Evaluation order of transactions in a block

[dahlia]

Currently the evaluation order of transactions signed by multiple signers in a block are arbitrary and can be reoredered (fabricated) by a miner. This indeterministic order makes games unfair.

For example, suppose there are two actions, Attack which decreases target's health and Defend which invalidates one succeeding Attack against on the signer. A signs a transaction t^A to Attack B, and B signs a transaction t^B to Defend itself. If two transactions try to belong to a block, a block miner can determine whether to decrease B's health or not. If t^A is followed by t^B B gets damage, whereas if t^B is followed by t^A A loses its chance.

Therefore, the evaluation order of transactions in a block should not be determined by any certain party.

One idea I have is to make the evaluation order determined after a block is mined so that a miner can't deal with it:

1. First of all, make Block<T>.Mine() to take a set of transactions instead of a sequence of transactions. These transactions are unordered.
2. When transactions (actions) in a block is being evaluated, sort them in the order of each one's nonce, which is derived from both a block hash and a transaction's signature.

This makes the order of evaluation impossible to predict until a block is mined. The order is still arbitrary, but it makes game play in general more fair.

Any opinions are welcome.

[moreal]

I started to work on this issue 😁

[moreal]

It's resolved by #355.

[moreal]

In #355, I decided to order transactions by its nonce for hash. But it can cause undetermined hash, which also affects the order in which transactions are executed.