Prune Excess Peers Better (#8260)

* add method

* add changes

* formatting

* choose

* fix waitgroup

* add

* add debug logs

* gaz

* make it better

* fix

* godoc

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>
Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

beacon-chain/p2p/peers/status.go

@@ -614,6 +614,57 @@ func (p *Status) BestNonFinalized(minPeers int, ourHeadEpoch uint64) (uint64, []
 	return targetEpoch, potentialPIDs
 }
 
+// PeersToPrune selects the most sutiable inbound peers
+// to disconnect the host peer from. As of this moment
+// the pruning relies on simple heuristics such as
+// bad response count. In the future scoring will be used
+// to determine the most suitable peers to take out.
+func (p *Status) PeersToPrune() []peer.ID {
+	connLimit := p.ConnectedPeerLimit()
+	activePeers := p.Active()
+	// Exit early if we are still below our max
+	// limit.
+	if len(activePeers) <= int(connLimit) {
+		return []peer.ID{}
+	}
+	p.store.Lock()
+	defer p.store.Unlock()
+
+	type peerResp struct {
+		pid     peer.ID
+		badResp int
+	}
+	peersToPrune := make([]*peerResp, 0)
+	// Select disconnected peers with a smaller bad response count.
+	for pid, peerData := range p.store.Peers() {
+		if peerData.ConnState == PeerConnected && peerData.Direction == network.DirInbound {
+			peersToPrune = append(peersToPrune, &peerResp{
+				pid:     pid,
+				badResp: peerData.BadResponses,
+			})
+		}
+	}
+
+	// Sort in descending order to favour pruning peers with a
+	// higher bad response count.
+	sort.Slice(peersToPrune, func(i, j int) bool {
+		return peersToPrune[i].badResp > peersToPrune[j].badResp
+	})
+
+	// Determine amount of peers to prune using our
+	// max connection limit.
+	amountToPrune := len(activePeers) - int(connLimit)
+
+	if amountToPrune < len(peersToPrune) {
+		peersToPrune = peersToPrune[:amountToPrune]
+	}
+	ids := make([]peer.ID, 0, len(peersToPrune))
+	for _, pr := range peersToPrune {
+		ids = append(ids, pr.pid)
+	}
+	return ids
+}
+
 // HighestEpoch returns the highest epoch reported epoch amongst peers.
 func (p *Status) HighestEpoch() uint64 {
 	p.store.RLock()