Fork choice spec checklist

[JustinDrake]

• 1. Executable spec: Make the fork choice spec executable.
• 2. (post-freeze discussion) Maximum reversions: Enshrine dynamic weak subjectivity reversion period into fork choice rule ("second finality mechanism").
• 3. Slashed validators: Decide whether slashed validators should be ignored or not. (Current inclination is to not ignore slashed validators.)
• 4. Eth1 validity: Explicitly require the Eth1 fork pointed to by state. latest_eth1_data to be valid and available.
• 5. Manual forks: Specify how manual forks are enacted and the corresponding interaction with state.fork.
• 6. Genesis event: Revisit how the genesis event is defined, especially if Eth2Genesis is removed from the deposit contract (see CHAIN_START_FULL_DEPOSIT_THRESHOLD bypass #1048). Formalise the genesis event in the fork choice rule.
• 7. (post-freeze discussion) Flip-flop attack hardening: Decide if there is anything actionable regarding flip-flop attacks.
• 8. (post-freeze discussion) Revisit SECONDS_PER_SLOT: Potentially increase to 8 or 12 seconds.
• 9. Clocking: Formalise the notion of an "honest clock", e.g. maximum drift from "real" time. Decide if there is anything actionable from this research post.
• 10. Tie breakers: Make sure the fork choice is always well-defined. In particular, check that all ties are broken. For example, "Let finalized_head be the finalized block with the highest epoch." is not well-defined if there are two finalized heads with the highest epoch.

[djrtwo]

This also needs to be added. In the context of a non-executable spec, it is a one-liner
#768

[JustinDrake]

We also need to be mindful of the DoS where many forks need to be followed through epoch transitions.

[vbuterin]

10. Tie breakers: Make sure the fork choice is always well-defined. In particular, check that all ties are broken. For example, "Let finalized_head be the finalized block with the highest epoch." is not well-defined if there are two finalized heads with the highest epoch.

FWIW my current code ( https://github.com/ethereum/eth2.0-specs/pull/1068/files#diff-65ff52fa41121e465ff23d3b53fd0518R205 ) de facto makes the winner be "the first finalized head you discovered", ie. it's a memory-ful fork choice rule. And it has to be memory-ful because that's how weak subjectivity works anyway.....

7. Flip-flop attack hardening: Decide if there is anything actionable regarding flip-flop attacks.

I have been waiting for Yan to finish a liveness proof to finalize this.

[djrtwo]

all items except (2) are addressed. Closing in favor of #1520