Why is "msg_sender()" a Result?

[PaulRBerg]

While browsing through the examples in the docs, this piece of code caught my attention:

let sender: Result<Identity, AuthError> = msg_sender();
let sender = match sender.unwrap() {
    Identity::Address(addr) => addr,
    _ => revert(0),
};

Why is the msg_sender a Result and not always an Identity::Address? I presume this has to do with the FuelVM. If yes, in what cases is the msg_sender null?

For context, my background is in Solidity, and a little bit of Rust.

[otrho]

You can see here in the standard libarary where msg_sender() is implemented: https://github.com/FuelLabs/sway/blob/master/sway-lib-std/src/auth.sw#L34-L89

It can return AuthError::InputsNotAllOwnedBySameAddress if the caller is external. I'm not an application dev so I can't elaborate more on the full semantics there, but it should make sense?

[PaulRBerg]

Thanks Toby, this is helpful.

I am still unsure in what cases one would bump into the InputsNotAllOwnedBySameAddress error.

Does that happen when the signature is forged and one of the input UTXOs do not belong to the same caller?

[mohammadfawaz]

Does that happen when the signature is forged and one of the input UTXOs do not belong to the same caller?

Exactly. The inputs need to belong to the same caller which could be an "address" or another "contract", hence the type Identity wrapped in Result.