chainntnfs: handling for potential deadlocks during system shutdown

If the lnd daemon is shut down while multiple subsystems are attempting
to register for notifications, the blocking of those chain notifier
registrations may cause the daemon shutdown to deadlock. The additions
in this commit allow the registration functions to return errors rather
than potentially deadlock when the chain notifier is shut down.

chainntnfs/btcdnotify/btcd.go

@@ -2,6 +2,7 @@ package btcdnotify
 
 import (
 	"container/heap"
+	"errors"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -546,7 +547,12 @@ func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint) (*chainntnfs.S
 		spendChan:      make(chan *chainntnfs.SpendDetail, 1),
 	}
 
-	b.notificationRegistry <- ntfn
+	select {
+	case <-b.quit:
+		return nil, errors.New("chainntnfs: system interrupt while " +
+			"attempting to register for spend notification.")
+	case b.notificationRegistry <- ntfn:
+	}
 
 	// The following conditional checks to ensure that when a spend notification
 	// is registered, the output hasn't already been spent. If the output
@@ -603,12 +609,16 @@ func (b *BtcdNotifier) RegisterConfirmationsNtfn(txid *wire.ShaHash,
 		negativeConf:     make(chan int32, 1),
 	}
 
-	b.notificationRegistry <- ntfn
+	select {
-
+	case <-b.quit:
+		return nil, errors.New("chainntnfs: system interrupt while " +
+			"attempting to register for confirmation notification.")
+	case b.notificationRegistry <- ntfn:
 		return &chainntnfs.ConfirmationEvent{
 			Confirmed:    ntfn.finConf,
 			NegativeConf: ntfn.negativeConf,
 		}, nil
+	}
 }
 
 // blockEpochRegistration represents a client's intent to receive a
@@ -625,9 +635,13 @@ func (b *BtcdNotifier) RegisterBlockEpochNtfn() (*chainntnfs.BlockEpochEvent, er
 		epochChan: make(chan *chainntnfs.BlockEpoch, 20),
 	}
 
-	b.notificationRegistry <- registration
+	select {
-
+	case <-b.quit:
+		return nil, errors.New("chainntnfs: system interrupt while " +
+			"attempting to register for block epoch notification.")
+	case b.notificationRegistry <- registration:
 		return &chainntnfs.BlockEpochEvent{
 			Epochs: registration.epochChan,
 		}, nil
+	}
 }