V14 metadata doesn't provide enough information to decode extrinsic

[eldargab]

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• This is not a support question.

Description of bug

To properly decode extrinsic one needs to know

1. The type of Address
2. The type of Signature
3. The type of Call.

While all the above types are included in the .lookup.types, there is no way to know which one is which.

At Subsquid we look at paths and type parameters .

Polkadot.js uses its own set of hardcoded assumptions none of which is guaranteed to hold.

This is practical issue which hardens the maintenance of our stack.

Steps to reproduce

No response

[bkchr]

3. The type of Call.

Yeah the call was always there, but more implicit by giving you all pallets and all their calls. We should probably revisit now as we have scale-info.

[jsdw]

We actually use the metadata already to decode extrinsics:

https://github.com/paritytech/subxt/blob/3f16bb8d521031c7e13749d41711f54f490bb84c/subxt/src/blocks/extrinsic_types.rs#L205

To get the address, call, signature and extra details needed, we do this:

https://github.com/paritytech/subxt/blob/3f16bb8d521031c7e13749d41711f54f490bb84c/subxt/src/blocks/extrinsic_types.rs#L483

Currently this means that we do rely on the metadata.extrinsic.ty containing specific generics (in other words, that it's an UncheckedExtrinsic type).

Perhaps we should expose those type IDs directly under metadata.extrinsic though, which would ensure that they exist (and make it easier to get at them). ie have:

pub struct ExtrinsicMetadata<T: Form = MetaForm> {
	/// The type of the extrinsic.
	pub ty: T::Type,
	/// Extrinsic version.
	pub version: u8,
        /// Type of the outermost Call enum.
        pub call_ty: T::Type,
        /// Type of the address.
        pub address_ty: T::Type,
        /// Type of the extrinsic signature.
        pub signature_ty: T::Type,
	/// The signed extensions in the order they appear in the extrinsic.
	pub signed_extensions: Vec<SignedExtensionMetadata<T>>,
}

Does that sound good? In the absense of anything saying otherwise, we'll aim to get this in to V15 metadata, because it seems like a sensible addition to me.

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/stablising-v15-metadata/2819/1