[mypyc] Fix list.pop primitive on free-threaded builds (#19522)

The old implementation caused segfaults on free-threaded builds. Provide
a slower but working implementation for free-threaded builds. Also
improve test coverage, since it used to be spotty.

Tested on a manually compiled Python 3.14.0b4 free-threaded build on
macOS.

Author: JukkaL

mypyc/lib-rt/list_ops.c

@@ -235,19 +235,51 @@ void CPyList_SetItemUnsafe(PyObject *list, Py_ssize_t index, PyObject *value) {
     PyList_SET_ITEM(list, index, value);
 }
 
-PyObject *CPyList_PopLast(PyObject *obj)
+#ifdef Py_GIL_DISABLED
+// The original optimized list.pop implementation doesn't work on free-threaded
+// builds, so provide an alternative that is a bit slower but works.
+//
+// Note that this implementation isn't intended to be atomic.
+static inline PyObject *list_pop_index(PyObject *list, Py_ssize_t index) {
+    PyObject *item = PyList_GetItemRef(list, index);
+    if (item == NULL) {
+        return NULL;
+    }
+    if (PySequence_DelItem(list, index) < 0) {
+        Py_DECREF(item);
+        return NULL;
+    }
+    return item;
+}
+#endif
+
+PyObject *CPyList_PopLast(PyObject *list)
 {
+#ifdef Py_GIL_DISABLED
+    // The other implementation causes segfaults on a free-threaded Python 3.14b4 build.
+    Py_ssize_t index = PyList_GET_SIZE(list) - 1;
+    return list_pop_index(list, index);
+#else
     // I tried a specalized version of pop_impl for just removing the
     // last element and it wasn't any faster in microbenchmarks than
     // the generic one so I ditched it.
-    return list_pop_impl((PyListObject *)obj, -1);
+    return list_pop_impl((PyListObject *)list, -1);
+#endif
 }
 
 PyObject *CPyList_Pop(PyObject *obj, CPyTagged index)
 {
     if (CPyTagged_CheckShort(index)) {
         Py_ssize_t n = CPyTagged_ShortAsSsize_t(index);
+#ifdef Py_GIL_DISABLED
+        // We must use a slower implementation on free-threaded builds.
+        if (n < 0) {
+            n += PyList_GET_SIZE(obj);
+        }
+        return list_pop_index(obj, n);
+#else
         return list_pop_impl((PyListObject *)obj, n);
+#endif
     } else {
         PyErr_SetString(PyExc_OverflowError, CPYTHON_LARGE_INT_ERRMSG);
         return NULL;

mypyc/primitives/list_ops.py

@@ -219,7 +219,7 @@
 )
 
 # list.pop(index)
-list_pop = method_op(
+method_op(
     name="pop",
     arg_types=[list_rprimitive, int_rprimitive],
     return_type=object_rprimitive,

mypyc/test-data/run-lists.test

@@ -150,7 +150,9 @@ print(primes(13))
 \[0, 0, 1, 1]
 \[0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1]
 
-[case testListBuild]
+[case testListPrimitives]
+from testutil import assertRaises
+
 def test_list_build() -> None:
     # Currently LIST_BUILDING_EXPANSION_THRESHOLD equals to 10
     # long list built by list_build_op
@@ -169,9 +171,6 @@ def test_list_build() -> None:
     l3.append('a')
     assert l3 == ['a']
 
-[case testListPrims]
-from typing import List
-
 def test_append() -> None:
     l = [1, 2]
     l.append(10)
@@ -189,10 +188,28 @@ def test_pop_last() -> None:
 
 def test_pop_index() -> None:
     l = [1, 2, 10, 3]
-    l.pop(2)
+    assert l.pop(2) == 10
     assert l == [1, 2, 3]
-    l.pop(-2)
+    assert l.pop(-2) == 2
     assert l == [1, 3]
+    assert l.pop(-2) == 1
+    assert l.pop(0) == 3
+    assert l == []
+    l = [int() + 1000, int() + 1001, int() + 1002]
+    assert l.pop(0) == 1000
+    assert l.pop(-1) == 1002
+    assert l == [1001]
+
+def test_pop_index_errors() -> None:
+    l = [int() + 1000]
+    with assertRaises(IndexError):
+        l.pop(1)
+    with assertRaises(IndexError):
+        l.pop(-2)
+    with assertRaises(OverflowError):
+        l.pop(1 << 100)
+    with assertRaises(OverflowError):
+        l.pop(-(1 << 100))
 
 def test_count() -> None:
     l = [1, 3]