Create New Beacon State Data Structure

[rauljordan]

This PR resolves #2011

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Description

Write why you are making the changes in this pull request

We need a way to avoid cloning the beacon state 56 times per slot as we get closer to a mainnet release. This PR adds a new beacon state data structure that recomputes its hashtreeroot on writes to fields and provides getters which copy fields on reads to maintain immutability. This PR will be backwards compatible with our current beacon state, and will help runtime a lot.

Write a summary of the changes you are making

Adds a new beacon state struct which looks like this:

type BeaconState struct {
    merkleTrieLayers [][][]byte
    state *pbp2p.BeaconState
}

func (b *BeaconState) Balances() []uint64 {
    res := make([]uint64, len(b.state.Balances))
    copy(res, b.state.Balances)
    return res
}

func (b *BeaconState) SetBalances(vals []uint64) error {
    // Recompute the branch up the Merkle trie for the
    // state.Balances field and update the cached value of state.Balances...
    ...
}

Link anything that would be helpful or relevant to the reviewers

We should expect resource consumption and bottlenecks to reduce significantly. This PR only implements the new data structure with tests but does not yet integrate it into our runtime.

Next Steps

Next, we need to begin integrating this new type throughout the repo. We would first need to amend the db.SaveState and db.HeadState functions to take in and return this type. Then, we would to modify the head fetcher (from the chain service) to use this type instead. Afterwards, we can just go package by package, checking for any instances of .HeadState() and replacing with appropriate getters instead. The HeadState() function should no longer clone, but instead just return the new beacon state type because all its fields are protected behind getters that copy.

[codecov]

Codecov Report

Merging #4602 into master will not change coverage.
The diff coverage is n/a.

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  Hits          966      966           
  Misses       3150     3150           
  Partials      141      141

[prestonvanloon]

Awesome patch!

Two concerns:

• Please confirm that you are using safe copy for byte slices in getters.
• Please complete godoc comments.