_zoneinfo starts with:

static int
zoneinfomodule_exec(PyObject *m)
{
    PyDateTime_IMPORT;

with:

#define PyDateTime_CAPSULE_NAME "datetime.datetime_CAPI"
#define PyDateTime_IMPORT \
    PyDateTimeAPI = (PyDateTime_CAPI *)PyCapsule_Import(PyDateTime_CAPSULE_NAME, 0)

If PyCapsule_Import() fails, zoneinfomodule_exec() returns 0 (success) with an exception raised.

It should check if the import succeeded or not. Concrete example on AIX where datetime cannot be imported:
https://bugs.python.org/issue42604#msg385347

"./python setup.py build" fails with:

Assertion failed: (item != NULL) ^ (PyErr_Occurred() != NULL), file Objects/abstract.c, line 163

--

By the way, the import machinery should raise a SystemError if a module exec function raises an exception *and* reports a success: see _Py_CheckFunctionResult().

If PyCapsule_Import() fails, zoneinfomodule_exec() returns 0 (success) with an exception raised.

Why not return -1 directly when PyCapsule_Import() fails?
It looks like more simpler. Do I miss something?

Why not return -1 directly when PyCapsule_Import() fails?

That would work.

I opened an issue since there is a bug.

New changeset eeb701a by Hai Shi in branch 'master':
bpo-42979: _zoneinfo exec function checks for PyDateTime_IMPORT failure (GH-24333)
eeb701a

Thanks for the fix Hai Shi!

By the way, the import machinery should raise a SystemError if a module exec function raises an exception *and* reports a success: see _Py_CheckFunctionResult().

It's already the case. Example:

>>> import _zoneinfo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: execution of module _zoneinfo raised unreported exception

With this bug:

diff --git a/Modules/_zoneinfo.c b/Modules/_zoneinfo.c
index d0c462fb86..fc564b9587 100644
--- a/Modules/_zoneinfo.c
+++ b/Modules/_zoneinfo.c
@@ -2683,6 +2683,7 @@ zoneinfomodule_exec(PyObject *m)
         goto error;
     }
 
+    PyErr_SetString(PyExc_Exception, "BUG");
     return 0;
 
 error:

New changeset 895591c by Miss Islington (bot) in branch '3.9':
bpo-42979: _zoneinfo exec function checks for PyDateTime_IMPORT failure (GH-24333) (GH-24351)
895591c

It's already the case. Example

Nice, interesting case.

New changeset c9b8e9c by Victor Stinner in branch 'master':
bpo-42979: Enhance abstract.c assertions checking slot result (GH-24352)
c9b8e9c

New changeset a619263 by Victor Stinner in branch 'master':
bpo-42979: Use _Py_CheckSlotResult() to check slots result (GH-24356)
a619263