From c559135c931789ebc752ae68814858c398cb798b Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 12 Jan 2026 20:23:05 +0200
Subject: [PATCH] gh-143196: Fix crash in non-standard use of internal JSON
 encoder object (GH-143618)

The internal encoder object returned by undocumented function
json.encoder.c_make_encoder() (aka _json.make_encoder()) crashed
when it was called with non-zero second argument.
---
 Lib/test/test_json/test_speedups.py           | 31 +++++++++++++++++++
 ...-01-09-17-50-26.gh-issue-143196.WxKxzU.rst |  3 ++
 Modules/_json.c                               |  1 +
 3 files changed, 35 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-01-09-17-50-26.gh-issue-143196.WxKxzU.rst

diff --git a/Lib/test/test_json/test_speedups.py b/Lib/test/test_json/test_speedups.py
index 682014cfd5b3..4c0aa5f993b3 100644
--- a/Lib/test/test_json/test_speedups.py
+++ b/Lib/test/test_json/test_speedups.py
@@ -80,3 +80,34 @@ class TestEncode(CTest):
     def test_unsortable_keys(self):
         with self.assertRaises(TypeError):
             self.json.encoder.JSONEncoder(sort_keys=True).encode({'a': 1, 1: 'a'})
+
+    def test_current_indent_level(self):
+        enc = self.json.encoder.c_make_encoder(
+            markers=None,
+            default=str,
+            encoder=self.json.encoder.c_encode_basestring,
+            indent='\t',
+            key_separator=': ',
+            item_separator=', ',
+            sort_keys=False,
+            skipkeys=False,
+            allow_nan=False)
+        expected = (
+            '[\n'
+            '\t"spam", \n'
+            '\t{\n'
+            '\t\t"ham": "eggs"\n'
+            '\t}\n'
+            ']')
+        self.assertEqual(enc(['spam', {'ham': 'eggs'}], 0)[0], expected)
+        self.assertEqual(enc(['spam', {'ham': 'eggs'}], -3)[0], expected)
+        expected2 = (
+            '[\n'
+            '\t\t\t\t"spam", \n'
+            '\t\t\t\t{\n'
+            '\t\t\t\t\t"ham": "eggs"\n'
+            '\t\t\t\t}\n'
+            '\t\t\t]')
+        self.assertEqual(enc(['spam', {'ham': 'eggs'}], 3)[0], expected2)
+        self.assertRaises(TypeError, enc, ['spam', {'ham': 'eggs'}], 3.0)
+        self.assertRaises(TypeError, enc, ['spam', {'ham': 'eggs'}])
diff --git a/Misc/NEWS.d/next/Library/2026-01-09-17-50-26.gh-issue-143196.WxKxzU.rst b/Misc/NEWS.d/next/Library/2026-01-09-17-50-26.gh-issue-143196.WxKxzU.rst
new file mode 100644
index 000000000000..9143cf2aeccc
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-01-09-17-50-26.gh-issue-143196.WxKxzU.rst
@@ -0,0 +1,3 @@
+Fix crash when the internal encoder object returned by undocumented function
+:func:`!json.encoder.c_make_encoder` was called with non-zero second
+(*_current_indent_level*) argument.
diff --git a/Modules/_json.c b/Modules/_json.c
index 78a85496575a..c7e62c4fe55c 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1460,6 +1460,7 @@ encoder_call(PyObject *op, PyObject *args, PyObject *kwds)
             return NULL;
         }
     }
+    indent_level = 0;
     if (encoder_listencode_obj(self, writer, obj, indent_level, indent_cache)) {
         PyUnicodeWriter_Discard(writer);
         Py_XDECREF(indent_cache);
-- 
2.47.3