Add prevHash field to CompactBlock

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

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()
+        }
+    }
+}