refactor block_headers_from_best_block, better cli documentation

ethcore/src/blockchain/blockchain.rs

@@ -667,18 +667,17 @@ impl BlockChain {
 
 	/// fetches the list of blocks from best block to n, and n's parent hash
 	/// where n > 0
-	pub fn block_headers_from_best_block(&self, n: u64) -> (Vec<encoded::Header>, H256) {
-		(0..n)
-			.fold(
-				(vec![], self.best_block_hash()),
-				|(mut blocks, hash), _| {
-					let current_hash = self.block_header_data(&hash)
-						.expect("hash was gotten from previous block's parent_hash; qed");
-					let parent_hash = current_hash.parent_hash();
-					blocks.push(current_hash);
-					(blocks, parent_hash)
-				}
-			)
+	pub fn block_headers_from_best_block(&self, n: u64) -> Option<(Vec<encoded::Header>, H256)> {
+		let mut blocks = vec![];
+		let mut hash = self.best_block_hash();
+
+		for _ in 0..n {
+			let current_hash = self.block_header_data(&hash)?;
+			hash = current_hash.parent_hash();
+			blocks.push(current_hash);
+		}
+
+		Some((blocks, hash))
 	}
 
 	/// Returns a tree route between `from` and `to`, which is a tuple of:

ethcore/src/client/client.rs

@@ -40,7 +40,7 @@ use client::{
 	RegistryInfo, ReopenBlock, PrepareOpenBlock, ScheduleInfo, ImportSealedBlock,
 	BroadcastProposalBlock, ImportBlock, StateOrBlock, StateInfo, StateClient, Call,
 	AccountData, BlockChain as BlockChainTrait, BlockProducer, SealedBlockImporter,
-	ClientIoMessage,
+	ClientIoMessage, BlockChainReset
 };
 use client::{
 	BlockId, TransactionId, UncleId, TraceId, ClientConfig, BlockChainClient,
@@ -87,7 +87,6 @@ pub use types::blockchain_info::BlockChainInfo;
 pub use types::block_status::BlockStatus;
 pub use blockchain::CacheSize as BlockChainCacheSize;
 pub use verification::QueueInfo as BlockQueueInfo;
-use client::BlockChainReset;
 use db::Writable;
 
 use_contract!(registry, "res/contracts/registrar.json");
@@ -475,7 +474,7 @@ impl Importer {
 	// it is for reconstructing the state transition.
 	//
 	// The header passed is from the original block data and is sealed.
-	// TODO: should return an error if ImportRoute is none
+	// TODO: should return an error if ImportRoute is none, issue #9910
 	fn commit_block<B>(&self, block: B, header: &Header, block_data: encoded::Block, client: &Client) -> ImportRoute where B: Drain {
 		let hash = &header.hash();
 		let number = header.number();
@@ -1342,7 +1341,8 @@ impl BlockChainReset for Client {
 		}
 
 		let (blocks_to_delete, best_block_hash) = self.chain.read()
-			.block_headers_from_best_block(num);
+			.block_headers_from_best_block(num)
+			.ok_or("Attempted to reset past genesis block")?;
 
 		let mut db_transaction = DBTransaction::with_capacity(num as usize);
 
@@ -1362,9 +1362,13 @@ impl BlockChainReset for Client {
 			.write(db_transaction)
 			.map_err(|err| format!("could not complete reset operation; io error occured: {}", err))?;
 
-		info!("Deleting block hashes {}", Colour::Red.bold().paint(format!("{:#?}", blocks_to_delete.iter().map(|b| b
-			.hash()).collect::<Vec<_>>()
-		)));
+		let hashes = blocks_to_delete.iter().map(|b| b.hash()).collect::<Vec<_>>();
+
+		info!("Deleting block hashes {}",
+			Colour::Red
+				.bold()
+				.paint(format!("{:#?}", hashes))
+		);
 
 		info!("New best block hash {}", Colour::Green.bold().paint(format!("{:?}", best_block_hash)));
 

parity/cli/mod.rs

@@ -219,10 +219,10 @@ usage! {
 			}
 
 			CMD cmd_db_reset {
-				"Resets the blocks in the blockchain db by the number given",
+				"Removes NUM latests blocks from the db",",
 
 				ARG arg_db_reset_num: (Option<u64>) = None,
-				"--num=[NUM]",
+				"<NUM>",
 				"Number of blocks to revert",
 			}