Rename the NU5 block commitment variant based on what it commits to

This change helps avoid confusion with block commitment variants in
future network upgrades, which may add more hashes to the commitment
list.

zebra-chain/src/block/commitment.rs

@@ -31,11 +31,11 @@ pub enum Commitment {
     /// Subsequent `Commitment` variants also commit to the `FinalSaplingRoot`,
     /// via their `EarliestSaplingRoot` and `LatestSaplingRoot` fields.
     ///
-    /// TODO: this field is verified during semantic verification
-    ///
     /// Since Zebra checkpoints on Canopy, we don't need to validate this
     /// field, but since it's included in the ChainHistoryRoot, we are
     /// already calculating it, so we might as well validate it.
+    ///
+    /// TODO: this field is verified during semantic verification
     FinalSaplingRoot(sapling::tree::Root),
 
     /// [Heartwood activation block] Reserved field.
@@ -63,7 +63,7 @@ pub enum Commitment {
     /// chain history hash in their activation block, via the previous block
     /// hash field.)
     ///
-    /// TODO: this field is verified during semantic verification
+    /// TODO: this field is verified during contextual verification
     ChainHistoryRoot(ChainHistoryMmrRootHash),
 
     /// [NU5 activation onwards] A commitment to:
@@ -79,11 +79,8 @@ pub enum Commitment {
     ///
     /// See also the [`ChainHistoryRoot`] variant.
     ///
-    /// TODO: this field is verified during semantic verification
-    //
-    // TODO: Do block commitments activate at NU5 activation, or (NU5 + 1)?
-    // https://github.com/zcash/zips/pull/474
-    BlockCommitments(BlockCommitmentsHash),
+    /// TODO: this field is verified during contextual verification
+    ChainHistoryBlockTxAuthCommitment(ChainHistoryBlockTxAuthCommitmentHash),
 }
 
 /// The required value of reserved `Commitment`s.
@@ -116,7 +113,9 @@ impl Commitment {
                 }
             }
             Heartwood | Canopy => Ok(ChainHistoryRoot(ChainHistoryMmrRootHash(bytes))),
-            Nu5 => Ok(BlockCommitments(BlockCommitmentsHash(bytes))),
+            Nu5 => Ok(ChainHistoryBlockTxAuthCommitment(
+                ChainHistoryBlockTxAuthCommitmentHash(bytes),
+            )),
         }
     }
 
@@ -130,7 +129,7 @@ impl Commitment {
             FinalSaplingRoot(hash) => hash.0,
             ChainHistoryActivationReserved => RESERVED_BYTES,
             ChainHistoryRoot(hash) => hash.0,
-            BlockCommitments(hash) => hash.0,
+            ChainHistoryBlockTxAuthCommitment(hash) => hash.0,
         }
     }
 }
@@ -143,17 +142,19 @@ impl Commitment {
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub struct ChainHistoryMmrRootHash([u8; 32]);
 
-/// The Block Commitments for a block. As of NU5, these cover:
+/// A block commitment to chain history and transaction auth.
 /// - the chain history tree for all ancestors in the current network upgrade,
 ///   and
 /// - the transaction authorising data in this block.
+///
+/// Introduced in NU5.
 //
 // TODO:
 //    - add auth data type
 //    - add a method for hashing chain history and auth data together
 //    - move to a separate file
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
-pub struct BlockCommitmentsHash([u8; 32]);
+pub struct ChainHistoryBlockTxAuthCommitmentHash([u8; 32]);
 
 /// Errors that can occur when checking RootHash consensus rules.
 ///
@@ -186,8 +187,8 @@ pub enum CommitmentError {
         actual: [u8; 32],
     },
 
-    #[error("invalid block commitment: expected {expected:?}, actual: {actual:?}")]
-    InvalidBlockCommitment {
+    #[error("invalid chain history + block transaction auth commitment: expected {expected:?}, actual: {actual:?}")]
+    InvalidChainHistoryBlockTxAuthCommitment {
         expected: [u8; 32],
         actual: [u8; 32],
     },