Handle attempts to get block headers at invalid heights (#3683)

* handle attempts to get block headers at invalid heights

* compare requested height against header pmmr size

* gte

chain/src/error.rs

@@ -135,6 +135,10 @@ pub enum ErrorKind {
 	/// Error from underlying block handling
 	#[fail(display = "Block Validation Error: {:?}", _0)]
 	Block(block::Error),
+	/// Attempt to retrieve a header at a height greater than
+	/// the max allowed by u64 limits
+	#[fail(display = "Invalid Header Height: {}", _0)]
+	InvalidHeaderHeight(u64),
 	/// Anything else
 	#[fail(display = "Other Error: {}", _0)]
 	Other(String),

chain/src/txhashset/txhashset.rs

@@ -110,6 +110,9 @@ impl PMMRHandle<BlockHeader> {
 
 	/// Get the header hash at the specified height based on the current header MMR state.
 	pub fn get_header_hash_by_height(&self, height: u64) -> Result<Hash, Error> {
+		if height >= self.size {
+			return Err(ErrorKind::InvalidHeaderHeight(height).into());
+		}
 		let pos = pmmr::insertion_to_pmmr_index(height);
 		let header_pmmr = ReadonlyPMMR::at(&self.backend, self.size);
 		if let Some(entry) = header_pmmr.get_data(pos) {

servers/src/grin/server.rs

@@ -454,9 +454,15 @@ impl Server {
 					// We need to query again for the actual block hash, as
 					// the difficulty iterator doesn't contain enough info to
 					// create a hash
-					let block_hash = match self.chain.get_header_by_height(height as u64) {
-						Ok(h) => h.hash(),
-						Err(_) => ZERO_HASH,
+					// The diff iterator returns 59 block headers 'before' 0 to give callers
+					// enough detail to calculate initial block difficulties. Ignore these.
+					let block_hash = if height < 0 {
+						ZERO_HASH
+					} else {
+						match self.chain.get_header_by_height(height as u64) {
+							Ok(h) => h.hash(),
+							Err(_) => ZERO_HASH,
+						}
 					};
 
 					DiffBlock {