Problem: state transition code is duplicated

[yihuang]

The key thing here is to make ApplyMessage more self-contained and easier to use.

ApplyMessage is called in several sites:

• ApplyTransaction, tx execution
  • the fee is already deducted in ante handler, so should do gas refund after message executed
  • should commit the context stack.
• EthCall/EthEstimateGas query handler
  • don't need to refund gas,
  • don't need to commit context stack
• ApplyNativeMessage, called by native modules to execute evm message
  • no need to refund gas
  • gas is accumulated naturally by cosmos-sdk
  • should commit context stack

The other thing is the context stack commit/flatten logic is pretty complicated, one way to simplify it is to keep the context stack logic inside the ApplyMessage, when ApplyMessage returns, the stack is either committed or discarded.

Proposal

Extract the common ground of the above scenarios, we should make the following changes to ApplyMessage:

1. ApplyMessage only calculates the gas refund amount and adjusts the gasUsed, but don't do refund action, move that to ApplyTransaction.
2. Move EVM construction into ApplyMessage, so the caller doesn't need to do it by themselves.
3. Simplify the parameters of ApplyMessage, by encapsulating the common ones into a struct.
4. Always commit/revert the context stack when ApplyMessage returns, so the caller doesn't need to worry about the context stack commit, flatten, revert stuff.
5. Remove ApplyNativeMessage, since the implementation is so trivial.

// External interface to execute a message:

k.WithContext(ctx)
msg := build core.Message
res, err := k.ApplyMessage(msg, nil, true) // nil means use default tracer, true means commit the context stack.
if err != nil {
    return err
}