optimize back-reference filling

ethcore/light/src/on_demand/mod.rs

@@ -481,7 +481,7 @@ impl Handler for OnDemand {
 		// for each incoming response
 		//   1. ensure verification data filled. (still TODO since on_demand doesn't use back-references yet)
 		//   2. pending.requests.supply_response
-		//   3. if extracted on-demand response
+		//   3. if extracted on-demand response, keep it for later.
 		for response in responses {
 			match pending.requests.supply_response(&*self.cache, response) {
 				Ok(response) => {
@@ -497,12 +497,14 @@ impl Handler for OnDemand {
 			}
 		}
 
+		pending.requests.fill_unanswered();
 		if pending.requests.is_complete() {
 			let _ = pending.sender.send(pending.responses);
 
 			return;
 		}
 
+
 		// update network requests (unless we're done, in which case fulfill the future.)
 		let mut builder = basic_request::RequestBuilder::default();
 		let num_answered = pending.requests.num_answered();

ethcore/light/src/types/request/builder.rs

@@ -115,6 +115,15 @@ impl<T: IncompleteRequest + Clone> Requests<T> {
 				.expect("All outputs checked as invariant of `Requests` object; qed"))
 		}
 	}
+
+	/// Sweep through all unanswered requests, filling them as necessary.
+	pub fn fill_unanswered(&mut self) {
+		let outputs = &mut self.outputs;
+
+		for req in self.requests.iter_mut().skip(self.answered) {
+			req.fill(|req_idx, out_idx| outputs.get(&(req_idx, out_idx)).cloned().ok_or(NoSuchOutput))
+		}
+	}
 }
 
 impl<T: super::CheckedRequest> Requests<T> {
@@ -141,8 +150,8 @@ impl<T: super::CheckedRequest> Requests<T> {
 
 		self.answered += 1;
 
-		// fill as much of each remaining request as we can.
-		for req in self.requests.iter_mut().skip(self.answered) {
+		// fill as much of the next request as we can.
+		if let Some(ref mut req) = self.requests.get_mut(self.answered) {
 			req.fill(|req_idx, out_idx| outputs.get(&(req_idx, out_idx)).cloned().ok_or(NoSuchOutput))
 		}