Why is this assert here, rather than checking for an error in the line above and handling it normally?

because the buffer is filled sith hardcoded chars set in this module, and the PyUnicode_DecodeASCII docs say Return NULL if an exception was raised by the codec. ... so I thought ...

well I checked PyUnicode_DecodeASCII and it can return null also in other cases (for instance when PyUnicodeNew fails.

Will fix this now

fixed now, please note that in this error case (and in the one above) there still are refleaks.

When the referencs are stolen by the PyModule_AddObject (in the ok paths), nothing decrefs those in the following error paths...

Pretty sure those are cleaned up whenever the module is (the stolen references reside in its __dict__ now).

with this patch applied

diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index 5b58658709..ec8dd3d942 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -3067,7 +3067,7 @@ array_modexec(PyObject *m)
         return -1;
     }
 
-    return 0;
+    return -1;
 }
 
 static PyModuleDef_Slot arrayslots[] = {

the following program leaks

import time
import gc
import os
print(os.getpid())
gc.disable()
for i in range(1000):
    try:
        import array
    except Exception:
        time.sleep(0.1)

with and without the Py_DECREF

I'm not sure what you mean when you say "with and without the Py_DECREF"... does the same leak occur with:

-    return 0;
+    Py_DECREF(m);
+    return -1;

Either way, this looks good to me. This can be a question for the core reviewer.

I made this module to always fail on exec by returning -1

then, withor without the decrefs on error paths (see next patch), I get same leaking behaviour

diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index ec8dd3d942..aacef20f20 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -3041,12 +3041,10 @@ array_modexec(PyObject *m)
 
     Py_INCREF((PyObject *)&Arraytype);
     if (PyModule_AddObject(m, "ArrayType", (PyObject *)&Arraytype) < 0) {
-        Py_DECREF((PyObject *)&Arraytype);
         return -1;
     }
     Py_INCREF((PyObject *)&Arraytype);
     if (PyModule_AddObject(m, "array", (PyObject *)&Arraytype) < 0) {
-        Py_DECREF((PyObject *)&Arraytype);
         return -1;
     }
 
@@ -3063,7 +3061,6 @@ array_modexec(PyObject *m)
         return -1;
     }
     if (PyModule_AddObject(m, "typecodes", typecodes) < 0) {
-        Py_DECREF(typecodes);
         return -1;
     }

Well, you don't hit those branches, right? So I wouldn't expect a difference.

awwww yes you are right!

...anyways this seems to point to the fact that Py_CLEAR(module) is not needed on error...

It is not needed. PyModule_AddObject() checks the added object for NULL.

If you remove it, change also Py_DECREF(typecodes) to Py_XDECREF(typecodes).

done in 777fc61