spending conditions - make block number and time available in exec environment

[johannbarbie]

• read time and height from last period submitted into bridge

Execution time:

• p+0 -> p+1 -> p+2 -> p+3
• p+0 is submitted, p+1 not submitted.
• use time from p+0 for execution.

Challenge time:

• p+0 -> p+1(tx inclusion) -> p+2 -> p+3
• p+0 to p+3 all submitted.
• tx with proof referencing p+1 initiates computation game.
• solEVM verifier checks the proof, walks 1 period back, takes the time and includes time in initial state.

[troggy]

so a node needs to keep track of the timestamp and height of the last successful period submission, right?

[johannbarbie]

we are already storing the Ethereum block hash and time in each period submission: https://github.com/leapdao/leap-contracts/blob/master/contracts/Bridge.sol#L98

⚠️ but there is no way to prove the prevPeriod 🤔 ⚠️