Investigate extended usage of metadata

[haerdib]

We have full access to the metadata, but it's hardly used yet. Can we do more with that?

E.g. like this comments suggests:

substrate-api-client/compose-macros/src/lib.rs

Lines 38 to 39 in 7e5962b

	/// * 'args' - Optional sequence of arguments of the call. They are not checked against the metadata.
	/// As of now the user needs to check himself that the correct arguments are supplied.

Check out subxt example: https://github.com/paritytech/subxt/blob/master/subxt/src/tx/tx_client.rs#L56-L73

[clangenb]

It seems that what the comment suggests and what is done in subxt is something entirely different. Subxt does only verify the generic call metadata hash without the arguments of what has been generated at compile time in their macro matches the the hash of the pallet-prefix and the call-name of the supplied call.

Hence, subxt verifies the same thing as we already do with:

substrate-api-client/compose-macros/src/lib.rs

Line 46 in 7e5962b

let call_index = pallet.call_indexes.get($call_name).unwrap();

Be aware that subxt has more need for validation than us, as it relies on statically generated code that it wants to check against the actual metadata that the client connects to, while the api-client does only know the latter.

but I like the Idea. What could we do with the metadata? :)

[haerdib]

I see. Didn't look too deeply into that. Nonetheless, something like integritee-network/worker#970 (comment) was possible. Not sure if that's verifiable with the metadata, but I think it should be worth it to investigate a little.

[haerdib]

I suggest timeboxing this task to 8h. If there's a promising lead after investigation, it should be done in a different task.

[clangenb]

There is an extensive discussion on how to change the metadata, especially for (offline-) light clients, and there is an ongoing proposal on polkadot on how to implement this. I suggest waiting for some results there: https://forum.polkadot.network/t/the-ledger-app-debate-united-we-stand-divided-we-fall/3177/9.