gh-127065: Make methodcaller thread-safe and re-entrant (GH-127746)

The function `operator.methodcaller` was not thread-safe since the additional
of the vectorcall method in gh-89013. In the free threading build the issue
is easy to trigger, for the normal build harder.

This makes the `methodcaller` safe by:

* Replacing the lazy initialization with initialization in the constructor.
* Using a stack allocated space for the vectorcall arguments and falling back
  to `tp_call` for calls with more than 8 arguments.

4 files changed, 131 insertions, 96 deletions

diff --git a/Lib/test/test_free_threading/test_methodcaller.py b/Lib/test/test_free_threading/test_methodcaller.py
new file mode 100644
index 0000000..8846b00
--- /dev/null
+++ b/Lib/test/test_free_threading/test_methodcaller.py
@@ -0,0 +1,33 @@
+import unittest
+from threading import Thread
+from test.support import threading_helper
+from operator import methodcaller
+
+
+class TestMethodcaller(unittest.TestCase):
+    def test_methodcaller_threading(self):
+        number_of_threads = 10
+        size = 4_000
+
+        mc = methodcaller("append", 2)
+
+        def work(mc, l, ii):
+            for _ in range(ii):
+                mc(l)
+
+        worker_threads = []
+        lists = []
+        for ii in range(number_of_threads):
+            l = []
+            lists.append(l)
+            worker_threads.append(Thread(target=work, args=[mc, l, size]))
+        for t in worker_threads:
+            t.start()
+        for t in worker_threads:
+            t.join()
+        for l in lists:
+            assert len(l) == size
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Lib/test/test_operator.py b/Lib/test/test_operator.py
index 812d464..82578a0 100644
--- a/Lib/test/test_operator.py
+++ b/Lib/test/test_operator.py
@@ -482,6 +482,8 @@ class OperatorTestCase:
                 return f
             def baz(*args, **kwds):
                 return kwds['name'], kwds['self']
+            def return_arguments(self, *args, **kwds):
+                return args, kwds
         a = A()
         f = operator.methodcaller('foo')
         self.assertRaises(IndexError, f, a)
@@ -498,6 +500,17 @@ class OperatorTestCase:
         f = operator.methodcaller('baz', name='spam', self='eggs')
         self.assertEqual(f(a), ('spam', 'eggs'))
 
+        many_positional_arguments = tuple(range(10))
+        many_kw_arguments = dict(zip('abcdefghij', range(10)))
+        f = operator.methodcaller('return_arguments', *many_positional_arguments)
+        self.assertEqual(f(a), (many_positional_arguments, {}))
+
+        f = operator.methodcaller('return_arguments', **many_kw_arguments)
+        self.assertEqual(f(a), ((), many_kw_arguments))
+
+        f = operator.methodcaller('return_arguments', *many_positional_arguments, **many_kw_arguments)
+        self.assertEqual(f(a), (many_positional_arguments, many_kw_arguments))
+
     def test_inplace(self):
         operator = self.module
         class C(object):
diff --git a/Misc/NEWS.d/next/Library/2024-12-01-22-28-41.gh-issue-127065.tFpRer.rst b/Misc/NEWS.d/next/Library/2024-12-01-22-28-41.gh-issue-127065.tFpRer.rst
new file mode 100644
index 0000000..03d6953
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-12-01-22-28-41.gh-issue-127065.tFpRer.rst
@@ -0,0 +1 @@
+Make :func:`operator.methodcaller` thread-safe and re-entrant safe.
diff --git a/Modules/_operator.c b/Modules/_operator.c
index 6c19451..ce3ef01 100644
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -1595,78 +1595,75 @@ static PyType_Spec attrgetter_type_spec = {
 typedef struct {
     PyObject_HEAD
     PyObject *name;
-    PyObject *xargs; // reference to arguments passed in constructor
+    PyObject *args;
     PyObject *kwds;
-    PyObject **vectorcall_args;  /* Borrowed references */
+    PyObject *vectorcall_args;
     PyObject *vectorcall_kwnames;
     vectorcallfunc vectorcall;
 } methodcallerobject;
 
-#ifndef Py_GIL_DISABLED
-static int _methodcaller_initialize_vectorcall(methodcallerobject* mc)
-{
-    PyObject* args = mc->xargs;
-    PyObject* kwds = mc->kwds;
-
-    Py_ssize_t nargs = PyTuple_GET_SIZE(args);
-    assert(nargs > 0);
-    mc->vectorcall_args = PyMem_Calloc(
-        nargs + (kwds ? PyDict_Size(kwds) : 0),
-        sizeof(PyObject*));
-    if (!mc->vectorcall_args) {
-        PyErr_NoMemory();
-        return -1;
-    }
-    /* The first item of vectorcall_args will be filled with obj later */
-    if (nargs > 1) {
-        memcpy(mc->vectorcall_args, PySequence_Fast_ITEMS(args),
-            nargs * sizeof(PyObject*));
-    }
-    if (kwds) {
-        const Py_ssize_t nkwds = PyDict_Size(kwds);
-
-        mc->vectorcall_kwnames = PyTuple_New(nkwds);
-        if (!mc->vectorcall_kwnames) {
-            return -1;
-        }
-        Py_ssize_t i = 0, ppos = 0;
-        PyObject* key, * value;
-        while (PyDict_Next(kwds, &ppos, &key, &value)) {
-            PyTuple_SET_ITEM(mc->vectorcall_kwnames, i, Py_NewRef(key));
-            mc->vectorcall_args[nargs + i] = value; // borrowed reference
-            ++i;
-        }
-    }
-    else {
-        mc->vectorcall_kwnames = NULL;
-    }
-    return 1;
-}
 
