Tweak light client root hash definition.

This changes the `light_client_root_hash` field to `light_client_root_bytes` to
hint that it's unparsed, and makes it public to match the rest of the
`BlockHeader` fields.  The `LightClientRootHash` serialization methods are
hidden from the public API, so that the `LightClientRootHash` has to be
constructed by the method on the `Block`.

zebra-chain/src/block.rs

@@ -103,7 +103,7 @@ impl Block {
     pub fn light_client_root_hash(&self, network: Network) -> Option<LightClientRootHash> {
         match self.coinbase_height() {
             Some(height) => Some(LightClientRootHash::from_bytes(
-                self.header.light_client_root_hash,
+                self.header.light_client_root_bytes,
                 network,
                 height,
             )),

zebra-chain/src/block/header.rs

@@ -36,10 +36,12 @@ pub struct BlockHeader {
 
     /// The light client root hash.
     ///
-    /// This field is interpreted differently, based on the current
-    /// block height. Use `block.light_client_root_hash(network)` to get the
-    /// parsed `LightClientRootHash` for this block.
-    pub(super) light_client_root_hash: [u8; 32],
+    /// Unfortunately, the interpretation of this field was changed without
+    /// incrementing the version, so it cannot be parsed without the block height
+    /// and network. Use
+    /// [`Block::light_client_root_hash`](super::Block::light_client_root_hash)
+    /// to get the parsed [`LightClientRootHash`](super::LightClientRootHash).
+    pub light_client_root_bytes: [u8; 32],
 
     /// The block timestamp is a Unix epoch time (UTC) when the miner
     /// started hashing the header (according to the miner).

zebra-chain/src/block/light_client.rs

@@ -6,7 +6,7 @@ use crate::{Network, NetworkUpgrade, NetworkUpgrade::*};
 
 /// Light client root hashes.
 ///
-/// The `BlockHeader.light_client_root_hash` field is interpreted differently,
+/// The `BlockHeader.light_client_root_bytes` field is interpreted differently,
 /// based on the current block height. The interpretation changes at or after
 /// network upgrades.
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
@@ -45,7 +45,7 @@ pub enum LightClientRootHash {
 impl LightClientRootHash {
     /// Returns `bytes` as the LightClientRootHash variant for `network` and
     /// `height`.
-    pub fn from_bytes(
+    pub(super) fn from_bytes(
         bytes: [u8; 32],
         network: Network,
         height: BlockHeight,
@@ -64,7 +64,7 @@ impl LightClientRootHash {
 
     /// Returns the serialized bytes for this LightClientRootHash.
     #[allow(dead_code)]
-    pub fn to_bytes(self) -> [u8; 32] {
+    pub(super) fn to_bytes(self) -> [u8; 32] {
         use LightClientRootHash::*;
 
         match self {

zebra-chain/src/block/serialize.rs

@@ -18,7 +18,7 @@ impl ZcashSerialize for BlockHeader {
         writer.write_u32::<LittleEndian>(self.version)?;
         self.previous_block_hash.zcash_serialize(&mut writer)?;
         writer.write_all(&self.merkle_root_hash.0[..])?;
-        writer.write_all(&self.light_client_root_hash[..])?;
+        writer.write_all(&self.light_client_root_bytes[..])?;
         // this is a truncating cast, rather than a saturating cast
         // but u32 times are valid until 2106, and our block verification time
         // checks should detect any truncation.
@@ -66,7 +66,7 @@ impl ZcashDeserialize for BlockHeader {
             version,
             previous_block_hash: BlockHeaderHash::zcash_deserialize(&mut reader)?,
             merkle_root_hash: MerkleTreeRootHash(reader.read_32_bytes()?),
-            light_client_root_hash: reader.read_32_bytes()?,
+            light_client_root_bytes: reader.read_32_bytes()?,
             // This can't panic, because all u32 values are valid `Utc.timestamp`s
             time: Utc.timestamp(reader.read_u32::<LittleEndian>()? as i64, 0),
             difficulty_threshold: CompactDifficulty(reader.read_u32::<LittleEndian>()?),

zebra-chain/src/block/tests.rs

@@ -25,8 +25,8 @@ impl Arbitrary for LightClientRootHash {
 
     fn arbitrary_with(_args: ()) -> Self::Strategy {
         (any::<[u8; 32]>(), any::<Network>(), any::<BlockHeight>())
-            .prop_map(|(light_client_root_hash, network, block_height)| {
-                LightClientRootHash::from_bytes(light_client_root_hash, network, block_height)
+            .prop_map(|(light_client_root_bytes, network, block_height)| {
+                LightClientRootHash::from_bytes(light_client_root_bytes, network, block_height)
             })
             .boxed()
     }
@@ -55,7 +55,7 @@ impl Arbitrary for BlockHeader {
                     version,
                     previous_block_hash,
                     merkle_root_hash,
-                    light_client_root_hash,
+                    light_client_root_bytes,
                     timestamp,
                     difficulty_threshold,
                     nonce,
@@ -64,7 +64,7 @@ impl Arbitrary for BlockHeader {
                     version,
                     previous_block_hash,
                     merkle_root_hash,
-                    light_client_root_hash,
+                    light_client_root_bytes,
                     time: Utc.timestamp(timestamp, 0),
                     difficulty_threshold,
                     nonce,
@@ -255,7 +255,7 @@ proptest! {
         let light_hash = block.light_client_root_hash(network);
         if let Some(light_hash) = light_hash {
             let light_hash_bytes = light_hash.to_bytes();
-            prop_assert_eq![block.header.light_client_root_hash, light_hash_bytes];
+            prop_assert_eq![block.header.light_client_root_bytes, light_hash_bytes];
         } else {
             prop_assert_eq![block.coinbase_height(), None];
         }