state: Move block header hashing to block_index

Only hash block headers in the lowest-level block index code.

This design has a few benefits:
  - failures are obvious, because the hash is not available,
  - get_tip() returns a smaller object,
  - we avoid re-hashing block headers multiple times.

These efficiency changes may be needed to support chain reorganisations,
multiple tips, and heavy query loads.

zebra-state/src/in_memory.rs

@@ -30,8 +30,10 @@ impl Service<Request> for ZebraState {
     fn call(&mut self, req: Request) -> Self::Future {
         match req {
             Request::AddBlock { block } => {
-                let hash = block.as_ref().into();
-                let result = self.index.insert(block).map(|_| Response::Added { hash });
+                let result = self
+                    .index
+                    .insert(block)
+                    .map(|hash| Response::Added { hash });
 
                 async { result }.boxed()
             }
@@ -48,7 +50,6 @@ impl Service<Request> for ZebraState {
                 let result = self
                     .index
                     .get_tip()
-                    .map(|block| block.as_ref().into())
                     .map(|hash| Response::Tip { hash })
                     .ok_or_else(|| "zebra-state contains no blocks".into());
 

zebra-state/src/in_memory/block_index.rs

@@ -17,7 +17,7 @@ impl BlockIndex {
     pub(super) fn insert(
         &mut self,
         block: impl Into<Arc<Block>>,
-    ) -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
+    ) -> Result<BlockHeaderHash, Box<dyn Error + Send + Sync + 'static>> {
         let block = block.into();
         let hash = block.as_ref().into();
         let height = block.coinbase_height().unwrap();
@@ -26,7 +26,7 @@ impl BlockIndex {
             Entry::Vacant(entry) => {
                 let _ = entry.insert(block.clone());
                 let _ = self.by_hash.insert(hash, block);
-                Ok(())
+                Ok(hash)
             }
             Entry::Occupied(_) => Err("forks in the chain aren't supported yet")?,
         }
@@ -40,12 +40,12 @@ impl BlockIndex {
         .cloned()
     }
 
-    pub(super) fn get_tip(&self) -> Option<Arc<Block>> {
+    pub(super) fn get_tip(&self) -> Option<BlockHeaderHash> {
         self.by_height
             .iter()
             .next_back()
             .map(|(_key, value)| value)
-            .cloned()
+            .map(|block| block.as_ref().into())
     }
 }