+#define _METHODCALLER_MAX_ARGS 8
 
 static PyObject *
-methodcaller_vectorcall(
-        methodcallerobject *mc, PyObject *const *args, size_t nargsf, PyObject* kwnames)
+methodcaller_vectorcall(methodcallerobject *mc, PyObject *const *args,
+        size_t nargsf, PyObject* kwnames)
 {
     if (!_PyArg_CheckPositional("methodcaller", PyVectorcall_NARGS(nargsf), 1, 1)
         || !_PyArg_NoKwnames("methodcaller", kwnames)) {
         return NULL;
     }
-    if (mc->vectorcall_args == NULL) {
-        if (_methodcaller_initialize_vectorcall(mc) < 0) {
-            return NULL;
-        }
-    }
+    assert(mc->vectorcall_args != NULL);
+
+    PyObject *tmp_args[_METHODCALLER_MAX_ARGS];
+    tmp_args[0] = args[0];
+    assert(1 + PyTuple_GET_SIZE(mc->vectorcall_args) <= _METHODCALLER_MAX_ARGS);
+    memcpy(tmp_args + 1, _PyTuple_ITEMS(mc->vectorcall_args), sizeof(PyObject *) * PyTuple_GET_SIZE(mc->vectorcall_args));
 
-    assert(mc->vectorcall_args != 0);
-    mc->vectorcall_args[0] = args[0];
-    return PyObject_VectorcallMethod(
-            mc->name, mc->vectorcall_args,
-            (PyTuple_GET_SIZE(mc->xargs)) | PY_VECTORCALL_ARGUMENTS_OFFSET,
+    return PyObject_VectorcallMethod(mc->name, tmp_args,
+            (1 + PyTuple_GET_SIZE(mc->args)) | PY_VECTORCALL_ARGUMENTS_OFFSET,
             mc->vectorcall_kwnames);
 }
-#endif
 
+static int
+_methodcaller_initialize_vectorcall(methodcallerobject* mc)
+{
+    PyObject* args = mc->args;
+    PyObject* kwds = mc->kwds;
+
+    if (kwds && PyDict_Size(kwds)) {
+        PyObject *values = PyDict_Values(kwds);
+        if (!values) {
+            return -1;
+        }
+        PyObject *values_tuple = PySequence_Tuple(values);
+        Py_DECREF(values);
+        if (!values_tuple) {
+            return -1;
+        }
+        if (PyTuple_GET_SIZE(args)) {
+            mc->vectorcall_args = PySequence_Concat(args, values_tuple);
+            Py_DECREF(values_tuple);
+            if (mc->vectorcall_args == NULL) {
+                return -1;
+            }
+        }
+        else {
+            mc->vectorcall_args = values_tuple;
+        }
+        mc->vectorcall_kwnames = PySequence_Tuple(kwds);
+        if (!mc->vectorcall_kwnames) {
+            return -1;
+        }
+    }
+    else {
+        mc->vectorcall_args = Py_NewRef(args);
+        mc->vectorcall_kwnames = NULL;
+    }
+
+    mc->vectorcall = (vectorcallfunc)methodcaller_vectorcall;
+    return 0;
+}
 
 /* AC 3.5: variable number of arguments, not currently support by AC */
 static PyObject *
@@ -1694,25 +1691,30 @@ methodcaller_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     if (mc == NULL) {
         return NULL;
     }
+    mc->vectorcall = NULL;
+    mc->vectorcall_args = NULL;
+    mc->vectorcall_kwnames = NULL;
+    mc->kwds = Py_XNewRef(kwds);
 
     Py_INCREF(name);
     PyInterpreterState *interp = _PyInterpreterState_GET();
     _PyUnicode_InternMortal(interp, &name);
     mc->name = name;
 
-    mc->xargs = Py_XNewRef(args); // allows us to use borrowed references
-    mc->kwds = Py_XNewRef(kwds);
-    mc->vectorcall_args = 0;
-
+    mc->args = PyTuple_GetSlice(args, 1, PyTuple_GET_SIZE(args));
+    if (mc->args == NULL) {
+        Py_DECREF(mc);
+        return NULL;
+    }
 
-#ifdef Py_GIL_DISABLED
-    // gh-127065: The current implementation of methodcaller_vectorcall
-    // is not thread-safe because it modifies the `vectorcall_args` array,
-    // which is shared across calls.
-    mc->vectorcall = NULL;
-#else
-    mc->vectorcall = (vectorcallfunc)methodcaller_vectorcall;
-#endif
+    Py_ssize_t vectorcall_size = PyTuple_GET_SIZE(args)
+                                   + (kwds ? PyDict_Size(kwds) : 0);
+    if (vectorcall_size < (_METHODCALLER_MAX_ARGS)) {
+        if (_methodcaller_initialize_vectorcall(mc) < 0) {
+            Py_DECREF(mc);
+            return NULL;
+        }
+    }
 
