migrate blocks in batches (rework db iterator impl) (#3450)

* rework db iterator
more robust block migration based on low level iterator

* cleanup

* cleanup

* fixup

p2p/src/peers.rs

@@ -369,15 +369,16 @@ impl Peers {
 		}
 	}
 
-	/// All peer information we have in storage
+	/// Iterator over all peers we know about (stored in our db).
+	pub fn peers_iter(&self) -> Result<impl Iterator<Item = PeerData>, Error> {
+		self.store.peers_iter().map_err(From::from)
+	}
+
+	/// Convenience for reading all peers.
 	pub fn all_peers(&self) -> Vec<PeerData> {
-		match self.store.all_peers() {
-			Ok(peers) => peers,
-			Err(e) => {
-				error!("all_peers failed: {:?}", e);
-				vec![]
-			}
-		}
+		self.peers_iter()
+			.map(|peers| peers.collect())
+			.unwrap_or(vec![])
 	}
 
 	/// Find peers in store (not necessarily connected) and return their data

p2p/src/store.rs

@@ -152,21 +152,27 @@ impl PeerStore {
 		cap: Capabilities,
 		count: usize,
 	) -> Result<Vec<PeerData>, Error> {
-		let key = to_key(PEER_PREFIX, "");
 		let peers = self
-			.db
-			.iter::<PeerData>(&key)?
-			.map(|(_, v)| v)
+			.peers_iter()?
 			.filter(|p| p.flags == state && p.capabilities.contains(cap))
 			.choose_multiple(&mut thread_rng(), count);
 		Ok(peers)
 	}
 
+	/// Iterator over all known peers.
+	pub fn peers_iter(&self) -> Result<impl Iterator<Item = PeerData>, Error> {
+		let key = to_key(PEER_PREFIX, "");
+		let protocol_version = self.db.protocol_version();
+		self.db.iter(&key, move |_, mut v| {
+			ser::deserialize(&mut v, protocol_version).map_err(From::from)
+		})
+	}
+
 	/// List all known peers
 	/// Used for /v1/peers/all api endpoint
 	pub fn all_peers(&self) -> Result<Vec<PeerData>, Error> {
-		let key = to_key(PEER_PREFIX, "");
-		Ok(self.db.iter::<PeerData>(&key)?.map(|(_, v)| v).collect())
+		let peers: Vec<PeerData> = self.peers_iter()?.collect();
+		Ok(peers)
 	}
 
 	/// Convenience method to load a peer data, update its status and save it
@@ -194,7 +200,7 @@ impl PeerStore {
 	{
 		let mut to_remove = vec![];
 
-		for x in self.all_peers()? {
+		for x in self.peers_iter()? {
 			if predicate(&x) {
 				to_remove.push(x)
 			}