Reading arguments in chain extension

Question

I have a question about reading data in a chain extension. To be precise, I'm concerned about methods Environment<_,_,_,S: BufIn>::read*. Do I understand correctly, that every such function doesn't move starting pointer and thus I can make only a single read?

If so, then I would like to ask whether there are any reasons for that. My use case where it is kinda onerous, is when my chain extension has such an API:

fn extension(a: SomeSizedType, b: Vec<u8>) {...}

The approach that would be natural for me would be to do parsing like:

let env = env.buf_in_buf_out();

let a = env.read_as<SomeSizedType>()?;
// The caller is expected to pass length hint
let b_len = env.read_as<u32>()?;
let b = env.read(b_len);

However, when I can invoke read* only once, I have to handle decoding and byte array mess on my site. Please notice, that I cannot pack a and b into a single struct, and use read_as, because Vec<_> is not MaxEncodedLen.

Answer 1

The way you would handle that is to pack a and b into a single struct. As you pointed out you can't use read_as in that case since that type will not be bounded. Do this instead:

let env = env.buf_in_buf_out();
let len = env.in_len();
// TODO: sanity check len and maybe charge some weight based on that
let input: MyStruct = env.read_as_unbounded(len)?;