gh-141563: make PyDateTime_IMPORT thread-safe (#144210)

Include/datetime.h

@@ -196,8 +196,23 @@ typedef struct {
 /* Define global variable for the C API and a macro for setting it. */
 static PyDateTime_CAPI *PyDateTimeAPI = NULL;
 
-#define PyDateTime_IMPORT \
-    PyDateTimeAPI = (PyDateTime_CAPI *)PyCapsule_Import(PyDateTime_CAPSULE_NAME, 0)
+static inline PyDateTime_CAPI *
+_PyDateTime_IMPORT(void) {
+    PyDateTime_CAPI *val = _Py_atomic_load_ptr(&PyDateTimeAPI);
+    if (val == NULL) {
+        PyDateTime_CAPI *capi = (PyDateTime_CAPI *)PyCapsule_Import(
+            PyDateTime_CAPSULE_NAME, 0);
+        if (capi != NULL) {
+            /* if the compare exchange fails then in that case
+               another thread would have initialized it */
+            _Py_atomic_compare_exchange_ptr(&PyDateTimeAPI, &val, (void *)capi);
+            return capi;
+        }
+    }
+    return val;
+}
+
+#define PyDateTime_IMPORT _PyDateTime_IMPORT()
 
 /* Macro for access to the UTC singleton */
 #define PyDateTime_TimeZone_UTC PyDateTimeAPI->TimeZone_UTC

Misc/NEWS.d/next/Core_and_Builtins/2026-02-02-17-07-34.gh-issue-141563.GheXjr.rst

@@ -0,0 +1 @@
+Fix thread safety of :c:macro:`! PyDateTime_IMPORT`.