bpo-47117: Don't crash if we fail to decode characters when the tokenizer buffers are uninitialized (GH-32129)

Automerge-Triggered-By: GH:pablogsal

Author: pablogsal

Misc/NEWS.d/next/Core and Builtins/2022-03-26-15-45-57.bpo-47117.60W6GQ.rst

@@ -0,0 +1,2 @@
+Fix a crash if we fail to decode characters in interactive mode if the
+tokenizer buffers are uninitialized. Patch by Pablo Galindo.

Parser/pegen_errors.c

@@ -248,7 +248,12 @@ get_error_line_from_tokenizer_buffers(Parser *p, Py_ssize_t lineno)
     assert((p->tok->fp == NULL && p->tok->str != NULL) || p->tok->fp == stdin);
 
     char *cur_line = p->tok->fp_interactive ? p->tok->interactive_src_start : p->tok->str;
-    assert(cur_line != NULL);
+    if (cur_line == NULL) {
+        assert(p->tok->fp_interactive);
+        // We can reach this point if the tokenizer buffers for interactive source have not been
+        // initialized because we failed to decode the original source with the given locale.
+        return PyUnicode_FromStringAndSize("", 0);
+    }
 
     Py_ssize_t relative_lineno = p->starting_lineno ? lineno - p->starting_lineno + 1 : lineno;
     const char* buf_end = p->tok->fp_interactive ? p->tok->interactive_src_end : p->tok->inp;
@@ -311,7 +316,7 @@ _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
         goto error;
     }
 
-    if (p->tok->fp_interactive) {
+    if (p->tok->fp_interactive && p->tok->interactive_src_start != NULL) {
         error_line = get_error_line_from_tokenizer_buffers(p, lineno);
     }
     else if (p->start_rule == Py_file_input) {