lnd_test+lntest: keep btcd debug logs from test run

Useful for debugging 🤷

lnd_test.go

@@ -12516,7 +12516,13 @@ func TestLightningNetworkDaemon(t *testing.T) {
 	// setting of accepting non-standard transactions on simnet to reject them.
 	// Transactions on the lightning network should always be standard to get
 	// better guarantees of getting included in to blocks.
-	args := []string{"--rejectnonstd", "--txindex"}
+	logDir := "./.backendlogs"
+	args := []string{
+		"--rejectnonstd",
+		"--txindex",
+		"--debuglevel=debug",
+		"--logdir=" + logDir,
+	}
 	handlers := &rpcclient.NotificationHandlers{
 		OnTxAccepted: func(hash *chainhash.Hash, amt btcutil.Amount) {
 			lndHarness.OnTxAccepted(hash)
@@ -12526,7 +12532,21 @@ func TestLightningNetworkDaemon(t *testing.T) {
 	if err != nil {
 		ht.Fatalf("unable to create mining node: %v", err)
 	}
-	defer btcdHarness.TearDown()
+	defer func() {
+		btcdHarness.TearDown()
+
+		// After shutting down the chain backend, we'll make a copy of
+		// the log file before deleting the temporary log dir.
+		logFile := logDir + "/" + harnessNetParams.Name + "/btcd.log"
+		err := lntest.CopyFile("./output_btcd_chainbackend.log",
+			logFile)
+		if err != nil {
+			fmt.Printf("unable to copy file: %v\n", err)
+		}
+		if err = os.RemoveAll(logDir); err != nil {
+			fmt.Printf("Cannot remove dir %s: %v\n", logDir, err)
+		}
+	}()
 
 	// First create the network harness to gain access to its
 	// 'OnTxAccepted' call back.