allow Wallet impls provide tip change tracking where possible

client/asset/dcr/dcr.go

@@ -537,11 +537,17 @@ func (dcr *ExchangeWallet) Connect(ctx context.Context) (*sync.WaitGroup, error)
 
 	success = true // All good, don't disconnect the wallet when this method returns.
 
+	// NotifyOnTipChange will return false if the wallet does not support
+	// tip change notification. We'll use dcr.monitorBlocks below if so.
+	monitoringBlocks := dcr.wallet.NotifyOnTipChange(ctx, dcr.handleTipChange)
+
 	var wg sync.WaitGroup
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		dcr.monitorBlocks(ctx)
+		if !monitoringBlocks {
+			dcr.monitorBlocks(ctx)
+		}
 		dcr.shutdown()
 	}()
 	return &wg, nil
@@ -2691,25 +2697,30 @@ func (dcr *ExchangeWallet) checkForNewBlocks() {
 	defer cancel()
 	newTip, err := dcr.getBestBlock(ctx)
 	if err != nil {
-		go dcr.tipChange(fmt.Errorf("failed to get best block: %w", err))
+		dcr.handleTipChange(nil, 0, fmt.Errorf("failed to get best block: %w", err))
 		return
 	}
 
-	// This method is called frequently. Don't hold write lock
-	// unless tip has changed.
 	dcr.tipMtx.RLock()
 	sameTip := dcr.currentTip.hash.IsEqual(newTip.hash)
 	dcr.tipMtx.RUnlock()
-	if sameTip {
+	if !sameTip {
+		dcr.handleTipChange(newTip.hash, newTip.height, nil)
+	}
+}
+
+func (dcr *ExchangeWallet) handleTipChange(newTipHash *chainhash.Hash, newTipHeight int64, err error) {
+	if err != nil {
+		go dcr.tipChange(err)
 		return
 	}
 
 	dcr.tipMtx.Lock()
 	defer dcr.tipMtx.Unlock()
 
 	prevTip := dcr.currentTip
-	dcr.currentTip = newTip
-	dcr.log.Debugf("tip change: %d (%s) => %d (%s)", prevTip.height, prevTip.hash, newTip.height, newTip.hash)
+	dcr.currentTip = &block{newTipHeight, newTipHash}
+	dcr.log.Debugf("tip change: %d (%s) => %d (%s)", prevTip.height, prevTip.hash, newTipHeight, newTipHash)
 	go dcr.tipChange(nil)
 
 	// Search for contract redemption in new blocks if there
@@ -2765,7 +2776,7 @@ func (dcr *ExchangeWallet) checkForNewBlocks() {
 			if aBlock.Confirmations > -1 {
 				// Found the mainchain ancestor of previous tip.
 				startHeight = int64(aBlock.Height)
-				dcr.log.Debugf("reorg detected from height %d to %d", aBlock.Height, newTip.height)
+				dcr.log.Debugf("reorg detected from height %d to %d", aBlock.Height, newTipHeight)
 				break
 			}
 			if aBlock.Height == 0 {
@@ -2785,7 +2796,7 @@ func (dcr *ExchangeWallet) checkForNewBlocks() {
 
 	// Run the redemption search from the startHeight determined above up
 	// till the current tip height.
-	go dcr.findRedemptionsInBlockRange(startHeight, newTip.height, contractOutpoints)
+	go dcr.findRedemptionsInBlockRange(startHeight, newTipHeight, contractOutpoints)
 }
 
 func (dcr *ExchangeWallet) getBestBlock(ctx context.Context) (*block, error) {

client/asset/dcr/rpcwallet.go

@@ -257,6 +257,18 @@ func (w *rpcWallet) Network(ctx context.Context) (wire.CurrencyNet, error) {
 	return net, translateRPCCancelErr(err)
 }
 
+// NotifyOnTipChange registers a callback function that the should be invoked
+// when the wallet sees new mainchain blocks. The return value indicates if this
+// notification can be provided.
+// Part of the Wallet interface.
+func (w *rpcWallet) NotifyOnTipChange(ctx context.Context, cb TipChangeCallback) bool {
+	// TODO: Could use the ctx provided to poll getbestblock and detect tip changes
+	// in order to invoke the provided callback. This is currently already being handled
+	// by the dcr ExchangeWallet to accomodate other implementations that may also not
+	// have an out-of-the-box provision for block change notifications.
+	return false
+}
+
 // AccountOwnsAddress uses the validateaddress rpc to check if the provided
 // address belongs to the specified account.
 // Part of the Wallet interface.

client/asset/dcr/wallet.go

@@ -42,6 +42,8 @@ func RegisterCustomWallet(constructor WalletConstructor, info *asset.WalletInfo)
 	chaincfg.TestNet3Params()
 }
 
+type TipChangeCallback func(*chainhash.Hash, int64, error)
+
 type Wallet interface {
 	// Connect establishes a connection to the wallet.
 	Connect(ctx context.Context) error
@@ -51,6 +53,12 @@ type Wallet interface {
 	Disconnected() bool
 	// Network returns the network of the connected wallet.
 	Network(ctx context.Context) (wire.CurrencyNet, error)
+	// NotifyOnTipChange registers a callback function that the should be
+	// invoked when the wallet sees new mainchain blocks. The return value
+	// indicates if this notification can be provided. Where this tip change
+	// notification is unimplemented, monitorBlocks should be used to track
+	// tip changes.
+	NotifyOnTipChange(ctx context.Context, cb TipChangeCallback) bool
 	// AccountOwnsAddress checks if the provided address belongs to the
 	// specified account.
 	AccountOwnsAddress(ctx context.Context, account, address string) (bool, error)