height (header version) specific output PMMR root rules (#3147)

* height (header version) specific output PMMR root rules

* cleanup

* cleanup based on PR feedback

* address feedback

api/src/types.rs

@@ -116,13 +116,16 @@ pub struct TxHashSet {
 }
 
 impl TxHashSet {
-	pub fn from_head(head: Arc<chain::Chain>) -> TxHashSet {
-		let roots = head.get_txhashset_roots();
-		TxHashSet {
-			output_root_hash: roots.output_root().to_hex(),
-			range_proof_root_hash: roots.rproof_root.to_hex(),
-			kernel_root_hash: roots.kernel_root.to_hex(),
-		}
+	/// A TxHashSet in the context of the api is simply the collection of PMMR roots.
+	/// We can obtain these in a lightweight way by reading them from the head of the chain.
+	/// We will have validated the roots on this header against the roots of the txhashset.
+	pub fn from_head(chain: Arc<chain::Chain>) -> Result<TxHashSet, chain::Error> {
+		let header = chain.head_header()?;
+		Ok(TxHashSet {
+			output_root_hash: header.output_root.to_hex(),
+			range_proof_root_hash: header.range_proof_root.to_hex(),
+			kernel_root_hash: header.kernel_root.to_hex(),
+		})
 	}
 }