netsync: add peerSubscription

peerSubscription is added to Manager which will allow it subscribers to
receive peers through the channel whenever the Manager is aware of a new
peer that it's been connected to.  This is useful to alert
query.Workmanager that a new peer that's been connected to is eligible
to download blocks from.

go.mod

@@ -25,6 +25,7 @@ require (
 	github.com/decred/dcrd/crypto/blake256 v1.0.0 // indirect
 	github.com/golang/snappy v0.0.4 // indirect
 	github.com/kkdai/bstream v0.0.0-20161212061736-f391b8402d23 // indirect
+	github.com/lightninglabs/neutrino v0.16.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/objx v0.5.0 // indirect
 	golang.org/x/net v0.24.0 // indirect

go.sum

@@ -63,6 +63,8 @@ github.com/jrick/logrotate v1.0.0 h1:lQ1bL/n9mBNeIXoTUoYRlK4dHuNJVofX9oWqBtPnSzI
 github.com/jrick/logrotate v1.0.0/go.mod h1:LNinyqDIJnpAur+b8yyulnQw/wDuN1+BYKlTRt3OuAQ=
 github.com/kkdai/bstream v0.0.0-20161212061736-f391b8402d23 h1:FOOIBWrEkLgmlgGfMuZT83xIwfPDxEI2OHu6xUmJMFE=
 github.com/kkdai/bstream v0.0.0-20161212061736-f391b8402d23/go.mod h1:J+Gs4SYgM6CZQHDETBtE9HaSEkGmuNXF86RwHhHUvq4=
+github.com/lightninglabs/neutrino v0.16.0 h1:YNTQG32fPR/Zg0vvJVI65OBH8l3U18LSXXtX91hx0q0=
+github.com/lightninglabs/neutrino v0.16.0/go.mod h1:x3OmY2wsA18+Kc3TSV2QpSUewOCiscw2mKpXgZv2kZk=
 github.com/nxadm/tail v1.4.4 h1:DQuhQpB1tVlglWS2hLQ5OV6B5r8aGxSrPc5Qo6uTN78=
 github.com/nxadm/tail v1.4.4/go.mod h1:kenIhsEOeOJmVchQTgglprH7qJGnHDVpk1VPCcaMI8A=
 github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=

netsync/manager.go

@@ -20,6 +20,7 @@ import (
 	"github.com/btcsuite/btcd/mempool"
 	peerpkg "github.com/btcsuite/btcd/peer"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightninglabs/neutrino/query"
 )
 
 const (
@@ -203,6 +204,7 @@ type SyncManager struct {
 	headerList       *list.List
 	startHeader      *list.Element
 	nextCheckpoint   *chaincfg.Checkpoint
+	peerSubscribers  []*peerSubscription
 
 	// An optional fee estimator.
 	feeEstimator *mempool.FeeEstimator
@@ -452,6 +454,31 @@ func (sm *SyncManager) isSyncCandidate(peer *peerpkg.Peer) bool {
 	return true
 }
 
+// notifyPeerSubscribers notifies all the current peer subscribers of the peer
+// that was passed in.
+func (sm *SyncManager) notifyPeerSubscribers(peer *peerpkg.Peer) {
+	// Loop for alerting subscribers to the new peer that was connected to.
+	n := 0
+	for i, sub := range sm.peerSubscribers {
+		select {
+		// Quickly check whether this subscription has been canceled.
+		case <-sub.cancel:
+			// Avoid GC leak.
+			sm.peerSubscribers[i] = nil
+			continue
+		default:
+		}
+
+		// Keep non-canceled subscribers around.
+		sm.peerSubscribers[n] = sub
+		n++
+
+		sub.peers <- peer
+	}
+	// Re-align the slice to only active subscribers.
+	sm.peerSubscribers = sm.peerSubscribers[:n]
+}
+
 // handleNewPeerMsg deals with new peers that have signalled they may
 // be considered as a sync peer (they have already successfully negotiated).  It
 // also starts syncing if needed.  It is invoked from the syncHandler goroutine.
@@ -471,6 +498,13 @@ func (sm *SyncManager) handleNewPeerMsg(peer *peerpkg.Peer) {
 		requestedBlocks: make(map[chainhash.Hash]struct{}),
 	}
 
+	// Only pass the peer off to the subscribers if we're able to sync off of
+	// the peer.
+	bestHeight := sm.chain.BestSnapshot().Height
+	if isSyncCandidate && peer.LastBlock() > bestHeight {
+		sm.notifyPeerSubscribers(peer)
+	}
+
 	// Start syncing by choosing the best candidate if needed.
 	if isSyncCandidate && sm.syncPeer == nil {
 		sm.startSync()
@@ -1666,6 +1700,13 @@ func (sm *SyncManager) Pause() chan<- struct{} {
 	return c
 }
 
+// peerSubscription holds a peer subscription which we'll notify about any
+// connected peers.
+type peerSubscription struct {
+	peers  chan<- query.Peer
+	cancel <-chan struct{}
+}
+
 // New constructs a new SyncManager. Use Start to begin processing asynchronous
 // block, tx, and inv updates.
 func New(config *Config) (*SyncManager, error) {