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 c3f6bdc332d23588102eba749a5929dd5bb67c9d by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
https://github.com/python/cpython/commit/c3f6bdc332d23588102eba749a5929dd5bb67c9d

New changeset 48f4f75baeee8ade1fbfab1b0aa6a21a3b13a2f2 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
https://github.com/python/cpython/commit/48f4f75baeee8ade1fbfab1b0aa6a21a3b13a2f2

New changeset 825e91be0407d6fc7fa034286b4e90634f181fab by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _asyncio initialization. (GH-17195)
https://github.com/python/cpython/commit/825e91be0407d6fc7fa034286b4e90634f181fab

New changeset 143a97f64128070386b12a0ee589bdaad5e51f40 by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
https://github.com/python/cpython/commit/143a97f64128070386b12a0ee589bdaad5e51f40

New changeset 8a334af13368573cc645488481b1173d65eeeb9a by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
https://github.com/python/cpython/commit/8a334af13368573cc645488481b1173d65eeeb9a

New changeset 1fe79a43400d092a074c6f9ae5eb290ea3e4f281 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _contextvars initialization. (GH-17198)
https://github.com/python/cpython/commit/1fe79a43400d092a074c6f9ae5eb290ea3e4f281

Is anything left to be done for the issue?

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

New changeset 289cf0fbf78c4f38c38ac71ac8b772be7ec2672f by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
https://github.com/python/cpython/commit/289cf0fbf78c4f38c38ac71ac8b772be7ec2672f

New changeset 9e4d0312101cc2bc44a9549974d4a25f80e1dc12 by Miss Islington (bot) in branch '3.7':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
https://github.com/python/cpython/commit/9e4d0312101cc2bc44a9549974d4a25f80e1dc12

New changeset 42a4359390b78b3360e100fc9c075495e48354b2 by Miss Islington (bot) in branch '3.8':
bpo-38823: Clean up refleaks in _tkinter initialization. (GH-17206)
https://github.com/python/cpython/commit/42a4359390b78b3360e100fc9c075495e48354b2

New changeset 54b32c987146123f2237f0e21b1d02b1c1ebdf6f by T. Wouters (Brandt Bucher) in branch 'master':
bpo-38823: Clean up refleak in fcntl module initialization. (GH-17236)
https://github.com/python/cpython/commit/54b32c987146123f2237f0e21b1d02b1c1ebdf6f

New changeset ac2235432c607ce2c0faf6dff5d9b2534d2f6652 by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
https://github.com/python/cpython/commit/ac2235432c607ce2c0faf6dff5d9b2534d2f6652

New changeset 5bd2af9c79796aa03b06d1e35ab6df7e28364e24 by Miss Islington (bot) in branch '3.7':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
https://github.com/python/cpython/commit/5bd2af9c79796aa03b06d1e35ab6df7e28364e24

New changeset a5ed2fe0eedefa1649aa93ee74a0bafc8e628a10 by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleaks in faulthandler init error path on Windows (GH-17250)
https://github.com/python/cpython/commit/a5ed2fe0eedefa1649aa93ee74a0bafc8e628a10

New changeset 33b671e72450bf4b5a946ce0dde6b7fe21150108 by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
https://github.com/python/cpython/commit/33b671e72450bf4b5a946ce0dde6b7fe21150108

The Azure Pipelines are sick tonight (unable to publish test results). PR #17272 and PR #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 #17235 (_tracemalloc) too.

New changeset d51a363a4379385fdfe9c09a56324631465ede29 by Victor Stinner (Brandt Bucher) in branch 'master':
bpo-38823: Fix refleak in _tracemalloc init error handling (GH-17235)
https://github.com/python/cpython/commit/d51a363a4379385fdfe9c09a56324631465ede29

New changeset 63f09e7628bd72f1bb2106226655b1f775757806 by Miss Islington (bot) in branch '3.7':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
https://github.com/python/cpython/commit/63f09e7628bd72f1bb2106226655b1f775757806

New changeset 2ea4c37c1ecf05a8495211d55ed6888439b1b9cf by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleak in marshal init error path (GH-17260)
https://github.com/python/cpython/commit/2ea4c37c1ecf05a8495211d55ed6888439b1b9cf

New changeset daf7a082b20e59a0518cda1500add42c36ab058f by Miss Islington (bot) in branch '3.8':
bpo-38823: Fix refleak in _tracemalloc init error handling (GH-17235)
https://github.com/python/cpython/commit/daf7a082b20e59a0518cda1500add42c36ab058f

New changeset 1d7245c3e0cfe4508855c5025b25d8894155ecc5 by Miss Islington (bot) in branch '3.7':
bpo-38823: Fix refleak in _tracemalloc init error handling (GH-17235)
https://github.com/python/cpython/commit/1d7245c3e0cfe4508855c5025b25d8894155ecc5

New changeset e5d1f734db135e284af8e8868e7ccc85355952b9 by Miss Islington (bot) (Brandt Bucher) in branch 'master':
bpo-38823: Clean up _xxtestfuzz initialization. (GH-17216)
https://github.com/python/cpython/commit/e5d1f734db135e284af8e8868e7ccc85355952b9

New changeset d2f96672642cc51b92f61b951ca1b11d615c12d1 by Brandt Bucher in branch 'master':
bpo-38823: Fix refleaks in _ast initialization error path (GH-17276)
https://github.com/python/cpython/commit/d2f96672642cc51b92f61b951ca1b11d615c12d1

New changeset d19fa7a337d829e3dab3e9f919f5dcf09cf6f6ba by Victor Stinner in branch 'master':
bpo-38823: Fix refleaks in _ctypes extension init (GH-23247)
https://github.com/python/cpython/commit/d19fa7a337d829e3dab3e9f919f5dcf09cf6f6ba

New changeset b5cc05bbe681dbe06d5ec6d34318815d1c1ad6c5 by Victor Stinner in branch 'master':
bpo-38823: Always build _ctypes with wchar_t (GH-23248)
https://github.com/python/cpython/commit/b5cc05bbe681dbe06d5ec6d34318815d1c1ad6c5

New changeset 0cec97eb6a3e31493c29ef9398fcf39a5ec445a6 by Victor Stinner in branch 'master':
bpo-38823: Fix compiler warning in _ctypes on Windows (GH-23258)
https://github.com/python/cpython/commit/0cec97eb6a3e31493c29ef9398fcf39a5ec445a6