Multi-line arguments in a function call crashes CPython

[xiaxinmeng]

Crash report

In a function call, if one argument is split in two lines, CPython crashes. For example, we split the lambda expression, i.e., lamda_exp=lambda:1, in test2.py into two lines (See test1.py) . CPython reports a segmentation fault.

This crash just occurs on the latest main branch(e.g.commit a1f08f5, 8a2b7ee ). The older version of CPython(e.g. CPython 3.10.8, CPython 3.9.0 ) does not report any crash.

test1.py (Segmentation fault)

def foo(param, lambda_exp):
    pass

foo(param=0,
    lambda_exp=lambda:
    1)

test2.py (work well )

def foo(param, lambda_exp):
    pass

foo(param=0,
    lambda_exp=lambda:1)

Error messages

Segmentation Fault

Your environment

• CPython versions tested on: main branch from GitHub (commit a1f08f5)
• Operating system and architecture: Ubuntu 18.04

Linked PRs

• gh-101928: fix crash in compiler on multi-line lambda in function call #101933

[hauntsaninja]

Thanks for the report! I bisected this to #101633 cc @penguin-wwy @iritkatriel

[iritkatriel]

Thank you @hauntsaninja.

I see the problem - we are now creating a basicblock with

RETURN_CONST followed by NOP, and the redundant NOP is not being removed before propagate_line_numbers() uses BB_HAS_FALLTHROUGH on the block, which only looks at the last instruction to see if it exits the block.

Possible fixes:

1. (easiest) replace LOAD_CONST, RETURN_VALUE by NOP, RETURN_CONST (instead of RETURN_CONST, NOP).
2. make sure the redundant NOP is removed before propagate_line_numbers is called

I'll fix it tomorrow if nobody else will beat me to it.

[penguin-wwy]

Thank you @hauntsaninja.

I see the problem - we are now creating a basicblock with

RETURN_CONST followed by NOP, and the redundant NOP is not being removed before propagate_line_numbers() uses BB_HAS_FALLTHROUGH on the block, which only looks at the last instruction to see if it exits the block.

Possible fixes:

1. (easiest) replace LOAD_CONST, RETURN_VALUE by NOP, RETURN_CONST (instead of RETURN_CONST, NOP).

2. make sure the redundant NOP is removed before propagate_line_numbers is called

I'll fix it tomorrow if nobody else will beat me to it.

@iritkatriel Thanks for the fix suggestion, I tried to fix it using the first method and it seems to have minimal impact and is effective.

[iritkatriel]

Thanks for the report @xiaxinmeng, and for the fix @penguin-wwy .