lnd/lntemp/node/harness_node.go

package node

import (
	"bytes"
	"context"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"testing"
	"time"

	"github.com/jackc/pgx/v4/pgxpool"
	"github.com/lightningnetwork/lnd/lnrpc"
	"github.com/lightningnetwork/lnd/lntemp/rpc"
	"github.com/lightningnetwork/lnd/lntest"
	"github.com/lightningnetwork/lnd/lntest/wait"
	"github.com/lightningnetwork/lnd/macaroons"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/status"
	"gopkg.in/macaroon.v2"
)

const (
	// logPubKeyBytes is the number of bytes of the node's PubKey that will
	// be appended to the log file name. The whole PubKey is too long and
	// not really necessary to quickly identify what node produced which
	// log file.
	logPubKeyBytes = 4

	// trickleDelay is the amount of time in milliseconds between each
	// release of announcements by AuthenticatedGossiper to the network.
	trickleDelay = 50

	postgresDsn = "postgres://postgres:postgres@localhost:" +
		"6432/%s?sslmode=disable"

	// commitInterval specifies the maximum interval the graph database
	// will wait between attempting to flush a batch of modifications to
	// disk(db.batch-commit-interval).
	commitInterval = 10 * time.Millisecond
)

// HarnessNode represents an instance of lnd running within our test network
// harness. It's responsible for managing the lnd process, grpc connection, and
// wallet auth. A HarnessNode is built upon its rpc clients, represented in
// `HarnessRPC`. It also has a `State` which holds its internal state, and a
// `Watcher` that keeps track of its topology updates.
type HarnessNode struct {
	*testing.T

	// Cfg holds the config values for the node.
	Cfg *BaseNodeConfig

	// RPC holds a list of RPC clients.
	RPC *rpc.HarnessRPC

	// State records the current state of the node.
	State *State

	// Watcher watches the node's topology updates.
	Watcher *nodeWatcher

	// PubKey is the serialized compressed identity public key of the node.
	// This field will only be populated once the node itself has been
	// started via the start() method.
	PubKey    [33]byte
	PubKeyStr string

	// conn is the underlying connection to the grpc endpoint of the node.
	conn *grpc.ClientConn

	// runCtx is a context with cancel method. It's used to signal when the
	// node needs to quit, and used as the parent context when spawning
	// children contexts for RPC requests.
	runCtx context.Context
	cancel context.CancelFunc

	// filename is the log file's name.
	filename string

	cmd     *exec.Cmd
	logFile *os.File
}

// NewHarnessNode creates a new test lightning node instance from the passed
// config.
func NewHarnessNode(t *testing.T, cfg *BaseNodeConfig) (*HarnessNode, error) {
	if cfg.BaseDir == "" {
		var err error
		cfg.BaseDir, err = ioutil.TempDir("", "lndtest-node")
		if err != nil {
			return nil, err
		}
	}
	cfg.DataDir = filepath.Join(cfg.BaseDir, "data")
	cfg.LogDir = filepath.Join(cfg.BaseDir, "logs")
	cfg.TLSCertPath = filepath.Join(cfg.BaseDir, "tls.cert")
	cfg.TLSKeyPath = filepath.Join(cfg.BaseDir, "tls.key")

	networkDir := filepath.Join(
		cfg.DataDir, "chain", "bitcoin", cfg.NetParams.Name,
	)
	cfg.AdminMacPath = filepath.Join(networkDir, "admin.macaroon")
	cfg.ReadMacPath = filepath.Join(networkDir, "readonly.macaroon")
	cfg.InvoiceMacPath = filepath.Join(networkDir, "invoice.macaroon")

	cfg.GenerateListeningPorts()

	// Create temporary database.
	var dbName string
	if cfg.DbBackend == lntest.BackendPostgres {
		var err error
		dbName, err = createTempPgDb()
		if err != nil {
			return nil, err
		}
		cfg.PostgresDsn = postgresDatabaseDsn(dbName)
	}

	cfg.OriginalExtraArgs = cfg.ExtraArgs
	cfg.postgresDbName = dbName

	return &HarnessNode{
		T:   t,
		Cfg: cfg,
	}, nil
}

// InitRPCClients initializes a list of RPC clients for the node.
func (hn *HarnessNode) InitRPCClients(c *grpc.ClientConn) {
	hn.conn = c

