Some logging and clear stalled blocks head

ethcore/sync/src/block_sync.rs

@@ -292,6 +292,10 @@ impl BlockDownloader {
 				if !headers.is_empty() {
 					// TODO: validate heads better. E.g. check that there is enough distance between blocks.
 					trace_sync!(self, target: "sync", "Received {} subchain heads, proceeding to download", headers.len());
+					match self.block_set {
+						BlockSet::NewBlocks => (),
+						BlockSet::OldBlocks => trace_sync!(self, target: "sync", "ChainHead: reset_to {:?}", hashes),
+					}
 					self.blocks.reset_to(hashes);
 					self.state = State::Blocks;
 					return Ok(DownloadAction::Reset);

ethcore/sync/src/blocks.rs

@@ -387,6 +387,15 @@ impl BlockCollection {
 		}
 
 		trace!(target: "sync", "Drained {} blocks, new head :{:?}", drained.len(), self.head);
+
+		// reset if head stuck, not downloading any block bodies or receipts
+		// for now a crude heuristic that may not cover all cases
+		// assumes that at least some bodies will be downloaded before all headers
+		// 256 = subchain heads length and 128 = subchain length. This limit could be an argument.
+		if drained.len() == 0 && self.blocks.len() > 128 * 256 {
+			info!(target: "sync", "Resetting blocks. Current head {:?}, blocks length {:?}, heads {:?}", self.head, self.blocks.len(), self.heads);
+		}
+
 		drained
 	}
 
@@ -569,6 +578,9 @@ impl BlockCollection {
 				}
 			}
 		}
+		if self.need_receipts {
+			trace!(target: "sync", "update_heads: {:?}", new_heads);
+		}
 		self.heads = new_heads;
 	}
 }