From 5cfdbab1175aec596e16eeafe418eba21e29c768 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Mon, 25 May 2020 18:33:09 +0200
Subject: [PATCH] [WIP] bpo-39465: Mark _Py_Identifier.object as atomic

This PR is basically a test to check if C compilers used by CPython
support C11 _Atomic specifier.
---
 Include/cpython/object.h |  5 ++++-
 Objects/unicodeobject.c  | 28 +++++++++++++++++-----------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 444f832f5bd8d3..255b31210874ea 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -45,7 +45,10 @@ PyAPI_FUNC(Py_ssize_t) _Py_GetRefTotal(void);
 typedef struct _Py_Identifier {
     struct _Py_Identifier *next;
     const char* string;
-    PyObject *object;
+    // bpo-39465: _PyUnicode_FromId() can be called by multiple threads
+    // running in different interpreters. Use an atomic variable rather
+    // than a lock for better parallelism.
+    _Atomic PyObject *object;
 } _Py_Identifier;
 
 #define _Py_static_string_init(value) { .next = NULL, .string = value, .object = NULL }
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index ea46a44bf5faac..c8421a3a53e7fa 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2275,18 +2275,24 @@ PyUnicode_FromString(const char *u)
 PyObject *
 _PyUnicode_FromId(_Py_Identifier *id)
 {
-    if (!id->object) {
-        id->object = PyUnicode_DecodeUTF8Stateful(id->string,
-                                                  strlen(id->string),
-                                                  NULL, NULL);
-        if (!id->object)
-            return NULL;
-        PyUnicode_InternInPlace(&id->object);
-        assert(!id->next);
-        id->next = static_strings;
-        static_strings = id;
+    if (id->object) {
+        return (PyObject *)id->object;
     }
-    return id->object;
+
+    PyObject *object;
+    object = PyUnicode_DecodeUTF8Stateful(id->string,
+                                          strlen(id->string),
+                                          NULL, NULL);
+    if (!object) {
+        return NULL;
+    }
+
+    PyUnicode_InternInPlace(&object);
+    id->object = (_Atomic PyObject *)object;
+    assert(!id->next);
+    id->next = static_strings;
+    static_strings = id;
+    return object;
 }
 
 static void