	// Init all the rpc clients.
	hn.RPC = rpc.NewHarnessRPC(hn.runCtx, hn.T, c, hn.Name())

	// Init the node's internal state.
	hn.State = newState(hn.RPC)

	// Init the topology watcher.
	hn.Watcher = newNodeWatcher(hn.RPC, hn.State)
}

// Name returns the name of this node set during initialization.
func (hn *HarnessNode) Name() string {
	return hn.Cfg.Name
}

// UpdateState updates the node's internal state.
func (hn *HarnessNode) UpdateState() {
	hn.State.updateState()
}

// String gives the internal state of the node which is useful for debugging.
func (hn *HarnessNode) String() string {
	type nodeCfg struct {
		LogFilenamePrefix string
		ExtraArgs         []string
		HasSeed           bool
		P2PPort           int
		RPCPort           int
		RESTPort          int
		ProfilePort       int
		AcceptKeySend     bool
		FeeURL            string
	}

	nodeState := struct {
		NodeID  uint32
		Name    string
		PubKey  string
		State   *State
		NodeCfg nodeCfg
	}{
		NodeID: hn.Cfg.NodeID,
		Name:   hn.Cfg.Name,
		PubKey: hn.PubKeyStr,
		State:  hn.State,
		NodeCfg: nodeCfg{
			LogFilenamePrefix: hn.Cfg.LogFilenamePrefix,
			ExtraArgs:         hn.Cfg.ExtraArgs,
			P2PPort:           hn.Cfg.P2PPort,
			RPCPort:           hn.Cfg.RPCPort,
			RESTPort:          hn.Cfg.RESTPort,
		},
	}

	stateBytes, err := json.MarshalIndent(nodeState, "", "\t")
	if err != nil {
		return fmt.Sprintf("\n encode node state with err: %v", err)
	}

	return fmt.Sprintf("\nnode state: %s", stateBytes)
}

// WaitUntilStarted waits until the wallet state flips from "WAITING_TO_START".
func (hn *HarnessNode) WaitUntilStarted() error {
	return hn.waitTillServerState(func(s lnrpc.WalletState) bool {
		return s != lnrpc.WalletState_WAITING_TO_START
	})
}

// WaitUntilServerActive waits until the lnd daemon is fully started.
func (hn *HarnessNode) WaitUntilServerActive() error {
	return hn.waitTillServerState(func(s lnrpc.WalletState) bool {
		return s == lnrpc.WalletState_SERVER_ACTIVE
	})
}

// Unlock attempts to unlock the wallet of the target HarnessNode. This method
// should be called after the restart of a HarnessNode that was created with a
// seed+password. Once this method returns, the HarnessNode will be ready to
// accept normal gRPC requests and harness command.
func (hn *HarnessNode) Unlock(unlockReq *lnrpc.UnlockWalletRequest) error {
	// Otherwise, we'll need to unlock the node before it's able to start
	// up properly.
	hn.RPC.UnlockWallet(unlockReq)

	// Now that the wallet has been unlocked, we'll wait for the RPC client
	// to be ready, then establish the normal gRPC connection.
	return hn.InitNode(nil)
}

// AddToLogf adds a line of choice to the node's logfile. This is useful
// to interleave test output with output from the node.
func (hn *HarnessNode) AddToLogf(format string, a ...interface{}) {
	// If this node was not set up with a log file, just return early.
	if hn.logFile == nil {
		return
	}

	desc := fmt.Sprintf("itest: %s\n", fmt.Sprintf(format, a...))
	if _, err := hn.logFile.WriteString(desc); err != nil {
		hn.printErrf("write to log err: %v", err)
	}
}

// ReadMacaroon waits a given duration for the macaroon file to be created. If
// the file is readable within the timeout, its content is de-serialized as a
// macaroon and returned.
func (hn *HarnessNode) ReadMacaroon(macPath string, timeout time.Duration) (
	*macaroon.Macaroon, error) {

