Add Tx Middleware for Tips

[amaury1093]

Part of the Tx working group.

Summary

The Tips proto definitions have been merged in #9406. Once we switch to the middleware design (#9585), we should add a new TxMiddleware which consumes tips.

Problem definition

The use case is the following (as an example): user1 has Regen tokens, and wants to execute a Msg on the hub (e.g. MsgVote on a proposal that would impact regen) but does not have Atoms. Tips try to solve this issue by allowing user1 to tip someone (user2) on the Hub using regen tokens. Basically user1 sends regen tokens to user2 to cover the Tx fees on the hub.

user1 == tipper
user2 == fee payer

We assume that user1's regen tokens for the Tip are already on the Hub's bank module, via IBC.

To do so, user1 builds a Tx where the TxBody contains MsgVote, adds a Tip (and populates relevant fields like accSeq, pk...), and signs it using SIGN_MODE_DIRECT_AUX. But user1 does NOT broadcast the tx. Instead, user1 sends the tx_bytes to user2. Now user2 builds the following Tx:

• body_bytes == user1's tx's body_bytes
• auth_info with Tip and Fee both set:
  • Tip is the tip chosen (and signed over) by user1
  • Fee is the fee user2 chooses to pay for the Tx on the hub
  • two signer_infos: the 1st one is the tipper's SIGN_MODE_AUX info, the second one is user2's info (can be DIRECT or LEGACY_AMINO_JSON).
• signatures: 2 sigs, corresponding to the two signer_infos.

user2 then sends this tx on the Hub, and if it passes, then the Tips middleware will automatically transfer the regen tokens (on the hub) from user2 to user1.

Below are the requirements to make tips work. Given an AuthInfo with both Fee and Tip set, we should check:

• the tipper signs over Tip
• the feePayer signs over fees
• make the Tip transfer only if there was enough gas during the tx execution phase

Depends on:

• Migrate antehandlers to middleware based design #9585

Proposal

Create a TipsTxMiddleware in x/auth/middleware/tips.go with the following features:

type tipsTxHandler struct {
  bankKeeper bank.Keeper
}

• In the pre-hook:
  • Make sure the tipper signs over the Tip. In practice, it means that we need to check the tipper's signMode must be SIGN_MODE_AUX. Note: the feePayer is always the last signer_info
  • Make sure the feePayer signs over the Fee. In practice, it means that we need to check the feePayer's signMode must be either SIGN_MODE_DIRECT or SIGN_MODE_AMINO.
• run the next middleware
• In the post-hook
  • do a bank transfer of the Tip coins from tipper to feePayer.

[amaury1093]

I added some acceptance tests in #9997. When working on this issue, I believe it would be good to have some of those tests as CLI integration tests too.