chain: enable syncTree() to process multiple tree intervals

lib/blockchain/chain.js

@@ -140,17 +140,28 @@ class Chain extends AsyncEmitter {
     // The tree root in this block header will be the first block that
     // commits to the new tree root hash, but the transactions it contains
     // have not yet been added to the tree.
-    const last = this.height - (this.height % treeInterval);
-    const entry = await this.db.getEntryByHeight(last + 1);
-
-    // Require that tree is less than one tree interval behind the chain.
-    // We can skip this check only if the blockchain is less than
+    let last = this.height - (this.height % treeInterval);
+    let entry = await this.db.getEntryByHeight(last + 1);
+
+    // Using the current tree root hash, rewind the blockchain to the beginning
+    // of the corresponding tree interval. If the tree has been compacted,
+    // this may be SEVERAL tree intervals behind the current chain tip
+    // (this is required to support chain reorgs to a limited depth,
+    // similar to pruning nodes keeping the last 288 blocks on disk).
+    // We can skip this only if the blockchain is less than
     // one tree interval old, OR the chain is already at the tree interval,
     // meaning the tree is already in sync with the chain and there is no delta.
-    if (entry) {
-      assert(entry.treeRoot.equals(currentRoot));
-    } else {
+    if (!entry) {
       assert(last === 0 || last === this.height);
+    } else {
+      for (;;) {
+        if (entry.treeRoot.equals(currentRoot))
+          break;
+
+        last -= treeInterval;
+        entry = await this.db.getEntryByHeight(last + 1);
+        assert(entry);
+      }
     }
 
     // Replay all blocks since the last tree interval to rebuild
@@ -171,8 +182,8 @@ class Chain extends AsyncEmitter {
       for (const tx of block.txs)
         await this.verifyCovenants(tx, view, height, hardened);
 
-      assert((height % this.network.names.treeInterval) !== 0);
-
+      // If the chain replay crosses a tree interval, it will commit
+      // and write to disk in saveNames(), resetting the `txn` like usual.
       await this.db.saveNames(view, entry, false);
     }