Minor fix in chain supplier and light provider (#8906)

* fix chain supplier increment

* fix light provider block_headers

ethcore/light/src/provider.rs

@@ -54,6 +54,7 @@ pub trait Provider: Send + Sync {
  /// results within must adhere to the `skip` and `reverse` parameters.
  fn block_headers(&self, req: request::CompleteHeadersRequest) -> Option<request::HeadersResponse> {
  use request::HashOrNumber;
+ const MAX_HEADERS_TO_SEND: u64 = 512;
 
  if req.max == 0 { return None }
 
@@ -82,10 +83,12 @@ pub trait Provider: Send + Sync {
  }
  };
 
- let headers: Vec<_> = (0u64..req.max as u64)
- .map(|x: u64| x.saturating_mul(req.skip + 1))
+ let max = ::std::cmp::min(MAX_HEADERS_TO_SEND, req.max);
+
+ let headers: Vec<_> = (0u64..max)
+ .map(|x: u64| x.saturating_mul(req.skip.saturating_add(1)))
  .take_while(|x| if req.reverse { x < &start_num } else { best_num.saturating_sub(start_num) >= *x })
- .map(|x| if req.reverse { start_num - x } else { start_num + x })
+ .map(|x| if req.reverse { start_num.saturating_sub(x) } else { start_num.saturating_add(x) })
  .map(|x| self.block_header(BlockId::Number(x)))
  .take_while(|x| x.is_some())
  .flat_map(|x| x)

ethcore/sync/src/chain/supplier.rs

@@ -133,7 +133,7 @@ impl SyncSupplier {
  let max_count = cmp::min(MAX_HEADERS_TO_SEND, max_headers);
  let mut count = 0;
  let mut data = Bytes::new();
- let inc = (skip + 1) as BlockNumber;
+ let inc = skip.saturating_add(1) as BlockNumber;
  let overlay = io.chain_overlay().read();
 
  // We are checking the `overlay` as well since it's where the ForkBlock
@@ -155,9 +155,9 @@ impl SyncSupplier {
  if number <= inc || number == 0 {
  break;
  }
- number -= inc;
+ number = number.saturating_sub(inc);
  } else {
- number += inc;
+ number = number.saturating_add(inc);
  }
  }
  let mut rlp = RlpStream::new_list(count as usize);