Implement Tributary syncing

Also adds a forwards-lookup to the Tributary blockchain.

coordinator/src/tributary/scanner.rs

@@ -1,5 +1,5 @@
 use core::ops::Deref;
-use std::collections::{VecDeque, HashMap};
+use std::collections::HashMap;
 
 use zeroize::Zeroizing;
 
@@ -297,43 +297,11 @@ pub async fn handle_new_blocks<D: Db, Pro: Processor, P: P2p>(
   spec: &TributarySpec,
   tributary: &Tributary<D, Transaction, P>,
 ) {
-  let last_block = db.last_block(tributary.genesis());
-
-  // Check if there's been a new Tributary block
-  let latest = tributary.tip().await;
-  if latest == last_block {
-    return;
-  }
-
-  let mut blocks = VecDeque::new();
-  // This is a new block, as per the prior if check
-  blocks.push_back(tributary.block(&latest).unwrap());
-
-  let mut block = None;
-  while {
-    let parent = blocks.back().unwrap().parent();
-    // If the parent is the genesis, we've reached the end
-    if parent == tributary.genesis() {
-      false
-    } else {
-      // Get this block
-      block = Some(tributary.block(&parent).unwrap());
-      // If it's the last block we've scanned, it's the end. Else, push it
-      block.as_ref().unwrap().hash() != last_block
-    }
-  } {
-    blocks.push_back(block.take().unwrap());
-
-    // Prevent this from loading the entire chain into RAM by setting a limit of 1000 blocks at a
-    // time (roughly 350 MB under the current block size limit)
-    if blocks.len() > 1000 {
-      blocks.pop_front();
-    }
-  }
-
-  while let Some(block) = blocks.pop_back() {
-    let hash = block.hash();
+  let mut last_block = db.last_block(tributary.genesis());
+  while let Some(next) = tributary.block_after(&last_block) {
+    let block = tributary.block(&next).unwrap();
     handle_block(db, key, processor, spec, tributary, block).await;
-    db.set_last_block(tributary.genesis(), hash);
+    last_block = next;
+    db.set_last_block(tributary.genesis(), next);
   }
 }