Deposit Cache Finalization & Fast WS Sync (#2915)

## Summary

The deposit cache now has the ability to finalize deposits. This will cause it to drop unneeded deposit logs and hashes in the deposit Merkle tree that are no longer required to construct deposit proofs. The cache is finalized whenever the latest finalized checkpoint has a new `Eth1Data` with all deposits imported.

This has three benefits:

1. Improves the speed of constructing Merkle proofs for deposits as we can just replay deposits since the last finalized checkpoint instead of all historical deposits when re-constructing the Merkle tree.
2. Significantly faster weak subjectivity sync as the deposit cache can be transferred to the newly syncing node in compressed form. The Merkle tree that stores `N` finalized deposits requires a maximum of `log2(N)` hashes. The newly syncing node then only needs to download deposits since the last finalized checkpoint to have a full tree.
3. Future proofing in preparation for [EIP-4444](https://eips.ethereum.org/EIPS/eip-4444) as execution nodes will no longer be required to store logs permanently so we won't always have all historical logs available to us.

## More Details

Image to illustrate how the deposit contract merkle tree evolves and finalizes along with the resulting `DepositTreeSnapshot`
![image](https://user-images.githubusercontent.com/37123614/151465302-5fc56284-8a69-4998-b20e-45db3934ac70.png)

## Other Considerations

I've changed the structure of the `SszDepositCache` so once you load & save your database from this version of lighthouse, you will no longer be able to load it from older versions.

Co-authored-by: ethDreamer <37123614+ethDreamer@users.noreply.github.com>

consensus/types/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "types"
-version = "0.2.0"
+version = "0.2.1"
 authors = ["Paul Hauner <paul@paulhauner.com>", "Age Manning <Age@AgeManning.com>"]
 edition = "2021"
 

consensus/types/src/deposit_tree_snapshot.rs

@@ -0,0 +1,83 @@
+use crate::*;
+use eth2_hashing::{hash32_concat, ZERO_HASHES};
+use int_to_bytes::int_to_bytes32;
+use serde_derive::{Deserialize, Serialize};
+use ssz_derive::{Decode, Encode};
+use test_random_derive::TestRandom;
+use test_utils::TestRandom;
+use DEPOSIT_TREE_DEPTH;
+
+#[derive(Encode, Decode, Deserialize, Serialize, Clone, Debug, PartialEq, TestRandom)]
+pub struct FinalizedExecutionBlock {
+    pub deposit_root: Hash256,
+    pub deposit_count: u64,
+    pub block_hash: Hash256,
+    pub block_height: u64,
+}
+
+impl From<&DepositTreeSnapshot> for FinalizedExecutionBlock {
+    fn from(snapshot: &DepositTreeSnapshot) -> Self {
+        Self {
+            deposit_root: snapshot.deposit_root,
+            deposit_count: snapshot.deposit_count,
+            block_hash: snapshot.execution_block_hash,
+            block_height: snapshot.execution_block_height,
+        }
+    }
+}
+
+#[derive(Encode, Decode, Deserialize, Serialize, Clone, Debug, PartialEq, TestRandom)]
+pub struct DepositTreeSnapshot {
+    pub finalized: Vec<Hash256>,
+    pub deposit_root: Hash256,
+    pub deposit_count: u64,
+    pub execution_block_hash: Hash256,
+    pub execution_block_height: u64,
+}
+
+impl Default for DepositTreeSnapshot {
+    fn default() -> Self {
+        let mut result = Self {
+            finalized: vec![],
+            deposit_root: Hash256::default(),
+            deposit_count: 0,
+            execution_block_hash: Hash256::zero(),
+            execution_block_height: 0,
+        };
+        // properly set the empty deposit root
+        result.deposit_root = result.calculate_root().unwrap();
+        result
+    }
+}
+
+impl DepositTreeSnapshot {
+    // Calculates the deposit tree root from the hashes in the snapshot
+    pub fn calculate_root(&self) -> Option<Hash256> {
+        let mut size = self.deposit_count;
+        let mut index = self.finalized.len();
+        let mut deposit_root = [0; 32];
+        for height in 0..DEPOSIT_TREE_DEPTH {
+            deposit_root = if (size & 1) == 1 {
+                index = index.checked_sub(1)?;
+                hash32_concat(self.finalized.get(index)?.as_bytes(), &deposit_root)
+            } else {
+                hash32_concat(&deposit_root, ZERO_HASHES.get(height)?)
+            };
+            size /= 2;
+        }
+        // add mix-in-length
+        deposit_root = hash32_concat(&deposit_root, &int_to_bytes32(self.deposit_count));
+
+        Some(Hash256::from_slice(&deposit_root))
+    }
+    pub fn is_valid(&self) -> bool {
+        self.calculate_root()
+            .map_or(false, |calculated| self.deposit_root == calculated)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    ssz_tests!(DepositTreeSnapshot);
+}

consensus/types/src/lib.rs

@@ -36,6 +36,7 @@ pub mod contribution_and_proof;
 pub mod deposit;
 pub mod deposit_data;
 pub mod deposit_message;
+pub mod deposit_tree_snapshot;
 pub mod enr_fork_id;
 pub mod eth1_data;
 pub mod eth_spec;
@@ -120,6 +121,7 @@ pub use crate::contribution_and_proof::ContributionAndProof;
 pub use crate::deposit::{Deposit, DEPOSIT_TREE_DEPTH};
 pub use crate::deposit_data::DepositData;
 pub use crate::deposit_message::DepositMessage;
+pub use crate::deposit_tree_snapshot::{DepositTreeSnapshot, FinalizedExecutionBlock};
 pub use crate::enr_fork_id::EnrForkId;
 pub use crate::eth1_data::Eth1Data;
 pub use crate::eth_spec::EthSpecId;