bpo-46707: Avoid potential exponential backtracking in some syntax errors (GH-31241)

Grammar/python.gram

@@ -1095,13 +1095,15 @@ invalid_expression:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
 
+invalid_left_assignment_prefixes(memo): list|tuple|genexp|'True'|'None'|'False'
+
 invalid_named_expression:
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
     | a=NAME '=' b=bitwise_or !('='|':=') {
         p->in_raw_rule ? NULL : RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
-    | !(list|tuple|genexp|'True'|'None'|'False') a=bitwise_or b='=' bitwise_or !('='|':=') {
+    | !invalid_left_assignment_prefixes a=bitwise_or b='=' bitwise_or !('='|':=') {
         p->in_raw_rule ? NULL : RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
                                           _PyPegen_get_expr_name(a)) }
 

Lib/test/test_syntax.py

@@ -1770,6 +1770,14 @@ while 1:
                 with self.assertRaises(MemoryError):
                     compile(source, "<string>", mode)
 
+    @support.cpython_only
+    def test_deep_invalid_rule(self):
+        # Check that a very deep invalid rule in the PEG
+        # parser doesn't have exponential backtracking.
+        source = "d{{{{{{{{{{{{{{{{{{{{{{{{{```{{{{{{{ef f():y"
+        with self.assertRaises(SyntaxError):
+            compile(source, "<string>", "exec")
+
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

Misc/NEWS.d/next/Core and Builtins/2022-02-10-03-13-18.bpo-46707.xeSEh0.rst

@@ -0,0 +1,2 @@
+Avoid potential exponential backtracking when producing some syntax errors
+involving lots of brackets. Patch by Pablo Galindo.