Use of unitialized variable `fields` along error path in code generated from asdl_c.py

[bradlarsen]

BPO	41204
Nosy	@vstinner, @bradlarsen, @isidentical
PRs	bpo-41204: Fix compiler warning in ast_type_init() #21307

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-07-04.21:21:07.601>
created_at = <Date 2020-07-03.21:47:00.691>
labels = ['interpreter-core', 'build', '3.10']
title = 'Use of unitialized variable `fields` along error path in code generated from asdl_c.py'
updated_at = <Date 2020-08-10.13:56:02.164>
user = 'https://github.com/bradlarsen'

bugs.python.org fields:

activity = <Date 2020-08-10.13:56:02.164>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-04.21:21:07.601>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2020-07-03.21:47:00.691>
creator = 'blarsen'
dependencies = []
files = []
hgrepos = []
issue_num = 41204
keywords = ['patch']
message_count = 4.0
messages = ['372963', '372997', '372998', '375115']
nosy_count = 3.0
nosy_names = ['vstinner', 'blarsen', 'BTaskaya']
pr_nums = ['21307']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue41204'
versions = ['Python 3.10']

[bradlarsen]

In commit b1cc6ba from earlier today, an error-handling path can now read an uninitialized variable.

b1cc6ba#diff-fa7f27df4c8df1055048e78340f904c4R695-R697

In particular, asdl_c.py is used to generate C source, and when building that code with Clang 10, there is the attached warning.

Likely fix: initialize fields to NULL. Also, perhaps a CI loop that has -Werror=sometimes-uninitialized would help detect these.

Compiler warning:

Python/Python-ast.c:1147:9: warning: variable 'fields' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
    if (state == NULL) {
        ^~~~~~~~~~~~~
Python/Python-ast.c:1210:16: note: uninitialized use occurs here
    Py_XDECREF(fields);
               ^~~~~~
./Include/object.h:520:51: note: expanded from macro 'Py_XDECREF'
#define Py_XDECREF(op) _Py_XDECREF(_PyObject_CAST(op))
                                                  ^~
./Include/object.h:112:41: note: expanded from macro '_PyObject_CAST'
#define _PyObject_CAST(op) ((PyObject*)(op))
                                        ^~
Python/Python-ast.c:1147:5: note: remove the 'if' if its condition is always false
    if (state == NULL) {
    ^~~~~~~~~~~~~~~~~~~~
Python/Python-ast.c:1145:35: note: initialize the variable 'fields' to silence this warning
    PyObject *key, *value, *fields;
                                  ^
                                   = NULL
1 warning generated.

[vstinner]

New changeset 1f76453 by Victor Stinner in branch 'master':
bpo-41204: Fix compiler warning in ast_type_init() (GH-21307)
1f76453

[vstinner]

Thanks for the report, it's not fixed.

[vstinner]

New changeset d2bea26 by Victor Stinner in branch '3.9':
[3.9] bpo-41194: Convert _ast extension to PEP-489 (GH-21807)
d2bea26