[3.12] pythongh-130618: Fix parser error when using lambdas inside f-strings (pythonGH-130638) (python#130644)

(cherry picked from commit e06bebb)

Author: pablogsal

Lib/test/test_grammar.py

@@ -2030,6 +2030,18 @@ async def foo():
         with self.assertRaises(Done):
             foo().send(None)
 
+    def test_complex_lambda(self):
+        def test1(foo, bar):
+            return ""
+
+        def test2():
+            return f"{test1(
+                foo=lambda: '、、、、、、、、、、、、、、、、、',
+                bar=lambda: 'abcdefghijklmnopqrstuvwxyz 123456789 123456789',
+            )}"
+
+        self.assertEqual(test2(), "")
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-02-27-15-07-06.gh-issue-130618.JTcsRB.rst

@@ -0,0 +1,3 @@
+Fix a bug that was causing ``UnicodeDecodeError`` or ``SystemError`` to be
+raised when using f-strings with ``lambda`` expressions with non-ASCII
+characters. Patch by Pablo Galindo

Parser/tokenizer.c

@@ -479,8 +479,11 @@ static int update_fstring_expr(struct tok_state *tok, char cur) {
     break;
   case '}':
   case '!':
-  case ':':
     tok_mode->last_expr_end = strlen(tok->start);
+  case ':':
+    if (tok_mode->last_expr_end == -1) {
+        tok_mode->last_expr_end = strlen(tok->start);
+    }
     break;
   default:
     Py_UNREACHABLE();