[3.9] bpo-45820: Fix a segfault when the parser fails without reading any input (GH-29580) (GH-29584)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Author: 3 people

.gitignore

@@ -136,5 +136,7 @@ Tools/ssl/win32
 !/Python/
 
 # Artifacts generated by 3.11 lying around when switching branches:
+/_bootstrap_python
 /Programs/_freeze_module
+/Python/deepfreeze/
 /Python/frozen_modules/ 

Misc/NEWS.d/next/Core and Builtins/2021-11-16-19-00-27.bpo-45820.2X6Psr.rst

@@ -0,0 +1,2 @@
+Fix a segfault when the parser fails without reading any input. Patch by
+Pablo Galindo

Parser/pegen/pegen.c

@@ -364,6 +364,14 @@ tokenizer_error(Parser *p)
 void *
 _PyPegen_raise_error(Parser *p, PyObject *errtype, const char *errmsg, ...)
 {
+    if (p->fill == 0) {
+        va_list va;
+        va_start(va, errmsg);
+        _PyPegen_raise_error_known_location(p, errtype, 0, 0, errmsg, va);
+        va_end(va);
+        return NULL;
+    }
+
     Token *t = p->known_err_token != NULL ? p->known_err_token : p->tokens[p->fill - 1];
     Py_ssize_t col_offset;
     if (t->col_offset == -1) {