The following code causes the interpreter to crash:
import os
import time
import resource
new_pid = os.fork()
if new_pid == 0:
    time.sleep(0.5)
else:
    resource.struct_rusage = None
    os.wait3(0)
    
We would get a crash also if we replaced 'os.wait3(0)' with 'os.wait4(new_pid, 0)'.

This is because wait_helper() (in in Modules/posixmodule.c) assumes that
resource.struct_rusage is a type object, and passes it to PyStructSequence_New(),
which tries to access the n_fields attribute, and crashes.


In addition, the following code causes a SystemError:
class BadStructRusage:
    n_fields = None

import os
import time
import resource
new_pid = os.fork()
if new_pid == 0:
    time.sleep(0.5)
else:
    resource.struct_rusage = BadStructRusage
    os.wait3(0)

This is because PyStructSequence_New() (in Objects/structseq.c) assumes that it
received a type with a valid n_fields attribute.


Similarly, the following code causes the interpreter to crash:
class BadStructRusage:
    n_fields = 16
    n_sequence_fields = None

import os
import time
import resource
new_pid = os.fork()
if new_pid == 0:
    time.sleep(0.5)
else:
    resource.struct_rusage = BadStructRusage
    os.wait3(0)


ISTM that we can fix these problems by adding checks to wait_helper() and to
PyStructSequence_New().
However, maybe a more simple solution would be to either:
    - Make wait_helper() always use StructRUsageType (defined in Modules/resource.c).
    - Disable assigning to resource.struct_rusage.

Moreover, I don't understand the comment before calling PyStructSequence_New():
/* XXX(nnorwitz): Copied (w/mods) from resource.c, there should be only one. */
Is it relevant to this issue?


Lastly, I am not sure about tests (as I found almost no tests of wait3() and
wait4()).
Should I add to Lib/test/test_wait3.py and Lib/test/test_wait4.py each a class
to test this issue? Or is it too much of a corner case, and a test is not needed?

Attached PR 3750 tries to validate the input type. The problem is that structseq is not exactly a type. It's hard to really validate everything. See the comment test in my PR.