feat: add RefundGas function to GasMeter

[fedekunze]

Adds support for refunding gas directly to the gas meter. Otherwise, you need to create a new GasMeter with the same gas limit and consume the desired amount:

// before
gasMeter := NewGasMeter(ctx.GasMeter().Limit())
gasMeter.ConsumeGas(uint64(ctx.GasMeter().GasConsumed()) - amount)
ctx.WithGasMeter(gasMeter)

// now
ctx.GasMeter().RefundGas(amount, "gas refunded from EVM execution")

Note: this PR doesn't affect how fees are managed

[codecov]

Codecov Report

Merging #9403 (6d09288) into master (33c045c) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 6d09288 differs from pull request most recent head 757053d. Consider uploading reports for the commit 757053d to get more accurate results

@@            Coverage Diff             @@
##           master    #9403      +/-   ##
==========================================
+ Coverage   60.39%   60.41%   +0.02%     
==========================================
  Files         589      589              
  Lines       37099    37113      +14     
==========================================
+ Hits        22405    22423      +18     
+ Misses      12715    12712       -3     
+ Partials     1979     1978       -1     

Impacted Files	Coverage Δ
store/types/gas.go	81.48% <100.00%> (+2.31%)	⬆️
x/staking/keeper/grpc_query.go	62.87% <0.00%> (+0.50%)	⬆️
x/bank/keeper/send.go	84.87% <0.00%> (+0.52%)	⬆️
x/auth/legacy/v043/store.go	67.14% <0.00%> (+2.85%)	⬆️

[ethanfrey]

Can you please show a real world use-case? Both in the description as well as a godoc on the method. Not sure when this would be useful.

I have done gas-limited calls to CosmWasm contracts via submessages, which is an interesting use-case and I would be happy if there was a primitive that would accomplish some of this. But Refund doesn't make much sense there I don't think.

Please check out https://github.com/CosmWasm/wasmd/blob/master/x/wasm/keeper/msg_dispatcher.go#L46-L71

Happy to see the code that would use this.

[fedekunze]

@ethanfrey great question. So go-ethereum uses the StateDb interface to execute state transitions on the EVM. One core functionality that this interface supports is adding and subtracting gas to/from the gas pool available for the transaction (see Refund funcs).

This PR adds the functionality to subtract a given amount of gas from the gas pool to fully support this use case within Ethermint.

[ethanfrey]

@ethanfrey great question. So to go-ethereum uses the StateDb interface to execute state transitions on the EVM. One core functionality that this interface supports is adding and subtracting gas to/from the gas pool available for the transaction (see Refund funcs).

This PR adds the functionality to subtract a given amount of gas from the gas pool to fully support this use case within Ethermint.

Thanks for the explanation. Can you add a few lines of docs above the RefundGas function saying so. Or it might be refactored away in 6 months when no one remembers why it is there

[fedekunze]

Can you add a few lines of docs above the RefundGas function saying so. Or it might be refactored away in 6 months when no one remembers why it is there

@ethanfrey done

[ethanfrey]

thanks for the update

[aaronc]

ACK. Okay to merge this into master at this point in the release process @AmauryM and @robert-zaremba ?

[amaury1093]

Nothing's breaking in this PR, so I'm fine to merge this into master.

We're manually testing v0.43 on the regen side right now, I added a note to possibly test this regen-network/regen-ledger#352.

[iramiller]

Why is this change not merged into the 0.43 series? Our chain is currently running with 0.42.6 which required an update to include the GasMeter in our custom antehandler ... this is now broken during our testing on 0.43-beta1 because it was not ported over.

[ryanchristo]

Everything currently in master will be included in RC0. Once we cut RC0, we will create a separate release branch for v0.43. This change did not make it into the initial beta release and we have not provided an updated release since. Apologies for the wait but rest assured this will be included in RC0.

[aaronc]

@AmauryM how is this missing in 0.43?

[amaury1093]

it's not in 0.43-beta1, but it's in 0.43-rc0

[albertchon]

I'm wondering if there's some restrictions/limitations on how much gas one can refund?

Empirically, I've found that when the entire gas consumed is refunded, the tx will timeout with ERRO[0051] failed to commit msg batch: null while if a smaller amount is used, it'll succeed.

// always times out
ctx.GasMeter().RefundGas(ctx.GasMeter().GasConsumed(), "test")
// succeeds
ctx.GasMeter().RefundGas(ctx.GasMeter().GasConsumed() / 10, "test")

Any ideas why?

EDIT: discovered that one can only safely refund the gas consumed in the handler execution itself.
So the following works:

// beginning of handler
startGas := ctx.GasMeter().GasConsumed()
...
ctx.GasMeter().RefundGas(ctx.GasMeter().GasConsumed() - startGas, "test")