gh-143379: fix UAF in struct.Struct.pack() when object modified by dunder methods

[skirpichev]

• Issue: Use-after-free in s_pack_internal via re-entrant __bool__ #143379

[skirpichev]

Alternative approach: use some flag in PyStructObject struct to forbid mutation during pack().

[vstinner]

Alternative approach: use some flag in PyStructObject struct to forbid mutation during pack().

Would it be possible to always disallow mutation? Raise an exception if __init__() is called twice.

[skirpichev]

Why there is __init__() method at all? The Struct expected to be an immutable type.

Maybe all logic in Struct___init__ (tp_init) should be moved to s_new (tp_new)? I think this will fix issue. Looking in the history, the custom __init__() was re-introduced back in #112358.

Here is the plan:

1. create a flag to temporary forbid mutation
2. deprecate Struct.__new__() calls without one required argument
3. eventually drop Struct.__init__() together with a flag

CC @serhiy-storchaka