Add prevHash field to CompactBlock

This enables basic verification of chain validity when CompactBlocks are
received without the full header.

zcash_client_backend/proto/compact_formats.proto

@@ -13,9 +13,10 @@ message CompactBlock {
     uint32 protoVersion = 1; // the version of this wire format, for storage
     uint64 height = 2; // the height of this block
     bytes hash = 3;
-    uint32 time = 4;
-    bytes header = 5; // (hash and time) OR (full header)
-    repeated CompactTx vtx = 6; // compact transactions from this block
+    bytes prevHash = 4;
+    uint32 time = 5;
+    bytes header = 6; // (hash, prevHash, and time) OR (full header)
+    repeated CompactTx vtx = 7; // compact transactions from this block
 }
 
 message CompactTx {

zcash_client_backend/src/proto/mod.rs

@@ -1,3 +1,54 @@
 //! Generated code for handling light client protobuf structs.
 
+use zcash_primitives::block::{BlockHash, BlockHeader};
+
 pub mod compact_formats;
+
+impl compact_formats::CompactBlock {
+    /// Returns the [`BlockHash`] for this block.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if [`CompactBlock.header`] is not set and
+    /// [`CompactBlock.hash`] is not exactly 32 bytes.
+    ///
+    /// [`CompactBlock.header`]: #structfield.header
+    /// [`CompactBlock.hash`]: #structfield.hash
+    pub fn hash(&self) -> BlockHash {
+        if let Some(header) = self.header() {
+            header.hash()
+        } else {
+            BlockHash::from_slice(&self.hash)
+        }
+    }
+
+    /// Returns the [`BlockHash`] for this block's parent.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if [`CompactBlock.header`] is not set and
+    /// [`CompactBlock.prevHash`] is not exactly 32 bytes.
+    ///
+    /// [`CompactBlock.header`]: #structfield.header
+    /// [`CompactBlock.prevHash`]: #structfield.prevHash
+    pub fn prev_hash(&self) -> BlockHash {
+        if let Some(header) = self.header() {
+            header.prev_block
+        } else {
+            BlockHash::from_slice(&self.prevHash)
+        }
+    }
+
+    /// Returns the [`BlockHeader`] for this block if present.
+    ///
+    /// A convenience method that parses [`CompactBlock.header`] if present.
+    ///
+    /// [`CompactBlock.header`]: #structfield.header
+    pub fn header(&self) -> Option<BlockHeader> {
+        if self.header.is_empty() {
+            None
+        } else {
+            BlockHeader::read(&self.header[..]).ok()
+        }
+    }
+}

zcash_primitives/src/block.rs

@@ -22,6 +22,20 @@ impl fmt::Display for BlockHash {
     }
 }
 
+impl BlockHash {
+    /// Constructs a [`BlockHash`] from the given slice.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the slice is not exactly 32 bytes.
+    pub fn from_slice(bytes: &[u8]) -> Self {
+        assert_eq!(bytes.len(), 32);
+        let mut hash = [0; 32];
+        hash.copy_from_slice(&bytes);
+        BlockHash(hash)
+    }
+}
+
 /// A Zcash block header.
 pub struct BlockHeader {
     hash: BlockHash,