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

Include/cpython/critical_section.h

@@ -85,6 +85,7 @@ PyCriticalSection2_End(PyCriticalSection2 *c);
 #ifndef Py_GIL_DISABLED
 # define Py_BEGIN_CRITICAL_SECTION(op)      \
     {
+# define Py_EXIT_CRITICAL_SECTION()
 # define Py_END_CRITICAL_SECTION()          \
     }
 # define Py_BEGIN_CRITICAL_SECTION2(a, b)   \
@@ -118,6 +119,10 @@ struct PyCriticalSection2 {
         PyCriticalSection _py_cs;                                       \
         PyCriticalSection_Begin(&_py_cs, _PyObject_CAST(op))
 
+# define Py_EXIT_CRITICAL_SECTION()                                     \
+        _PyCriticalSection_End(&_py_cs);
+
+
 # define Py_END_CRITICAL_SECTION()                                      \
         PyCriticalSection_End(&_py_cs);                                 \
     }

Include/internal/pycore_critical_section.h

@@ -51,6 +51,11 @@ extern "C" {
         }                                                               \
     }
 
+# define Py_EXIT_CRITICAL_SECTION_SEQUENCE_FAST()                       \
+        if (_should_lock_cs) {                                          \
+            _PyCriticalSection_End(&_cs);                               \
+        }
+
 // 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.
@@ -78,6 +83,7 @@ extern "C" {
 # define Py_BEGIN_CRITICAL_SECTION_MUT(mut) {
 # define Py_BEGIN_CRITICAL_SECTION2_MUT(m1, m2) {
 # define Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(original) {
+# define Py_EXIT_CRITICAL_SECTION_SEQUENCE_FAST()
 # define Py_END_CRITICAL_SECTION_SEQUENCE_FAST() }
 # define _Py_CRITICAL_SECTION_ASSERT_MUTEX_LOCKED(mutex)
 # define _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op)

Misc/NEWS.d/next/Core and Builtins/2024-06-04-20-26-21.gh-issue-116738.q_hPYq.rst

@@ -0,0 +1 @@
+Make the module :mod:`json` safe to use under the free-theading build.

Modules/_json.c

@@ -10,6 +10,7 @@
 
 #include "Python.h"
 #include "pycore_ceval.h"           // _Py_EnterRecursiveCall()
+#include "pycore_critical_section.h"  // Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_runtime.h"         // _PyRuntime
 #include "pycore_pyerrors.h"        // _PyErr_FormatNote
 
@@ -1618,12 +1619,16 @@ encoder_listencode_dict(PyEncoderObject *s, _PyUnicodeWriter *writer,
 
     } else {
         Py_ssize_t pos = 0;
+        Py_BEGIN_CRITICAL_SECTION(dct);
         while (PyDict_Next(dct, &pos, &key, &value)) {
             if (encoder_encode_key_value(s, writer, &first, dct, key, value,
                                          new_newline_indent,
-                                         current_item_separator) < 0)
+                                         current_item_separator) < 0) {
+                Py_EXIT_CRITICAL_SECTION();
                 goto bail;
+            }
         }
+        Py_END_CRITICAL_SECTION();
     }
 
     if (ident != NULL) {
@@ -1666,8 +1671,12 @@ encoder_listencode_list(PyEncoderObject *s, _PyUnicodeWriter *writer,
     s_fast = PySequence_Fast(seq, "_iterencode_list needs a sequence");
     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);
+        Py_EXIT_CRITICAL_SECTION_SEQUENCE_FAST();
         return _PyUnicodeWriter_WriteASCIIString(writer, "[]", 2);
     }
 
@@ -1735,10 +1744,12 @@ encoder_listencode_list(PyEncoderObject *s, _PyUnicodeWriter *writer,
     if (_PyUnicodeWriter_WriteChar(writer, ']'))
         goto bail;
     Py_DECREF(s_fast);
+    Py_EXIT_CRITICAL_SECTION_SEQUENCE_FAST();
     return 0;
 
 bail:
     Py_XDECREF(ident);
+    Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
     Py_DECREF(s_fast);
     Py_XDECREF(separator_indent);
     Py_XDECREF(new_newline_indent);