Make get_dispatch_info and weight_to_fee Available via state_call

[joepetrowski]

When constructing XCM Transact calls, it's useful to have an idea of how much weight some call will consume and how much that weight will cost in order to BuyExecution. query_info only takes a fully encoded extrinsic and returns the weight and fee, but requires some hacks, namely:

• Users must add some bytes to represent an extrinsic (signature/signed extensions), but an XCM Transact will just have the call.
• The return includes the length fee and the ExtrinsicBaseWeight, which are not relevant in Transact.

The addition of these two endpoints would allow submission of a Call and obtaining its weight and a fee estimate (with the user knowing that the weight to fee rate can be variable so they'll want to supply a little extra):

• fn get_dispatch_info(call) -> DispatchInfo { weight, ... }; (useful for specifying required weight)
• fn weight_to_fee(weight) -> Balance; (useful for buying execution)

Related: polkadot-js/apps#7688

[bkchr]

• fn get_dispatch_info(call) -> DispatchInfo { weight, ... }; (useful for specifying required weight)

This is already returned by payment_querInfo.

• fn weight_to_fee(weight) -> Balance; (useful for buying execution)

And should that not be doable with payment_queryFeeDetails?

[joepetrowski]

This is already returned by payment_querInfo

AFAIK, no, because it expects an extrinsic, not just a call.

[athei]

But is there any reason why you couldn't turn your call into an extrinsic and then submit it to these RPCs? It shouldn't matter with which key the extrinsic is signed.

[joepetrowski]

It's possible but it's also kind of a hack IMO. @TarikGul wrote this script to do that and that's what I use now.

It also doesn't give you the weight and execution cost of the call, because it includes ExtrinsicBaseWeight and the length fee. Calls made over XCM don't have this overhead. Again, it's OK-ish as it's an overestimate, but IMO we expect an increase in para-to-para interaction with instructions that are not signed by users, so we should expose weight and fee info related to executing the instructions and not on verifying signatures.

[xlc]

One thing I would like to add is that as we want to start using light client more, which doesn't supports custom RPC, we should avoid adding more custom RPC instead. The alternative solution is add new runtime API and use state_call to call the runtime API.

[joepetrowski]

Yes, that's why I linked to the runtime API source in the main body.
https://github.com/paritytech/substrate/blob/ef66142/frame/transaction-payment/rpc/runtime-api/src/lib.rs

[muharem]

Had a chat with @bkontur
Picking up the issue

[athei]

Please keep in mind what @xlc said: Don't add a new RPC. Having a new runtime API should be enough in order to call it through state_call.