chainntnfs/btcdnotify: recognize JSON-RPC error in RegisterSpendNtfn

This commit fixes a prior bug in the logic for registering a new spend
notification. Previously, if the transaction wasn’t found in the
mempool or already confirmed within the chain, then
GetRawTransactionVerbose would return an error which would cause the
function itself to exit with an error.

This issue would then cause the server to be unable to start up as the
breach arbiter would be unable to register for spend notifications for
all the channels that it needed to be watching.

We fix this error simply by recognizing the particular JSON-RPC error
that will be returned in this scenario and treating it as a benign
error.

chainntnfs/btcdnotify/btcd.go

@@ -506,6 +506,8 @@ func (b *BtcdNotifier) notifyBlockEpochs(newHeight int32, newSha *chainhash.Hash
 		go func(ntfnChan chan *chainntnfs.BlockEpoch, cancelChan chan struct{},
 			clientWg *sync.WaitGroup) {
 
+			// TODO(roasbeef): move to goroutine per client, use sync queue
+
 			defer clientWg.Done()
 			defer b.wg.Done()
 
@@ -664,18 +666,26 @@ func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint,
 	if txout == nil {
 		transaction, err := b.chainConn.GetRawTransactionVerbose(&outpoint.Hash)
 		if err != nil {
-			return nil, err
+			jsonErr, ok := err.(*btcjson.RPCError)
+			switch {
+			case ok && jsonErr.Code == -5:
+			default:
+				return nil, err
+			}
 		}
 
-		blockhash, err := chainhash.NewHashFromStr(transaction.BlockHash)
-		if err != nil {
-			return nil, err
-		}
+		if transaction != nil {
+			blockhash, err := chainhash.NewHashFromStr(transaction.BlockHash)
+			if err != nil {
+				return nil, err
+			}
 
-		ops := []*wire.OutPoint{outpoint}
-		if err := b.chainConn.Rescan(blockhash, nil, ops); err != nil {
-			chainntnfs.Log.Errorf("Rescan for spend notification txout failed: %v", err)
-			return nil, err
+			ops := []*wire.OutPoint{outpoint}
+			if err := b.chainConn.Rescan(blockhash, nil, ops); err != nil {
+				chainntnfs.Log.Errorf("Rescan for spend "+
+					"notification txout failed: %v", err)
+				return nil, err
+			}
 		}
 	}