apply pending changes to chain after DB commit

ethcore/light/src/client/header_chain.rs

@@ -126,6 +126,11 @@ fn era_key(number: u64) -> String {
 	format!("candidates_{}", number)
 }
 
+/// Pending changes from `insert` to be applied after the database write has finished.
+pub struct PendingChanges {
+	best_block: Option<BlockDescriptor>, // new best block.
+}
+
 /// Header chain. See module docs for more details.
 pub struct HeaderChain {
 	genesis_header: encoded::Header, // special-case the genesis.
@@ -203,10 +208,15 @@ impl HeaderChain {
 	/// Insert a pre-verified header.
 	///
 	/// This blindly trusts that the data given to it is sensible.
-	pub fn insert(&self, transaction: &mut DBTransaction, header: Header) -> Result<(), BlockError> {
+	/// Returns a set of pending changes to be applied with `apply_pending`
+	/// before the next call to insert and after the transaction has been written.
+	pub fn insert(&self, transaction: &mut DBTransaction, header: Header) -> Result<PendingChanges, BlockError> {
 		let hash = header.hash();
 		let number = header.number();
 		let parent_hash = *header.parent_hash();
+		let mut pending = PendingChanges {
+			best_block: None,
+		};
 
 		// hold candidates the whole time to guard import order.
 		let mut candidates = self.candidates.write();
@@ -286,11 +296,11 @@ impl HeaderChain {
 			}
 
 			trace!(target: "chain", "New best block: ({}, {}), TD {}", number, hash, total_difficulty);
-			*self.best_block.write() = BlockDescriptor {
+			pending.best_block = Some(BlockDescriptor {
 				hash: hash,
 				number: number,
 				total_difficulty: total_difficulty,
-			};
+			});
 
 			// produce next CHT root if it's time.
 			let earliest_era = *candidates.keys().next().expect("at least one era just created; qed");
@@ -334,7 +344,15 @@ impl HeaderChain {
 			stream.append(&best_num).append(&latest_num);
 			transaction.put(self.col, CURRENT_KEY, &stream.out())
 		}
-		Ok(())
+		Ok(pending)
+	}
+
+	/// Apply pending changes from a previous `insert` operation.
+	/// Must be done before the next `insert` call.
+	pub fn apply_pending(&self, pending: PendingChanges) {
+		if let Some(best_block) = pending.best_block {
+			*self.best_block.write() = best_block;
+		}
 	}
 
 	/// Get a block header. In the case of query by number, only canonical blocks
@@ -360,6 +378,9 @@ impl HeaderChain {
 					.and_then(load_from_db)
 			}
 			BlockId::Latest | BlockId::Pending => {
+				// hold candidates hear to prevent deletion of the header
+				// as we read it.
+				let _candidates = self.candidates.read();
 				let hash = {
 					let best = self.best_block.read();
 					if best.number == 0 {

ethcore/light/src/client/mod.rs

@@ -223,18 +223,20 @@ impl Client {
 			let (num, hash) = (verified_header.number(), verified_header.hash());
 
 			let mut tx = self.db.transaction();
-			match self.chain.insert(&mut tx, verified_header) {
-				Ok(()) => {
+			let pending = match self.chain.insert(&mut tx, verified_header) {
+				Ok(pending) => {
 					good.push(hash);
 					self.report.write().blocks_imported += 1;
+					pending
 				}
 				Err(e) => {
 					debug!(target: "client", "Error importing header {:?}: {}", (num, hash), e);
 					bad.push(hash);
+					break;
 				}
-			}
+			};
 			self.db.write_buffered(tx);
-
+			self.chain.apply_pending(pending);
 			if let Err(e) = self.db.flush() {
 				panic!("Database flush failed: {}. Check disk health and space.", e);
 			}