gh-159672: Keep typo budgets off the normal parser path

Author: pablogsal

Parser/pegen.c

@@ -1045,6 +1045,19 @@ _PyPegen_Parser_Free(Parser *p)
     PyMem_Free(p);
 }
 
+int
+_PyPegen_stack_exhausted_slow(Parser *p)
+{
+    if (p->keyword_trial != NULL) {
+        if (p->keyword_trial->budget == 0) {
+            return 1;
+        }
+        p->keyword_trial->budget--;
+        p->keyword_trial->furthest = Py_MAX(p->keyword_trial->furthest, p->mark);
+    }
+    return _Py_ReachedRecursionLimitWithMargin(p->tstate, 1);
+}
+
 static void
 reset_parser_state_for_error_pass(Parser *p)
 {
@@ -1156,6 +1169,14 @@ try_keyword_typo(Parser *p, KeywordTypo *candidate)
     trial.error_indicator = 0;
     trial.call_invalid_rules = 0;
     trial.keyword_trial = &state;
+    // Force trials through the existing stack-check slow path to count rule
+    // entries without adding a branch to normal parsing. The slow path still
+    // checks the thread's real stack limit.
+#if _Py_STACK_GROWS_DOWN
+    trial.stack_soft_limit = UINTPTR_MAX;
+#else
+    trial.stack_soft_limit = 0;
+#endif
     trial.known_err_token = NULL;
     trial.errcode = NULL;
     trial.identifier_cache = identifiers;

Parser/pegen.h

@@ -111,19 +111,14 @@ typedef struct {
     uintptr_t stack_soft_limit;
 } Parser;
 
+int _PyPegen_stack_exhausted_slow(Parser *p);
+
 // C-stack exhaustion check run on every rule entry. The fast path compares
 // the machine stack pointer against the soft limit cached in the Parser;
-// only when the stack is nearly exhausted is the full check called.
+// trials force the slow path to account for their rule budget.
 static inline int
 _PyPegen_stack_exhausted(Parser *p)
 {
-    if (p->keyword_trial != NULL) {
-        if (p->keyword_trial->budget == 0) {
-            return 1;
-        }
-        p->keyword_trial->budget--;
-        p->keyword_trial->furthest = Py_MAX(p->keyword_trial->furthest, p->mark);
-    }
 #if _Py_STACK_GROWS_DOWN
     if (_Py_get_machine_stack_pointer() > p->stack_soft_limit) {
 #else
@@ -132,7 +127,7 @@ _PyPegen_stack_exhausted(Parser *p)
         assert(!_Py_ReachedRecursionLimitWithMargin(p->tstate, 1));
         return 0;
     }
-    return _Py_ReachedRecursionLimitWithMargin(p->tstate, 1);
+    return _PyPegen_stack_exhausted_slow(p);
 }
 
 typedef struct {