	// Wait until macaroon file is created and has valid content before
	// using it.
	var mac *macaroon.Macaroon
	err := wait.NoError(func() error {
		macBytes, err := ioutil.ReadFile(macPath)
		if err != nil {
			return fmt.Errorf("error reading macaroon file: %v",
				err)
		}

		newMac := &macaroon.Macaroon{}
		if err = newMac.UnmarshalBinary(macBytes); err != nil {
			return fmt.Errorf("error unmarshalling macaroon "+
				"file: %v", err)
		}
		mac = newMac

		return nil
	}, timeout)

	return mac, err
}

// ConnectRPCWithMacaroon uses the TLS certificate and given macaroon to
// create a gRPC client connection.
func (hn *HarnessNode) ConnectRPCWithMacaroon(mac *macaroon.Macaroon) (
	*grpc.ClientConn, error) {

	// Wait until TLS certificate is created and has valid content before
	// using it, up to 30 sec.
	var tlsCreds credentials.TransportCredentials
	err := wait.NoError(func() error {
		var err error
		tlsCreds, err = credentials.NewClientTLSFromFile(
			hn.Cfg.TLSCertPath, "",
		)
		return err
	}, DefaultTimeout)
	if err != nil {
		return nil, fmt.Errorf("error reading TLS cert: %v", err)
	}

	opts := []grpc.DialOption{
		grpc.WithBlock(),
		grpc.WithTransportCredentials(tlsCreds),
	}

	ctx, cancel := context.WithTimeout(hn.runCtx, DefaultTimeout)
	defer cancel()

	if mac == nil {
		return grpc.DialContext(ctx, hn.Cfg.RPCAddr(), opts...)
	}
	macCred, err := macaroons.NewMacaroonCredential(mac)
	if err != nil {
		return nil, fmt.Errorf("error cloning mac: %v", err)
	}
	opts = append(opts, grpc.WithPerRPCCredentials(macCred))

	return grpc.DialContext(ctx, hn.Cfg.RPCAddr(), opts...)
}

// ConnectRPC uses the TLS certificate and admin macaroon files written by the
// lnd node to create a gRPC client connection.
func (hn *HarnessNode) ConnectRPC() (*grpc.ClientConn, error) {
	// If we should use a macaroon, always take the admin macaroon as a
	// default.
	mac, err := hn.ReadMacaroon(hn.Cfg.AdminMacPath, DefaultTimeout)
	if err != nil {
		return nil, err
	}

	return hn.ConnectRPCWithMacaroon(mac)
}

// SetExtraArgs assigns the ExtraArgs field for the node's configuration. The
// changes will take effect on restart.
func (hn *HarnessNode) SetExtraArgs(extraArgs []string) {
	hn.Cfg.ExtraArgs = extraArgs
}

// StartLndCmd handles the startup of lnd, creating log files, and possibly
// kills the process when needed.
func (hn *HarnessNode) StartLndCmd() error {
	// Init the runCtx.
	hn.runCtx, hn.cancel = context.WithCancel(context.Background())

	args := hn.Cfg.GenArgs()
	hn.cmd = exec.Command(hn.Cfg.LndBinary, args...) //nolint:gosec

	// Redirect stderr output to buffer
	var errb bytes.Buffer
	hn.cmd.Stderr = &errb

	// If the logoutput flag is passed, redirect output from the nodes to
	// log files.
	if *lntest.LogOutput {
		err := addLogFile(hn)
		if err != nil {
			return err
		}
	}

	// Start the process.
	if err := hn.cmd.Start(); err != nil {
		return err
	}

	return nil
}

// StartWithSeed will start the lnd process, creates the grpc connection
// without macaroon auth, and waits until the server is reported as waiting to
// start.
//
// NOTE: caller needs to take extra step to create and unlock the wallet.
func (hn *HarnessNode) StartWithSeed() error {
	// Start lnd process and prepare logs.
	if err := hn.StartLndCmd(); err != nil {
		return fmt.Errorf("start lnd error: %w", err)
	}

	// Create an unauthed connection.
	conn, err := hn.ConnectRPCWithMacaroon(nil)
	if err != nil {
		return fmt.Errorf("ConnectRPCWithMacaroon err: %w", err)
	}

