[light client] RPC eth_estimateGas don't behave as in full client

[niklasad1]

What is the problem

In the context of a light-client, for example, we want to use the eth_estimateGas RPC for an unknown account and this doesn't work currently because the ExecutionProof always falls and this don't happen in full client implementation.

2018-09-12 10:54:02   TRACE rpc  Request: {"method":"eth_estimateGas","params":[{"from":"acc", "gas":"0xffff", "gasPrice":"0xffff", "nonce":"0x0"}, "latest"],"id":1,"jsonrpc":"2.0"}.           
2018-09-12 10:54:02   DEBUG on_demand  Execution request, Incomplete { block_hash: Scalar(0x97db89353cc3abb9d51a6690338cb66a2f10dd58f350394e9f1f980201320b2c), from: acc, action: Create, gas: 65535, gas_price: 65535, value: 0, data: [] }
execute_tx finished wait on the future
2018-09-12 10:54:04  IO Worker #1 DEBUG on_demand  #1 requests have not been dispatched
2018-09-12 10:54:04  IO Worker #1 DEBUG on_demand  ReqID: Request #11
2018-09-12 10:54:04  IO Worker #1 DEBUG on_demand  Request Batch { outputs: {}, requests: [Execution(Incomplete { block_hash: Scalar(0x97db89353cc3abb9d51a6690338cb66a2f10dd58f350394e9f1f980201320b2c), from: x, action: Create, gas: 65535, gas_price: 65535, value: 0, data: [] })], answered: 0 }
2018-09-12 10:54:07  IO Worker #2 DEBUG on_demand  Execution request completed Incomplete { block_hash: Scalar(0x97db89353cc3abb9d51a6690338cb66a2f10dd58f350394e9f1f980201320b2c), from: account, action: Create, gas: 65535, gas_price: 65535, value: 0, data: [] }
2018-09-12 10:54:07  IO Worker #2 DEBUG on_demand  BadExecution Proof
2018-09-12 10:54:07  IO Worker #2 DEBUG on_demand  Peer 43 gave bad response: Validity(BadProof)
2018-09-12 10:54:07  IO Worker #2 DEBUG on_demand  BadExecution Proof
2018-09-12 10:54:07  IO Worker #2 DEBUG on_demand  Peer 36 gave bad response: Validity(BadProof)
...
and so on!

¹ The request batch log is not part of parity, I manually added it which prints out the number of requests and indiviudal request which have not been anwered yet!

Educated Guess

I'm not exactly sure the reason why but I think it is because the actual execution in full client seems to boost the actual account balance to execute the transaction successfully (transact_virtual will provide enough funds), https://github.com/paritytech/parity-ethereum/blob/master/ethcore/src/client/client.rs#L1473-#L1545.

In the light client validation (executed by one or more full nodes) we use are using the actual account balance for validation and should be changed to do same as the full client (boost the account balance)!

However, in context as a validator, I'm not sure how to distinguish between different executions and it might be better to leave it as it is!

Related files:

• https://github.com/paritytech/parity-ethereum/blob/master/ethcore/light/src/provider.rs#L267-#L286 (call to provider full node)
• https://github.com/paritytech/parity-ethereum/blob/master/ethcore/src/client/client.rs#L2346-#L2364
• https://github.com/paritytech/parity-ethereum/blob/master/ethcore/src/state/mod.rs#L230-#L263