Minimal support for ERC2771 (GSNv2)

[Amxx]

This PR includes a ERC2771 compatible BaseRelayRecipient and a compatible, ERC712 based, relayer.

Fixes #2258

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

I don't know how to evaluate whether this TrustedForwarder implementation is compliant with the GSNv2 ERCs. I could only find a PR for an EIP that has been closed due to inactivity: ethereum/EIPs#2770. In this draft there is something weird about extending the ForwardRequest type that I don't understand where it fits in the GSNv2 picture. The signature for the verify and execute functions are different from what we have here.

Do you know if these two functions got changed at some point?

[Amxx]

I don't know how to evaluate whether this TrustedForwarder implementation is compliant with the GSNv2 ERCs. I could only find a PR for an EIP that has been closed due to inactivity: ethereum/EIPs#2770. In this draft there is something weird about extending the ForwardRequest type that I don't understand where it fits in the GSNv2 picture. The signature for the verify and execute functions are different from what we have here.

Do you know if these two functions got changed at some point?

AFAIK, there is no requierements about what the forwarder should or should not do. I went with a ERC712 metatx relayer, but anything is possible. My understanding is that the standard focus on the data encoding between the forwarder and the recipient. How the forwarder works, and how it verifies ownership before encoding data is not specified.

[frangio]

There is at least a requirement for the TrustedForwarder to follow an interface that the RelayHub will use, which if I understand correctly consists of verify and execute. However, these two functions are not specified in the EIP that was merged. We should report this to the OpenGSN team to at least get some clarity, because otherwise we would be merging a feature that we can't be sure is spec-compliant.

[frangio]

Actually, there is no EIP but I suppose the current de facto spec is the OpenGSN repo and it seems we're implementing a different Forwarder interface. Take a look at IForwarder.sol.

[forshtat]

We will get a verification audit of GSN v2 smart contracts, including Forwarder, in a very short time. I don't expect Forwarder to be affected, but still, after that I will update the ERCs with current interfaces.

In the meanwhile we accepted a proposal to add optional expiration date to the base ForwardRequest to prevent eternally valid signed messages (not on master yet):
opengsn/gsn@5d562f8

[frangio]

Thanks for the update @forshtat!

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@Amxx Given that ERC2771 can be used independently of the GSN we should consider renaming the GSNv2 directory. Perhaps something associated to meta-transactions would be a good name. One option is metatx.

[frangio]

Thank you!