## Proposed Changes
This change attempts to prevent failed re-orgs by:
1. Lowering the re-org cutoff from 2s to 1s. This is informed by a failed re-org attempted by @yorickdowne's node. The failed block was requested in the 1.5-2s window due to a Vouch failure, and failed to propagate to the majority of the network before the attestation deadline at 4s.
2. Allow users to adjust their re-org cutoff depending on observed network conditions and their risk profile. The static 2 second cutoff was too rigid.
3. Add a `--proposer-reorg-disallowed-offsets` flag which can be used to prohibit reorgs at certain slots. This is intended to help workaround an issue whereby reorging blocks at slot 1 are currently taking ~1.6s to propagate on gossip rather than ~500ms. This is suspected to be due to a cache miss in current versions of Prysm, which should be fixed in their next release.
## Additional Info
I'm of two minds about removing the `shuffling_stable` check which checks for blocks at slot 0 in the epoch. If we removed it users would be able to configure Lighthouse to try reorging at slot 0, which likely wouldn't work very well due to interactions with the proposer index cache. I think we could leave it for now and revisit it later.
## Proposed Changes
With proposer boosting implemented (#2822) we have an opportunity to re-org out late blocks.
This PR adds three flags to the BN to control this behaviour:
* `--disable-proposer-reorgs`: turn aggressive re-orging off (it's on by default).
* `--proposer-reorg-threshold N`: attempt to orphan blocks with less than N% of the committee vote. If this parameter isn't set then N defaults to 20% when the feature is enabled.
* `--proposer-reorg-epochs-since-finalization N`: only attempt to re-org late blocks when the number of epochs since finalization is less than or equal to N. The default is 2 epochs, meaning re-orgs will only be attempted when the chain is finalizing optimally.
For safety Lighthouse will only attempt a re-org under very specific conditions:
1. The block being proposed is 1 slot after the canonical head, and the canonical head is 1 slot after its parent. i.e. at slot `n + 1` rather than building on the block from slot `n` we build on the block from slot `n - 1`.
2. The current canonical head received less than N% of the committee vote. N should be set depending on the proposer boost fraction itself, the fraction of the network that is believed to be applying it, and the size of the largest entity that could be hoarding votes.
3. The current canonical head arrived after the attestation deadline from our perspective. This condition was only added to support suppression of forkchoiceUpdated messages, but makes intuitive sense.
4. The block is being proposed in the first 2 seconds of the slot. This gives it time to propagate and receive the proposer boost.
## Additional Info
For the initial idea and background, see: https://github.com/ethereum/consensus-specs/pull/2353#issuecomment-950238004
There is also a specification for this feature here: https://github.com/ethereum/consensus-specs/pull/3034
Co-authored-by: Michael Sproul <micsproul@gmail.com>
Co-authored-by: pawan <pawandhananjay@gmail.com>
