gh-143379: fix UAF in struct.Struct.pack() when object modified by dunder methods by skirpichev · Pull Request #143382 · python/cpython

Would not it prevent concurrent use of pack()? It would be undesirable. We need a counter (atomic for GIL-less build) which would allow concurrent operations, but block __init__(). Please test how it affects performance.

To be absolutely safe, we would need also a mutex which would block packing while __init__() is executed. Because there is a race condition between checking if it is safe to modify the struct state and modifying it. But it is very unlikely to happen in real world (why would anybody call __init__() concurrently with pack()?), so we can ignore this for now. Well, ignoring this issue until we forbid repeated calls of __init__() is also solution.