Clone InterfaceRegistry for migrations

[amaury1093]

Summary

Migrations need to register legacy interfaces. To not pollute the real interfaceRegistry, we should clone a separate interfaceRegistry for migrations only.

Problem Definition

Here's an example:

• In v042, the supply was stored in state as an interface SupplyI:

  cosmos-sdk/x/bank/keeper/keeper.go

  Line 174 in 7648bfc

  bz, err := k.MarshalSupply(supply)

  
  To do so, we needed to register SupplyI in the interfaceRegistry
• In v043, we now store the supply indexed by denom (Index supply by denom #7092). We removed the SupplyI interface, and also removed it from the interfaceRegistry.

However, to perform in-place store migrations from 0.42 to 0.43, we still need to register SupplyI inside an interfaceRegistry, just to be able to perform the migration.

Proposal

Three proposals:

1. Keep one unique interfaceRegistry like now, and register legacy interfaces on it

This is the approach taken in #8998. In the (AppModule) RegisterInterfaces, we also register legacy interfaces.

+ simple
- polluting the current interfaceRegistry with legacy interfaces

2. Clone the interfaceRegistry inside each keeper

Each keeper has a Migrator struct, which could receive a reference to the interfaceRegistry, clone it, and register legacy interfaces on that clones interfaceRegistry.

+ simple (maybe slightly less than 1/, but still okay)
- we are cloning N interfaceRegistries for migrations
- docs for module developers

3. Clone the interfaceRegistry once, and use this same instance for all migrations

An idea could be for the configurator to hold a migrationsInterfaceRegistry, which is a clone of the real interfaceRegistry. Also, when each module calls RegisterMigration, we can allow it to register legacy interfaces:

- RegisterMigration(moduleName string, forVersion uint64, handler MigrationHandler) error
+ RegisterMigration(moduleName string, forVersion uint64, handler MigrationHandler, func (registry types.InterfaceRegistry)) error

// and then, inside modules:

func (am AppModule) RegisterServices(cfg module.Configurator) {
	// -- snip --
-	cfg.RegisterMigration("bank", 1, m.Migrate1to2)
+	cfg.RegisterMigration("bank", 1, m.Migrate1to2, v040bank.RegisterInterfaces)
}

+ no cloning of N interfaceRegistries, no polluting the real interfaceRegistry
- makes code a bit harder to understand
- refactoring (possibly with breaking changes)

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[amaury1093]

While 3/ seems like the correct way, i actually prefer 2/ as its a good balance between code cleanliness and simplicity/understandability.