[issue46606] Large C stack usage of os.getgroups() and os.setgroups()

[Inada Naoki]

New submission from Inada Naoki <songofacandy at gmail.com>:

I checked stack usage for bpo-46600 and found this two functions use a lot of stack.

os_setgroups: 262200 bytes
os_getgroups_impl: 262184 bytes

Both function has local variable like this:

    gid_t grouplist[MAX_GROUPS];

MAX_GROUPS is defined as:

```
#ifdef NGROUPS_MAX
#define MAX_GROUPS NGROUPS_MAX
#else
    /* defined to be 16 on Solaris7, so this should be a small number */
#define MAX_GROUPS 64
#endif
```

NGROUPS_MAX is 65536 and sizeof(gid_t) is 4 on Ubuntu 20.04, so grouplist is 262144bytes.

It seems this grouplist is just for avoid allocation:

```
    } else if (n <= MAX_GROUPS) {
        /* groups will fit in existing array */
        alt_grouplist = grouplist;
    } else {
        alt_grouplist = PyMem_New(gid_t, n);
        if (alt_grouplist == NULL) {
            return PyErr_NoMemory();
        }
```

How about just using `#define MAX_GROUPS 64`?
Or should we remove this grouplist because os.grouplist() is not called so frequently?

----------
components: Library (Lib)
messages: 412335
nosy: methane
priority: normal
severity: normal
status: open
title: Large C stack usage of os.getgroups() and os.setgroups()
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46606>
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