[3.13] gh-127065: Make methodcaller thread-safe in free threading build (GH-127109) (GH-127150)

miss-islington · colesbury · web-flow · commit 5468d219df65 · 2024-11-22T14:51:40.000Z
The `methodcaller` C vectorcall implementation uses an arguments array that is shared across calls. The first argument is modified on every invocation. This isn't thread-safe in the free threading build. I think it's also not safe in general, but for now just disable it in the free threading build. (cherry picked from commit f83ca69) Co-authored-by: Sam Gross <colesbury@gmail.com>
diff --git a/Misc/NEWS.d/next/Library/2024-11-21-16-23-16.gh-issue-127065.cfL1zd.rst b/Misc/NEWS.d/next/Library/2024-11-21-16-23-16.gh-issue-127065.cfL1zd.rst
@@ -0,0 +1,2 @@
+Fix crash when calling a :func:`operator.methodcaller` instance from
+multiple threads in the free threading build.
diff --git a/Modules/_operator.c b/Modules/_operator.c
@@ -1572,6 +1572,7 @@ typedef struct {
     vectorcallfunc vectorcall;
 } methodcallerobject;
 
+#ifndef Py_GIL_DISABLED
 static int _methodcaller_initialize_vectorcall(methodcallerobject* mc)
 {
     PyObject* args = mc->xargs;
@@ -1634,6 +1635,7 @@ methodcaller_vectorcall(
             (PyTuple_GET_SIZE(mc->xargs)) | PY_VECTORCALL_ARGUMENTS_OFFSET,
             mc->vectorcall_kwnames);
 }
+#endif
 
 
 /* AC 3.5: variable number of arguments, not currently support by AC */
@@ -1673,7 +1675,14 @@ methodcaller_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     mc->vectorcall_args = 0;
 
 
+#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
 
     PyObject_GC_Track(mc);
     return (PyObject *)mc;

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Fix crash when calling a :func:`operator.methodcaller` instance from
	`2`	`+multiple threads in the free threading build.`