## Issue Addressed
- Resolves#1616
## Proposed Changes
Fixes a bug where we are unable to read the finalized block from fork choice.
## Detail
I had made an assumption that the finalized block always has a parent root of `None`:
https://github.com/sigp/lighthouse/blob/e5fc6bab485fa54d7e518b325f4eb9201ba5c6a1/consensus/fork_choice/src/fork_choice.rs#L749-L752
This was a faulty assumption, we don't set parent *roots* to `None`. Instead we *sometimes* set parent *indices* to `None`, depending if this pruning condition is satisfied:
https://github.com/sigp/lighthouse/blob/e5fc6bab485fa54d7e518b325f4eb9201ba5c6a1/consensus/proto_array/src/proto_array.rs#L229-L232
The bug manifested itself like this:
1. We attempt to get the finalized block from fork choice
1. We try to check that the block is descendant of the finalized block (note: they're the same block).
1. We expect the parent root to be `None`, but it's actually the parent root of the finalized root.
1. We therefore end up checking if the parent of the finalized root is a descendant of itself. (note: it's an *ancestor* not a *descendant*).
1. We therefore declare that the finalized block is not a descendant of (or eq to) the finalized block. Bad.
## Additional Info
In reflection, I made a poor assumption in the quest to obtain a probably negligible performance gain. The performance gain wasn't worth the risk and we got burnt.
Paul Hauner
a17f74896a