gh-116738: Make _json module safe in the free-threading build

Include/internal/pycore_critical_section.h

@@ -128,6 +128,12 @@ extern "C" {
         }                                                               \
     }
 
+# define Py_RETURN_CRITICAL_SECTION_SEQUENCE_FAST(value)                \
+        if (_should_lock_cs) {                                          \
+            _PyCriticalSection_End(&_cs);                               \
+        }                                                               \
+        return value;
+
 // Asserts that the mutex is locked.  The mutex must be held by the
 // top-most critical section otherwise there's the possibility
 // that the mutex would be swalled out in some code paths.
@@ -159,6 +165,7 @@ extern "C" {
 # define Py_END_CRITICAL_SECTION2()
 # define Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(original)
 # define Py_END_CRITICAL_SECTION_SEQUENCE_FAST()
+# define Py_RETURN_CRITICAL_SECTION_SEQUENCE_FAST(value)
 # define _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(mutex)
 # define _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op)
 #endif  /* !Py_GIL_DISABLED */

Modules/_json.c

@@ -1664,13 +1664,15 @@
 
     ident = NULL;
     s_fast = PySequence_Fast(seq, "_iterencode_list needs a sequence");
-    Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(seq);
-
     if (s_fast == NULL)
         return -1;
+
+    Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(seq);
+
     if (PySequence_Fast_GET_SIZE(s_fast) == 0) {
         Py_DECREF(s_fast);
-        return _PyUnicodeWriter_WriteASCIIString(writer, "[]", 2);
+        int r = _PyUnicodeWriter_WriteASCIIString(writer, "[]", 2);
+        Py_RETURN_CRITICAL_SECTION_SEQUENCE_FAST(r);
     }
 
     if (s->markers != Py_None) {
@@ -1718,13 +1720,11 @@
         if (encoder_listencode_obj(s, writer, obj, new_newline_indent))
             goto bail;
     }
-    Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
     if (ident != NULL) {
         if (PyDict_DelItem(s->markers, ident))
             goto bail;
         Py_CLEAR(ident);
     }
-
     if (s->indent != Py_None) {
         Py_CLEAR(new_newline_indent);
         Py_CLEAR(separator_indent);
@@ -1736,14 +1736,16 @@
     if (_PyUnicodeWriter_WriteChar(writer, ']'))
         goto bail;
     Py_DECREF(s_fast);
-    return 0;
+    Py_RETURN_CRITICAL_SECTION_SEQUENCE_FAST(0);
 
 bail:
+    Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
     Py_XDECREF(ident);
     Py_DECREF(s_fast);
     Py_XDECREF(separator_indent);
     Py_XDECREF(new_newline_indent);
     return -1;
+
 }
 
 static void