Fix on-disk consensus context format

beacon_node/store/src/consensus_context.rs

@@ -0,0 +1,56 @@
+use ssz_derive::{Decode, Encode};
+use state_processing::ConsensusContext;
+use types::{EthSpec, Hash256, Slot};
+
+/// The consensus context is stored on disk as part of the data availability overflow cache.
+///
+/// We use this separate struct to keep the on-disk format stable in the presence of changes to the
+/// in-memory `ConsensusContext`. You MUST NOT change the fields of this struct without
+/// superstructing it and implementing a schema migration.
+#[derive(Debug, PartialEq, Clone, Encode, Decode)]
+pub struct OnDiskConsensusContext {
+    /// Slot to act as an identifier/safeguard
+    slot: Slot,
+    /// Proposer index of the block at `slot`.
+    proposer_index: Option<u64>,
+    /// Block root of the block at `slot`.
+    current_block_root: Option<Hash256>,
+}
+
+impl OnDiskConsensusContext {
+    pub fn from_consensus_context<E: EthSpec>(ctxt: &ConsensusContext<E>) -> Self {
+        // Match exhaustively on fields here so we are forced to *consider* updating the on-disk
+        // format when the `ConsensusContext` fields change.
+        let &ConsensusContext {
+            slot,
+            previous_epoch: _,
+            current_epoch: _,
+            proposer_index,
+            current_block_root,
+            indexed_attestations: _,
+        } = ctxt;
+        OnDiskConsensusContext {
+            slot,
+            proposer_index,
+            current_block_root,
+        }
+    }
+
+    pub fn into_consensus_context<E: EthSpec>(self) -> ConsensusContext<E> {
+        let OnDiskConsensusContext {
+            slot,
+            proposer_index,
+            current_block_root,
+        } = self;
+
+        let mut ctxt = ConsensusContext::new(slot);
+
+        if let Some(proposer_index) = proposer_index {
+            ctxt = ctxt.set_proposer_index(proposer_index);
+        }
+        if let Some(block_root) = current_block_root {
+            ctxt = ctxt.set_current_block_root(block_root);
+        }
+        ctxt
+    }
+}