gh-116738: Make _json module thread-safe in the free-threading (#119438)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_free_threading/test_json.py

@@ -0,0 +1,79 @@
+from threading import Barrier, Thread
+from test.test_json import CTest
+from test.support import threading_helper
+
+
+def encode_json_helper(
+    json, worker, data, number_of_threads=12, number_of_json_encodings=100
+):
+    worker_threads = []
+    barrier = Barrier(number_of_threads)
+    for index in range(number_of_threads):
+        worker_threads.append(
+            Thread(target=worker, args=[barrier, data, index])
+        )
+    for t in worker_threads:
+        t.start()
+    for ii in range(number_of_json_encodings):
+        json.dumps(data)
+    data.clear()
+    for t in worker_threads:
+        t.join()
+
+
+class MyMapping(dict):
+    def __init__(self):
+        self.mapping = []
+
+    def items(self):
+        return self.mapping
+
+
+@threading_helper.reap_threads
+@threading_helper.requires_working_threading()
+class TestJsonEncoding(CTest):
+    # Test encoding json with concurrent threads modifying the data cannot
+    # corrupt the interpreter
+
+    def test_json_mutating_list(self):
+        def worker(barrier, data, index):
+            barrier.wait()
+            while data:
+                for d in data:
+                    if len(d) > 5:
+                        d.clear()
+                    else:
+                        d.append(index)
+
+        data = [[], []]
+        encode_json_helper(self.json, worker, data)
+
+    def test_json_mutating_exact_dict(self):
+        def worker(barrier, data, index):
+            barrier.wait()
+            while data:
+                for d in data:
+                    if len(d) > 5:
+                        try:
+                            key = list(d)[0]
+                            d.pop()
+                        except (KeyError, IndexError):
+                            pass
+                    else:
+                        d[index] = index
+
+        data = [{}, {}]
+        encode_json_helper(self.json, worker, data)
+
+    def test_json_mutating_mapping(self):
+        def worker(barrier, data, index):
+            barrier.wait()
+            while data:
+                for d in data:
+                    if len(d.mapping) > 3:
+                        d.mapping.clear()
+                    else:
+                        d.mapping.append((index, index))
+
+        data = [MyMapping(), MyMapping()]
+        encode_json_helper(self.json, worker, data)