Genesis is height 0 (not height 1) (#1896)

chain/src/txhashset/txhashset.rs

@@ -713,7 +713,7 @@ impl<'a> HeaderExtension<'a> {
 	pub fn validate_root(&self, header: &BlockHeader) -> Result<(), Error> {
 		// If we are validating the genesis block then we have no prev_root.
 		// So we are done here.
-		if header.height == 1 {
+		if header.height == 0 {
 			return Ok(());
 		}
 
@@ -1102,7 +1102,7 @@ impl<'a> Extension<'a> {
 	/// Validate the provided header by comparing its prev_root to the
 	/// root of the current header MMR.
 	pub fn validate_header_root(&self, header: &BlockHeader) -> Result<(), Error> {
-		if header.height == 1 {
+		if header.height == 0 {
 			return Ok(());
 		}
 

core/src/core/pmmr/pmmr.rs

@@ -17,7 +17,7 @@ use std::u64;
 
 use croaring::Bitmap;
 
-use core::hash::Hash;
+use core::hash::{Hash, ZERO_HASH};
 use core::merkle_proof::MerkleProof;
 use core::pmmr::{Backend, ReadonlyPMMR};
 use core::BlockHeader;
@@ -124,6 +124,9 @@ where
 	/// Computes the root of the MMR. Find all the peaks in the current
 	/// tree and "bags" them to get a single peak.
 	pub fn root(&self) -> Hash {
+		if self.is_empty() {
+			return ZERO_HASH;
+		}
 		let mut res = None;
 		for peak in self.peaks().iter().rev() {
 			res = match res {
@@ -350,6 +353,11 @@ where
 		Ok(())
 	}
 
+	/// Check if this PMMR is (unpruned_size == 0).
+	pub fn is_empty(&self) -> bool {
+		self.unpruned_size() == 0
+	}
+
 	/// Total size of the tree, including intermediary nodes and ignoring any
 	/// pruning.
 	pub fn unpruned_size(&self) -> u64 {