gh-152052: Fix misleading json error for \uXXXX escape at the end of input (#152053)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: tonghuaroot

Lib/test/test_json/test_fail.py

@@ -144,6 +144,13 @@ def test_truncated_input(self):
             ('"', 'Unterminated string starting at', 0),
             ('"spam', 'Unterminated string starting at', 0),
         ]
+        # A complete \uXXXX escape at end of input leaves it unterminated.
+        test_cases += [
+            (r'"\u0041', 'Unterminated string starting at', 0),
+            (r'"\ud834', 'Unterminated string starting at', 0),
+            (r'"\ud834\udd1e', 'Unterminated string starting at', 0),
+            (r'{"a": "\u0041', 'Unterminated string starting at', 6),
+        ]
         for data, msg, idx in test_cases:
             with self.assertRaises(self.JSONDecodeError) as cm:
                 self.loads(data)

Lib/test/test_json/test_scanstring.py

@@ -137,6 +137,9 @@ def test_bad_escapes(self):
             '"\\ud834\\u-123"',
             '"\\ud834\\u+123"',
             '"\\ud834\\u1_23"',
+            # Truncated or non-hex \uXXXX escape at end of input.
+            '"\\u004',
+            '"\\uXYZW',
         ]
         for s in bad_escapes:
             with self.assertRaises(self.JSONDecodeError, msg=s):

Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-152052.yBssDE.rst

@@ -0,0 +1,2 @@
+The :mod:`json` C accelerator now correctly reports an unterminated string for a
+``\uXXXX`` escape at the end of the input.

Modules/_json.c

@@ -576,7 +576,7 @@ scanstring_unicode(PyObject *pystr, Py_ssize_t end, int strict, Py_ssize_t *next
             c = 0;
             next++;
             end = next + 4;
-            if (end >= len) {
+            if (end > len) {
                 raise_errmsg("Invalid \\uXXXX escape", pystr, next - 1);
                 goto bail;
             }