`_set_str` missing `NULL` check after `PyUnicode_DecodeASCII` in `_csv.c`

[devdanzin]

Crash report

What happened?

It's possible to segfault the interpreter on OOM by trying to write a CSV row.

Automated diagnosis:

Bug: On OOM, PyUnicode_DecodeASCII returns NULL, _set_str stores NULL in *target and returns 0 (success). Later PyUnicode_GET_LENGTH(NULL) segfaults.

File: Modules/_csv.c, line 319

MRE:

import _testcapi, csv, io

# Persistent failure ensures the allocation inside _set_str fails
for n in range(1, 30):
    print(n)
    _testcapi.set_nomemory(n, 0)
    try:
        w = csv.writer(io.StringIO())
        w.writerow(["a", "b"])
        _testcapi.remove_mem_hooks()
        break
    except MemoryError:
        _testcapi.remove_mem_hooks()

Backtrace:

Program received signal SIGSEGV, Segmentation fault.
dialect_check_char (name=0x7ffff7fa39e5 "delimiter", c=44, dialect=dialect@entry=0x7ffff74dadb0, allowspace=true) at ./Modules/_csv.c:353
353             PyUnicode_GET_LENGTH(dialect->lineterminator), 1) >= 0)

#0  dialect_check_char (name=0x7ffff7fa39e5 "delimiter", c=44, dialect=dialect@entry=0x7ffff74dadb0, allowspace=true) at ./Modules/_csv.c:353
#1  0x00007ffff7f9ee52 in dialect_new (type=0x555555f539b0, args=0x555555d62c40 <_PyRuntime+135440>, kwargs=0x0) at ./Modules/_csv.c:543
#2  0x000055555576bd44 in type_call (self=0x555555f539b0, args=<optimized out>, kwds=<optimized out>) at Objects/typeobject.c:2441
#3  0x00005555556a34b4 in _PyObject_MakeTpCall (tstate=0x555555d99c08 <_PyRuntime+360664>, callable=0x555555f539b0, args=0x0, nargs=0, keywords=0x0) at Objects/call.c:242
#4  0x00007ffff7fa19d3 in _call_dialect (module_state=0x7ffff72d4cc0, dialect_inst=0x0, kwargs=0x0) at ./Modules/_csv.c:646
#5  csv_writer (module=0x7ffff72b47a0, args=0x7ffff7477cf0, keyword_args=0x0) at ./Modules/_csv.c:1581
#6  0x00005555557210e2 in cfunction_call (func=0x7ffff72cf170, args=0x7ffff7477cf0, kwargs=0x0) at Objects/methodobject.c:564
#7  0x00005555556a34b4 in _PyObject_MakeTpCall (tstate=0x555555d99c08 <_PyRuntime+360664>, callable=0x7ffff72cf170, args=0x7fffffffb628, nargs=1, keywords=0x0) at Objects/call.c:242
#8  0x000055555583f29d in _Py_VectorCall_StackRefSteal (callable=callable@entry=..., arguments=0x7ffff7fa7090, total_args=total_args@entry=1, kwnames=kwnames@entry=...) at Python/ceval.c:733
#9  0x00005555558556d6 in _PyEval_EvalFrameDefault (tstate=tstate@entry=0x555555d99c08 <_PyRuntime+360664>, frame=<optimized out>, frame@entry=0x7ffff7fa7020, throwflag=throwflag@entry=0)
    at Python/generated_cases.c.h:4170
#10 0x000055555583f08b in _PyEval_EvalFrame (tstate=0x555555d99c08 <_PyRuntime+360664>, frame=0x7ffff7fa7020, throwflag=0) at ./Include/internal/pycore_ceval.h:118
#11 _PyEval_Vector (tstate=tstate@entry=0x555555d99c08 <_PyRuntime+360664>, func=func@entry=0x7ffff7466690, locals=locals@entry=0x7ffff746a4b0, args=args@entry=0x0,
    argcount=argcount@entry=0, kwnames=kwnames@entry=0x0) at Python/ceval.c:2130
#12 0x000055555583ee1e in PyEval_EvalCode (co=co@entry=0x555555f396d0, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0) at Python/ceval.c:686
#13 0x00005555559c8f8e in run_eval_code_obj (tstate=0x555555d99c08 <_PyRuntime+360664>, co=co@entry=0x555555f396d0, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0)
    at Python/pythonrun.c:1368
#14 0x00005555559c8adb in run_mod (mod=mod@entry=0x555555f40fa0, filename=filename@entry=0x7ffff74e8580, globals=globals@entry=0x7ffff746a4b0, locals=locals@entry=0x7ffff746a4b0,
    flags=0x7fffffffc950, arena=arena@entry=0x7ffff74dad40, interactive_src=0x0, generate_new_source=0) at Python/pythonrun.c:1471

Found using cpython-review-toolkit with Claude Opus 4.6, using the /cpython-review-toolkit:explore Modules/_csv.c all deep command.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.15.0a7+ (heads/main:e0f7c1097e1, Mar 17 2026, 18:10:52) [Clang 21.1.2 (2ubuntu6)]

Linked PRs

• gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed #146113
• [3.14] gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed (GH-146113) #146130
• [3.13] gh-146093: Fix csv _set_str(): check if PyUnicode_DecodeASCII() failed (GH-146113) #146131

[vstinner]

I wrote #146113 to fix the issue.

[vstinner]

Issue fixed by #146113. Thanks for the bug report.