     PyObject_GC_Track(mc);
     return (PyObject *)mc;
@@ -1722,13 +1724,10 @@ static void
 methodcaller_clear(methodcallerobject *mc)
 {
     Py_CLEAR(mc->name);
-    Py_CLEAR(mc->xargs);
+    Py_CLEAR(mc->args);
     Py_CLEAR(mc->kwds);
-    if (mc->vectorcall_args != NULL) {
-        PyMem_Free(mc->vectorcall_args);
-        mc->vectorcall_args = 0;
-        Py_CLEAR(mc->vectorcall_kwnames);
-    }
+    Py_CLEAR(mc->vectorcall_args);
+    Py_CLEAR(mc->vectorcall_kwnames);
 }
 
 static void
@@ -1745,8 +1744,10 @@ static int
 methodcaller_traverse(methodcallerobject *mc, visitproc visit, void *arg)
 {
     Py_VISIT(mc->name);
-    Py_VISIT(mc->xargs);
+    Py_VISIT(mc->args);
     Py_VISIT(mc->kwds);
+    Py_VISIT(mc->vectorcall_args);
+    Py_VISIT(mc->vectorcall_kwnames);
     Py_VISIT(Py_TYPE(mc));
     return 0;
 }
@@ -1765,15 +1766,7 @@ methodcaller_call(methodcallerobject *mc, PyObject *args, PyObject *kw)
     if (method == NULL)
         return NULL;
 
-
-    PyObject *cargs = PyTuple_GetSlice(mc->xargs, 1, PyTuple_GET_SIZE(mc->xargs));
-    if (cargs == NULL) {
-        Py_DECREF(method);
-        return NULL;
-    }
-
-    result = PyObject_Call(method, cargs, mc->kwds);
-    Py_DECREF(cargs);
+    result = PyObject_Call(method, mc->args, mc->kwds);
     Py_DECREF(method);
     return result;
 }
@@ -1791,7 +1784,7 @@ methodcaller_repr(methodcallerobject *mc)
     }
 
     numkwdargs = mc->kwds != NULL ? PyDict_GET_SIZE(mc->kwds) : 0;
-    numposargs = PyTuple_GET_SIZE(mc->xargs) - 1;
+    numposargs = PyTuple_GET_SIZE(mc->args);
     numtotalargs = numposargs + numkwdargs;
 
     if (numtotalargs == 0) {
@@ -1807,7 +1800,7 @@ methodcaller_repr(methodcallerobject *mc)
     }
 
     for (i = 0; i < numposargs; ++i) {
-        PyObject *onerepr = PyObject_Repr(PyTuple_GET_ITEM(mc->xargs, i+1));
+        PyObject *onerepr = PyObject_Repr(PyTuple_GET_ITEM(mc->args, i));
         if (onerepr == NULL)
             goto done;
         PyTuple_SET_ITEM(argreprs, i, onerepr);
@@ -1859,14 +1852,14 @@ methodcaller_reduce(methodcallerobject *mc, PyObject *Py_UNUSED(ignored))
 {
     if (!mc->kwds || PyDict_GET_SIZE(mc->kwds) == 0) {
         Py_ssize_t i;
-        Py_ssize_t newarg_size = PyTuple_GET_SIZE(mc->xargs);
-        PyObject *newargs = PyTuple_New(newarg_size);
+        Py_ssize_t callargcount = PyTuple_GET_SIZE(mc->args);
+        PyObject *newargs = PyTuple_New(1 + callargcount);
         if (newargs == NULL)
             return NULL;
         PyTuple_SET_ITEM(newargs, 0, Py_NewRef(mc->name));
-        for (i = 1; i < newarg_size; ++i) {
-            PyObject *arg = PyTuple_GET_ITEM(mc->xargs, i);
-            PyTuple_SET_ITEM(newargs, i, Py_NewRef(arg));
+        for (i = 0; i < callargcount; ++i) {
+            PyObject *arg = PyTuple_GET_ITEM(mc->args, i);
+            PyTuple_SET_ITEM(newargs, i + 1, Py_NewRef(arg));
         }
         return Py_BuildValue("ON", Py_TYPE(mc), newargs);
     }
@@ -1884,12 +1877,7 @@ methodcaller_reduce(methodcallerobject *mc, PyObject *Py_UNUSED(ignored))
         constructor = PyObject_VectorcallDict(partial, newargs, 2, mc->kwds);
 
         Py_DECREF(partial);
-        PyObject *args = PyTuple_GetSlice(mc->xargs, 1, PyTuple_GET_SIZE(mc->xargs));
-        if (!args) {
-            Py_DECREF(constructor);
-            return NULL;
-        }
-        return Py_BuildValue("NO", constructor, args);
+        return Py_BuildValue("NO", constructor, mc->args);
     }
 }