chainntnfs: allow clients to pass in best block

Clients can optionally pass their best block known into RegisterBlockEpochNtfn. This enables the notifiers to catch up clients on blocks they may have missed.

chainntnfs/neutrinonotify/neutrino.go

@@ -20,7 +20,6 @@ import (
 )
 
 const (
-
 	// notifierType uniquely identifies this concrete implementation of the
 	// ChainNotifier interface.
 	notifierType = "neutrino"
@@ -781,6 +780,10 @@ type blockEpochRegistration struct {
 
 	cancelChan chan struct{}
 
+	bestBlock *chainntnfs.BlockEpoch
+
+	errorChan chan error
+
 	wg sync.WaitGroup
 }
 
@@ -790,14 +793,20 @@ type epochCancel struct {
 	epochID uint64
 }
 
-// RegisterBlockEpochNtfn returns a BlockEpochEvent which subscribes the caller
-// to receive notifications, of each new block connected to the main chain.
-func (n *NeutrinoNotifier) RegisterBlockEpochNtfn() (*chainntnfs.BlockEpochEvent, error) {
+// RegisterBlockEpochNtfn returns a BlockEpochEvent which subscribes the
+// caller to receive notifications, of each new block connected to the main
+// chain. Clients have the option of passing in their best known block, which
+// the notifier uses to check if they are behind on blocks and catch them up.
+func (n *NeutrinoNotifier) RegisterBlockEpochNtfn(
+	bestBlock *chainntnfs.BlockEpoch) (*chainntnfs.BlockEpochEvent, error) {
+
 	reg := &blockEpochRegistration{
 		epochQueue: chainntnfs.NewConcurrentQueue(20),
 		epochChan:  make(chan *chainntnfs.BlockEpoch, 20),
 		cancelChan: make(chan struct{}),
 		epochID:    atomic.AddUint64(&n.epochClientCounter, 1),
+		bestBlock:  bestBlock,
+		errorChan:  make(chan error, 1),
 	}
 	reg.epochQueue.Start()