Implement gas estimation

What is wrong?

We need to implement gas estimation for the JSON-RPC interface.

An ideal gas estimation API should take into account the following.

• total gas used isn’t always right due to gas refunds.
• transactions can alter execution flow based on the txn gas.

How can it be fixed

This should be implemented using a swappable backend API. Accurate gas estimations are likely to be computationally expensive where-as naive estimations based on gas used are likely to work for most use cases and should be computationally cheap.

The API can be something as simple as a function which takes VM instance and a Transaction instance and returns the gas estimation. It might be appropriate to include this as an API on the Chain and VM as Chain.estimate_transaction_gas() and VM.estimate_transaction_gas()

Strategies

We should first implement a PeekGasUsedStrategy which uses the peek gas usage

1. run the computation with the gas limit set at the full gas limit for the next pending block.
2. return the computation.gas_meter.start_gas - computation.gas_metea.gas_remaining for the outer computation.

Then we should implement what I believe is the current strategy used by go-ethereum which starts with running the transaction using the gas limit for the next pending block and then performs a binary search, re-running the transaction at each level of the search until it finds the threshold for which the transaction succeeds/fails.

Note that the binary search can be performed at a higher granularity than 1 gas. From @carver comment, a 1024 granularity should produce approximately a 2x speedup.

[pipermerriam]

I have no idea what I was looking at. Disregard.

[pipermerriam]

Thanks, I updated the spec to reflect the gas limit and the granularity.