Bug report

_winapi_WaitForMultipleObjects_impl in _winapi.c creates an array of MAXIMUM_WAIT_OBJECTS handles but then arbitrarily enforces a cap of MAXIMUM_WAIT_OBJECTS - 1 handles. This is incorrect. WaitForMultipleObjects is clearly documented as being able to handle MAXIMUM_WAIT_OBJECTS (64) handles. This simple C++ sample code demonstrates this:

#include <stdio.h>
#include <windows.h>

#include

int main(int argc, char* argv[]) {
for (DWORD count = 60; count < 66; ++count) {
printf("About to wait on %d handles.\n", count);
std::vector handles(count);
for (int i = 0; i < count; ++i)
handles[i] = CreateEventW(nullptr, FALSE, TRUE, nullptr);
DWORD wait_result = WaitForMultipleObjects(count, handles.data(), TRUE, INFINITE);
if (wait_result == WAIT_OBJECT_0)
printf(" Wait satisfied by object 0.\n");
else if (wait_result == WAIT_FAILED)
printf(" Wait failed.\n");
else
printf(" Result was %lu.\n", wait_result);
for (int i = 0; i < count; ++i)
CloseHandle(handles[i]);
}
}

This error also shows up here:
3988986#diff-eb5cd13d65d2f215d8dc0d95eb1a1341ff69b226ac56ec6099e39582edecc8cfR219
and here:
3988986#diff-84a955ea642d41d9f164e9f94499d8eb33c357ec7f4362a341951f6f67ab21a1R117

Note that this is in addition to the off-by-one error that was tracked by this issue:
#84444

That is, there used to be an off-by-two error (maximum of 60 processes could be waited on) and now there is an off-by-one error (maximum of 61 processes can be waited on). In Python 3.8.10 the limit is still 60, but in Python 3.9 the limit is (as far as I can tell) 61. It should be 62.

Your environment

Python 3.8.10 (tags/v3.8.10:3d8993a, May 3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)] on win32
Windows 10