Revert "bpo-46110: Add a recursion check to avoid stack overflow in the PEG parser (GH-30177)" (GH-30363)

This reverts commit e9898bf153 temporarily as we want to confirm if this commit is the cause of a slowdown at startup time.

Lib/test/test_syntax.py

@@ -1729,14 +1729,6 @@ while 1:
 """
         self._check_error(source, "too many statically nested blocks")
 
-    @support.cpython_only
-    def test_error_on_parser_stack_overflow(self):
-        source = "-" * 100000 + "4"
-        for mode in ["exec", "eval", "single"]:
-            with self.subTest(mode=mode):
-                with self.assertRaises(MemoryError):
-                    compile(source, "<string>", mode)
-
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

Misc/NEWS.d/next/Core and Builtins/2021-12-18-02-37-07.bpo-46110.B6hAfu.rst

@@ -1,2 +0,0 @@
-Add a maximum recursion check to the PEG parser to avoid stack overflow.
-Patch by Pablo Galindo

Parser/pegen.c

@@ -815,7 +815,6 @@ void *
 _PyPegen_run_parser(Parser *p)
 {
     void *res = _PyPegen_parse(p);
-    assert(p->level == 0);
     if (res == NULL) {
         if (PyErr_Occurred() && !PyErr_ExceptionMatches(PyExc_SyntaxError)) {
             return NULL;

Tools/peg_generator/pegen/c_generator.py

@@ -37,8 +37,6 @@ EXTENSION_PREFIX = """\
 #  define D(x)
 #endif
 
-# define MAXSTACK 6000
-
 """
 
 
@@ -366,14 +364,10 @@ class CParserGenerator(ParserGenerator, GrammarVisitor):
         self.skip_actions = skip_actions
 
     def add_level(self) -> None:
-        self.print("if (p->level++ == MAXSTACK) {")
-        with self.indent():
-            self.print("p->error_indicator = 1;")
-            self.print("PyErr_NoMemory();")
-        self.print("}")
+        self.print("D(p->level++);")
 
     def remove_level(self) -> None:
-        self.print("p->level--;")
+        self.print("D(p->level--);")
 
     def add_return(self, ret_val: str) -> None:
         self.remove_level()
@@ -550,10 +544,9 @@ class CParserGenerator(ParserGenerator, GrammarVisitor):
                 self.print("p->in_raw_rule++;")
                 self.print(f"void *_raw = {node.name}_raw(p);")
                 self.print("p->in_raw_rule--;")
-                self.print("if (p->error_indicator) {")
+                self.print("if (p->error_indicator)")
                 with self.indent():
-                    self.add_return("NULL")
-                self.print("}")
+                    self.print("return NULL;")
                 self.print("if (_raw == NULL || p->mark <= _resmark)")
                 with self.indent():
                     self.print("break;")