Init sync: conditional syncing to finalized slot

[farazdagi]

What type of PR is this?

Other

What does this PR do? Why is it needed?

• When node's head is already beyond the finalized slot, phase1 sync should be skipped (and phase2 -- sync to best-known non-finalized slot -- should commence).
• This will also help with More robust init-sync on detached head block #7990 (init-sync will skip the 1st phase, and continue immediately into the 2nd, re-syncing immediately; otherwise, it stays in 1st phase until chain sufficiently advances -- we check that requested slot is lower than finalized one in 1st phase, and do not do so in 2nd).
• This PR:
  • extracts phase1/phase2 sync code into the separate methods
  • adds additional check in phase1 code, to see if chain.HeadSlot() >= highestFinalizedSlot

Which issues(s) does this PR fix?

N/A

Other notes for review

[shayzluf]

how do we handle missing slot in this case?

[farazdagi]

It is not missing slot, we have a finalized epoch add +1 to it, and take the start slot of the next to finalized epoch. But since this method should sync to finalized slot only, we need to get the last slot of the finalized epoch, not the first slot of the next epoch.

Note: since we are adding +1 to finalized epoch, then even if it finalized epoch is zero, we get 1st epoch (so, can substract w/o underflow)

[farazdagi]

Although highestFinalizedEpoch cannot be zero (since we are using start slot of the next to finalized epoch), I still added check for zero, just in case some logic on how we first setting up the highestFinalizedSlot changes (and we will be open to underflow).

[shayzluf]

thanks for clarifying