Group Msg Pattern: expose tx messages introspection

[robert-zaremba]

Proposal

Currently we don't provide a clear way for handler to check other messages in the same transaction.

Proposal: put all decoded transaction in the context and allow handlers to know what's the current msg index, and browse other messages.

Motivation

Grouping messages in a transaction allows to make an atomic operations. This is a very powerful pattern. It allows to more advanced message composition, without creating more complex messages and handlers which would aggregate that operations.

This pattern is widely used by Algorand smart contracts: user groups many transactions in a single group transaction. Usually, one of the transaction is a smart contract call, which will check other transaction types. Example: a token vote. Vote call on a contract will only be validated if:

• it's called in a group transaction
• there is a exactly other transaction, where user is spending his coins to make a vote
• the vote "power" equals to the number of coins spent.
  Here is another example presented on Algorand with more details and design.

NOTE: in Algorand, there is no cross-smart contract calls, so this pattern is very important there.

Ref:

• Add Tx Signers information to the Context in AnteHandler, Add Tx Signers information to the Context in AnteHandler #4937 (comment)

[aaronc]

This sounds like interesting functionality but I'm not seeing how it's relevant for Cosmos. In general, I really prefer to spend design energy on ideas motivated by use cases arising in our ecosystem. I can also imagine downsides to this sort of design and we do allow cross-module calls in Cosmos so I'm not seeing what the need is.

[robert-zaremba]

This writing in in response to support direct access to other Msgs in same transaction (as requested in #4937). We don't need to refactor anything. The discussion is to decide if we want to enable this pattern or not.