Many of the C stdlib modules can benefit from improved error handling during initialization. I've now had two PRs where the authors had reference leaks on error conditions, but defended their decisions by pointing to examples of similar idioms all over the stdlib.

The problems fall into two related categories, mostly:

• Not DECREF'ing the new module object on failure.
• Not handling errors raised by the PyModule_Add* family of functions... specifically, the weird steal-on-success semantics of PyModule_AddObject. I've already improved the docs for this, so we should see the issue less, but our own code should still be fixed.

I intend to turn this into a longer term project. I'll be working my way through these modules bit-by-bit over time, using this issue to track all of them (since there are a few dozen cases). I'd rather not make one huge one that spams all of the code owners and is impossible to review.

If anybody want to make themselves available to review/merge these as I go along, that would be great! Many of the ones I'll start with are just adding trivial DECREFs to the more popular modules (_asyncio, _contextvars, _functools, _random, _warnings, etc...).

How do others feel about the creation of a new private API? It would keep these diffs smaller and ease refactoring... and it would probably be good to have around anyways:

/* Like PyModule_AddObject, but steals o on success AND failure. */

int
_PyModule_StealObject(PyObject *m, const char *name, PyObject *o)
{
    if (PyModule_AddObject(m, name, o) < 0) {
        Py_XDECREF(o);
        return -1;
    }
    return 0;
}

New changeset c3f6bdc by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
c3f6bdc

New changeset 48f4f75 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
48f4f75

New changeset 825e91b by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
825e91b

New changeset 143a97f by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
143a97f

New changeset 8a334af by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
8a334af

New changeset 1fe79a4 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
1fe79a4

Is anything left to be done for the issue?

Yes, there are still a few dozen modules I plan to update!

New changeset 289cf0f by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
289cf0f

New changeset 9e4d031 by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
9e4d031

New changeset 42a4359 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
42a4359

New changeset 54b32c9 by T. Wouters (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleak in fcntl module initialization. (GH-17236)
54b32c9

New changeset ac22354 by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
ac22354

New changeset 5bd2af9 by Miss Islington (bot) in branch '3.7':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
5bd2af9

New changeset a5ed2fe by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
a5ed2fe

New changeset 33b671e by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
33b671e

The Azure Pipelines are sick tonight (unable to publish test results). PR bpo-17272 and PR bpo-17271 are blocked by this CI.

Thanks Victor. These obviously aren’t urgent, so feel free to return to them whenever’s convenient.

I also pinged you on bpo-17235 (_tracemalloc) too.

New changeset d51a363 by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleak in _tracemalloc init error handling (GH-17235)
d51a363

New changeset 63f09e7 by Miss Islington (bot) in branch '3.7':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
63f09e7

New changeset 2ea4c37 by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
2ea4c37

New changeset daf7a08 by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleak in _tracemalloc init error handling (GH-17235)
daf7a08

A lot of issues were fixed by Serhiy in #106858. Are there more actionable items left? (I did not check the _decimal PR yet.)

I suggest to close the issue which has a long history, and maybe create a new issue with a better described scope (list C extensions) if more C extensions need to be fixed.

All right, there are no open PRs left (they were all obsoleted by Serhiy's recent work in #106858).

I suggest to close the issue which has a long history, and maybe create a new issue with a better described scope (list C extensions) if more C extensions need to be fixed.

I agree; let's close this :) Thanks to Brandt for starting this effort in the first place, and thanks to all who improved extension module error handling!