Issue 40903: Segfault in new PEG parser

The input `p=p=` causes python 3.10 to crash.

I bisected the change, and the behavior appears to have been introduced by 16ab07063cb564c1937714bd39d6915172f005b5 (bpo-40334: Correctly identify invalid target in assignment errors (GH-20076) )

Steps to reproduce:

$ echo 'p=p=' | /path/to/python3.10
=== SIGSEGV (Address boundary error)


Analysis:

This code is an invalid assignment, and the parser tries to generate a useful message for this case (invalid_assignment_rule).

However, the `target` of the assignment is a Name node.

The invalid_assignment_rule function tries to identify the target of the assignment, to create a useful description for the error menssage by calling `_PyPegen_get_invalid_target`, passing in the Name Node.

`PyPegen_get_invalid_target` returns NULL if the type is a Name type (pegen.c:2114).

The result of this call is then passed unconditionally to _PyPegen_get_expr_name, which is expecting a statement, not NULL.

Error happens here: pegen.c:164
`_PyPegen_get_expr_name(expr_ty e)` is being called with `e = 0x0`

New changeset 9f495908c5bd3645ed1af82d7bae6782720dab77 by Pablo Galindo in branch 'master':
bpo-40903: Handle multiple '=' in invalid assignment rules in the PEG parser (GH-20697)
https://github.com/python/cpython/commit/9f495908c5bd3645ed1af82d7bae6782720dab77

New changeset 8df4f3942faf05790efeaf62a8f493aabd181d3f by Miss Islington (bot) in branch '3.9':
bpo-40903: Handle multiple '=' in invalid assignment rules in the PEG parser (GH-20697)
https://github.com/python/cpython/commit/8df4f3942faf05790efeaf62a8f493aabd181d3f

https://buildbot.python.org/all/#builders/765/builds/51

The new test fails with the old parser:

**********************************************************************
File "/buildbot/buildarea/3.9.pablogsal-arch-x86_64.oldparser/build/Lib/test/test_syntax.py", line 66, in test.test_syntax
Failed example:
    yield = 1
Expected:
    Traceback (most recent call last):
    SyntaxError: assignment to yield expression not possible
Got:
    Traceback (most recent call last):
      File "/buildbot/buildarea/3.9.pablogsal-arch-x86_64.oldparser/build/Lib/doctest.py", line 1336, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest test.test_syntax[10]>", line 1
        yield = 1
              ^
    SyntaxError: invalid syntax
**********************************************************************
1 items had failures:
   1 of 108 in test.test_syntax

> The new test fails with the old parser:

Oh, it's already fixed by:
https://github.com/python/cpython/commit/2b33cc3a2509983c4fa7884fa2722bd2e5781e51

> The new test fails with the old parser:

I fixed it this morning in https://github.com/python/cpython/pull/20717

pull_requests: + pull_request19912
stage: patch review