downloader: added missing blocks catchup functionality

When a parent is missing in the block list an attempt should be made to
fetch the missing parent and grandparents.

eth/downloader/peer.go

@@ -13,9 +13,51 @@ const (
 	idleState    = 4
 )
 
+type hashFetcherFn func(common.Hash) error
+type blockFetcherFn func([]common.Hash) error
+
+// XXX make threadsafe!!!!
+type peers map[string]*peer
+
+func (p peers) get(state int) []*peer {
+	var peers []*peer
+	for _, peer := range p {
+		peer.mu.RLock()
+		if peer.state == state {
+			peers = append(peers, peer)
+		}
+		peer.mu.RUnlock()
+	}
+
+	return peers
+}
+
+func (p peers) setState(id string, state int) {
+	if peer, exist := p[id]; exist {
+		peer.mu.Lock()
+		defer peer.mu.Unlock()
+		peer.state = state
+	}
+}
+
+func (p peers) getPeer(id string) *peer {
+	return p[id]
+}
+
+func (p peers) bestPeer() *peer {
+	var peer *peer
+	for _, cp := range p {
+		if peer == nil || cp.td.Cmp(peer.td) > 0 {
+			peer = cp
+		}
+	}
+	return peer
+}
+
 // peer represents an active peer
 type peer struct {
-	state int
+	state int // Peer state (working, idle)
+	rep   int // TODO peer reputation
 
 	mu         sync.RWMutex
 	id         string