chain: implement urkel tree compaction to historical root

lib/blockchain/chain.js

@@ -2078,6 +2078,52 @@ class Chain extends AsyncEmitter {
     }
   }
 
+  /**
+   * Compact the Urkel Tree.
+   * Removes all historical state and all data not
+   * linked directly to the provided root node hash.
+   * @param {Hash} root
+   * @returns {Promise}
+   */
+
+  async compactTree() {
+    const unlock = await this.locker.lock();
+    this.logger.info('Compacting Urkel Tree...');
+
+    // To support chain reorgs of limited depth we compact the tree
+    // to some commitment point in recent history, then rebuild it from there
+    // back up to the current chain tip. In order to support pruning nodes,
+    // all blocks above this depth must be available on disk.
+    // This actually further reduces the ability for a pruning node to recover
+    // from a deep reorg. On mainnet, `keepBlocks` is 288. A normal pruning
+    // node can recover from a reorg up to that depth. Compacting the tree
+    // potentially reduces that depth to 288 - 36 = 252. A reorg deeper than
+    // that will result in a `MissingNodeError` thrown by Urkel inside
+    // chain.saveNames() as it tries to restore a deleted state.
+
+    // Oldest block available to a pruning node.
+    const oldestBlock = this.height - this.network.block.keepBlocks;
+
+    // Distance from that block to the start of the oldest tree interval.
+    const toNextInterval =
+      this.network.names.treeInterval -
+      (oldestBlock % this.network.names.treeInterval);
+
+    // Get the oldest Urkel Tree root state a pruning node can recover from.
+    const oldestTreeIntervalStart = oldestBlock + toNextInterval + 1;
+    const entry = await this.db.getEntryByHeight(oldestTreeIntervalStart);
+
+    try {
+      // Rewind Urkel Tree and delete all historical state.
+      await this.db.compactTree(entry.treeRoot);
+
+      // Replay the blockchain back up to the tip to rebuild tree.
+      return await this.syncTree();
+    } finally {
+      unlock();
+    }
+  }
+
   /**
    * Scan the blockchain for transactions containing specified address hashes.
    * @param {Hash} start - Block hash to start at.

lib/blockchain/chaindb.js

@@ -935,6 +935,25 @@ class ChainDB {
     return true;
   }
 
+  /**
+   * Compact the Urkel Tree.
+   * Removes all historical state and all data not
+   * linked directly to the provided root node hash.
+   * @param {Hash} root
+   * @returns {Promise}
+   */
+
+  async compactTree(root) {
+    // Rewind tree to historical commitment
+    await this.tree.inject(root);
+
+    // Delete historical data
+    await this.tree.compact();
+
+    // Reset in-memory tree delta
+    this.txn = this.tree.txn();
+  }
+
   /**
    * Get the _next_ block hash (does not work by height).
    * @param {Hash} hash