Remove panickers from trie iterators (#2209)

* port trie iterators to use error handling

* use new trie iterators in snapshot

allows proper recovery from a premature periodic snapshot

ethcore/src/client/client.rs

@@ -144,7 +144,9 @@ pub struct Client {
 	factories: Factories,
 }
 
-const HISTORY: u64 = 1200;
+/// The pruning constant -- how old blocks must be before we
+/// assume finality of a given candidate.
+pub const HISTORY: u64 = 1200;
 
 /// Append a path element to the given path and return the string.
 pub fn append_path<P>(path: P, item: &str) -> String where P: AsRef<Path> {

ethcore/src/snapshot/account.rs

@@ -92,7 +92,8 @@ impl Account {
 
 		let mut pairs = Vec::new();
 
-		for (k, v) in db.iter() {
+		for item in try!(db.iter()) {
+			let (k, v) = try!(item);
 			pairs.push((k, v));
 		}
 

ethcore/src/snapshot/mod.rs

@@ -358,7 +358,8 @@ pub fn chunk_state<'a>(db: &HashDB, root: &H256, writer: &Mutex<SnapshotWriter +
 	let mut used_code = HashSet::new();
 
 	// account_key here is the address' hash.
-	for (account_key, account_data) in account_trie.iter() {
+	for item in try!(account_trie.iter()) {
+		let (account_key, account_data) = try!(item);
 		let account = Account::from_thin_rlp(account_data);
 		let account_key_hash = H256::from_slice(&account_key);
 

ethcore/src/snapshot/service.rs

@@ -27,7 +27,7 @@ use super::{ManifestData, StateRebuilder, BlockRebuilder, RestorationStatus, Sna
 use super::io::{SnapshotReader, LooseReader, SnapshotWriter, LooseWriter};
 
 use blockchain::BlockChain;
-use client::Client;
+use client::{BlockChainClient, Client};
 use engines::Engine;
 use error::Error;
 use ids::BlockID;
@@ -345,7 +345,17 @@ impl Service {
 		let res = client.take_snapshot(writer, BlockID::Number(num), &self.progress);
 
 		self.taking_snapshot.store(false, Ordering::SeqCst);
-		try!(res);
+		if let Err(e) = res {
+			if client.chain_info().best_block_number >= num + ::client::HISTORY {
+				// "Cancelled" is mincing words a bit -- what really happened
+				// is that the state we were snapshotting got pruned out
+				// before we could finish.
+				info!("Cancelled prematurely-started periodic snapshot.");
+				return Ok(())
+			} else {
+				return Err(e);
+			}
+		}
 
 		info!("Finished taking snapshot at #{}", num);
 

ethcore/src/snapshot/tests/helpers.rs

@@ -52,8 +52,9 @@ impl StateProducer {
 		// modify existing accounts.
 		let mut accounts_to_modify: Vec<_> = {
 			let trie = TrieDB::new(&*db, &self.state_root).unwrap();
-			let temp = trie.iter() // binding required due to complicated lifetime stuff
+			let temp = trie.iter().unwrap() // binding required due to complicated lifetime stuff
 				.filter(|_| rng.gen::<f32>() < ACCOUNT_CHURN)
+				.map(Result::unwrap)
 				.map(|(k, v)| (H256::from_slice(&k), v.to_owned()))
 				.collect();