Implement missing host functions

[cmichi]

pallet-revive has added a number of host functions that we don't support in ink! yet. I've just compiled the list and am creating this tracking issue for them.

• fn get_immutable_data(output: &mut &mut [u8]);
• fn set_immutable_data(data: &[u8]);
• fn balance_of(addr: &[u8; 20], output: &mut [u8; 32]);
• fn chain_id(output: &mut [u8; 32]);
• fn gas_price() -> u64;
• fn base_fee(output: &mut [u8; 32]);
• fn call_data_size() -> u64;
• fn origin(output: &mut [u8; 20]);
• fn code_size(addr: &[u8; 20]) -> u64;
• fn call_data_load(output: &mut [u8; 32], offset: u32);
• fn gas_limit() -> u64;
• fn set_storage_or_clear(flags: StorageFlags, key: &[u8; 32], value: &[u8; 32]) -> Option<u32>;
• fn get_storage_or_zero(flags: StorageFlags, key: &[u8; 32], output: &mut [u8; 32]);
• fn return_data_size() -> u64;
• fn return_data_copy(output: &mut &mut [u8], offset: u32);
• fn ref_time_left() -> u64;
• fn block_author(output: &mut [u8; 20]);
• fn block_hash(block_number: &[u8; 32], output: &mut [u8; 32]);

Having the host functions supported is not a requirement for an ink! v6 release. Some are easy to implement, while others (e.g. immutable data) require more thought about the design.

[cmichi]

@LucasGrasso This is a tracking issue, so you don't have to implement all of these functions at all. Let's just start with a single one: gas_limit. This is a host function that pallet-revive exposes to us, it already exists there. It just doesn't exist in ink! yet.

You can take a look at the function transferred_value and how we've implemented it (just full-text search the codebase). It's a similar function, it just gets some value from pallet-revive and returns it to the smart contract.

You'll find a number of places where transferred_value is used and you can implement gas_limit by taking the transferred_value code as a blueprint.

We use transferred_value in a bunch of contract examples (those are in integration-tests). For gas_limit it's best if you add a new contract example integration-tests/internal/gas. This contract should have a contract function that uses gas_limit + an E2E test that tests this contract function.

Feel free to ask questions in case you have questions!

Edit: Some pointers

• https://github.com/paritytech/polkadot-sdk/blob/master/substrate/frame/revive/src/vm/pvm/env.rs#L625-L631 This is where pallet-revive defines gas_limit.
• https://github.com/use-ink/ink/blob/master/crates/env/src/engine/on_chain/pallet_revive.rs#L890-L896 This is how transferred_value calls pallet-revive. There are traits and trait implementations that call this function, hence you're best of doing a full-text search.

[LucasGrasso]

Thanks for the introduction and help.
How should one implement the hostfn in the off-chain engine? I suppose that the gas limit value should be stored in the Engine Database as one can not access revive? How are those db values configured?

[cmichi]

@LucasGrasso We are in the process of removing the off-chain testing environment. Hence you can just add the method there, but panic inside of it.

[LucasGrasso]

• gas_limit should be marked as completed.
• I will continue with the implementation of the rest of the 0-ary functions
  fn gas_price() -> u64
fn call_data_size() -> u64
fn return_data_size() -> u64
fn ref_time_left() -> u64