construct
-       a writable memoryview pointing to it. */
-    b = PyByteArray_FromStringAndSize(NULL, n);
+    b = PyBytes_FromStringAndSize(NULL, n);
     if (b == NULL)
         return NULL;
 
-    res = PyObject_CallMethodObjArgs(self, &_Py_ID(readinto), b, NULL);
+    mv = PyMemoryView_FromMemory(PyBytes_AS_STRING(b), n, PyBUF_WRITE);
+    if (mv == NULL) {
+        Py_DECREF(b);
+        return NULL;
+    }
+
+    res = PyObject_CallMethodObjArgs(self, &_Py_ID(readinto), mv, NULL);
+    Py_DECREF(mv);
+
     if (res == NULL || res == Py_None) {
         Py_DECREF(b);
         return res;
@@ -946,9 +952,12 @@ _io__RawIOBase_read_impl(PyObject *self, Py_ssize_t n)
         return NULL;
     }
 
-    res = PyBytes_FromStringAndSize(PyByteArray_AsString(b), n);
-    Py_DECREF(b);
-    return res;
+    if (_PyBytes_Resize(&b, n) < 0) {
+        Py_DECREF(b);
+        return NULL;
+    }
+
+    return b;
 }