GH-95909: Make `_PyArg_Parser` initialization thread safe (GH-95958)

1 files changed, 23 insertions, 6 deletions

diff --git a/Python/getargs.c b/Python/getargs.c
index 457dd99..f0b84b8 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -1974,15 +1974,10 @@ new_kwtuple(const char * const *keywords, int total, int pos)
 }
 
 static int
-parser_init(struct _PyArg_Parser *parser)
+_parser_init(struct _PyArg_Parser *parser)
 {
     const char * const *keywords = parser->keywords;
     assert(keywords != NULL);
-
-    if (parser->initialized) {
-        assert(parser->kwtuple != NULL);
-        return 1;
-    }
     assert(parser->pos == 0 &&
            (parser->format == NULL || parser->fname == NULL) &&
            parser->custom_msg == NULL &&
@@ -2035,6 +2030,28 @@ parser_init(struct _PyArg_Parser *parser)
     return 1;
 }
 
+static int
+parser_init(struct _PyArg_Parser *parser)
+{
+    // volatile as it can be modified by other threads
+    // and should not be optimized or reordered by compiler
+    if (*((volatile int *)&parser->initialized)) {
+        assert(parser->kwtuple != NULL);
+        return 1;
+    }
+    PyThread_acquire_lock(_PyRuntime.getargs.mutex, WAIT_LOCK);
+    // Check again if another thread initialized the parser
+    // while we were waiting for the lock.
+    if (*((volatile int *)&parser->initialized)) {
+        assert(parser->kwtuple != NULL);
+        PyThread_release_lock(_PyRuntime.getargs.mutex);
+        return 1;
+    }
+    int ret = _parser_init(parser);
+    PyThread_release_lock(_PyRuntime.getargs.mutex);
+    return ret;
+}
+
 static void
 parser_clear(struct _PyArg_Parser *parser)
 {