gh-150942: Speed up asyncio Future.remove_done_callback

[muhamedfazalps]

Summary

Use _PyList_AppendTakeRef instead of PyList_Append to avoid an extra incref/decref pair when compaction appends callbacks to the new list.

Fixes gh-150942

Motivation

The issue lists this as a possible performance improvement:

Modules/_asynciomodule.c:1213 | asyncio future done-callback compaction | _PyList_AppendTakeRef

The current code:

PyObject *item = PyList_GET_ITEM(self->fut_callbacks, i);
Py_INCREF(item);
ret = PyObject_RichCompareBool(PyTuple_GET_ITEM(item, 0), fn, Py_EQ);
if (ret == 0) {
    if (j < len) {
        PyList_SET_ITEM(newlist, j, item);
        j++;
        continue;
    }
    ret = PyList_Append(newlist, item);
}
Py_DECREF(item);

The item is already owned (Py_INCREF was called earlier), so PyList_Append increments the refcount and then Py_DECREF decrements it - an unnecessary pair.

Changes

PyObject *item = PyList_GET_ITEM(self->fut_callbacks, i);
Py_INCREF(item);
ret = PyObject_RichCompareBool(PyTuple_GET_ITEM(item, 0), fn, Py_EQ);
if (ret == 0) {
    if (j < len) {
        PyList_SET_ITEM(newlist, j, item);
        j++;
        continue;
    }
    ret = _PyList_AppendTakeRef((PyListObject *)newlist, item);
}
else {
    Py_DECREF(item);
}

_PyList_AppendTakeRef takes ownership of the reference without incrementing it, eliminating the unnecessary incref/decref pair.

Testing

• Existing asyncio tests pass
• The change is a performance optimization with no behavioral change

---

Discovered during open source research. https://buymeacoffee.com/muhamedfazalps

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[python-cla-bot]

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[picnixz]

Please, we need to see benchmarks and how it improved the resutl. JUst changing the code is not sufficient.

[picnixz]

See #150942 (comment).

[picnixz]

Please include benchmarks first on the issue, sign the CLA and add a NEWS entry.

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