	// Since the conn is not authed, only the `WalletUnlocker` and `State`
	// clients can be inited from this conn.
	hn.conn = conn
	hn.RPC = rpc.NewHarnessRPC(hn.runCtx, hn.T, conn, hn.Name())

	// Wait till the server is starting.
	return hn.WaitUntilStarted()
}

// Start will start the lnd process, creates the grpc connection, and waits
// until the server is fully started.
func (hn *HarnessNode) Start() error {
	// Start lnd process and prepare logs.
	if err := hn.StartLndCmd(); err != nil {
		return fmt.Errorf("start lnd error: %w", err)
	}

	// Since Stop uses the LightningClient to stop the node, if we fail to
	// get a connected client, we have to kill the process.
	conn, err := hn.ConnectRPC()
	if err != nil {
		err = fmt.Errorf("ConnectRPC err: %w", err)
		cmdErr := hn.kill()
		if cmdErr != nil {
			err = fmt.Errorf("kill process got err: %w: %v",
				cmdErr, err)
		}
		return err
	}

	// Init all the RPC clients.
	hn.InitRPCClients(conn)

	// Wait till the server is starting.
	if err := hn.WaitUntilStarted(); err != nil {
		return fmt.Errorf("waiting for start got: %v", err)
	}

	// Subscribe for topology updates.
	return hn.initLightningClient()
}

// InitNode waits until the main gRPC server is detected as active, then
// complete the normal HarnessNode gRPC connection creation. A non-nil
// `macBytes` indicates the node is initialized stateless, otherwise it will
// use the admin macaroon.
func (hn *HarnessNode) InitNode(macBytes []byte) error {
	var (
		conn *grpc.ClientConn
		err  error
	)

	// If the node has been initialized stateless, we need to pass the
	// macaroon to the client.
	if macBytes != nil {
		adminMac := &macaroon.Macaroon{}
		err := adminMac.UnmarshalBinary(macBytes)
		if err != nil {
			return fmt.Errorf("unmarshal failed: %w", err)
		}
		conn, err = hn.ConnectRPCWithMacaroon(adminMac)
		if err != nil {
			return err
		}
	} else {
		// Normal initialization, we expect a macaroon to be in the
		// file system.
		conn, err = hn.ConnectRPC()
		if err != nil {
			return err
		}
	}

	// Init all the RPC clients.
	hn.InitRPCClients(conn)

	return hn.initLightningClient()
}

// waitTillServerState makes a subscription to the server's state change and
// blocks until the server is in the targeted state.
func (hn *HarnessNode) waitTillServerState(
	predicate func(state lnrpc.WalletState) bool) error {

	client := hn.RPC.SubscribeState()

	errChan := make(chan error, 1)
	done := make(chan struct{})
	go func() {
		for {
			resp, err := client.Recv()
			if err != nil {
				errChan <- err
				return
			}

			if predicate(resp.State) {
				close(done)
				return
			}
		}
	}()

	for {
		select {
		case <-time.After(lntest.NodeStartTimeout):
			return fmt.Errorf("timeout waiting for server state")
		case err := <-errChan:
			return fmt.Errorf("receive server state err: %v", err)

		case <-done:
			return nil
		}
	}
}

// initLightningClient blocks until the lnd server is fully started and
// subscribes the harness node to graph topology updates. This method also
// spawns a lightning network watcher for this node, which watches for topology
// changes.
func (hn *HarnessNode) initLightningClient() error {
	// Wait until the server is fully started.
	if err := hn.WaitUntilServerActive(); err != nil {
		return err
	}

	// Set the harness node's pubkey to what the node claims in GetInfo.
	// The RPC must have been started at this point.
	if err := hn.attachPubKey(); err != nil {
		return err
	}

	// Launch the watcher that will hook into graph related topology change
	// from the PoV of this node.
	started := make(chan error, 1)
	go hn.Watcher.topologyWatcher(hn.runCtx, started)

	select {
	// First time reading the channel indicates the topology client is
	// started.
	case err := <-started:
		if err != nil {
			return fmt.Errorf("create topology client stream "+
				"got err: %v", err)
		}

	case <-time.After(DefaultTimeout):
		return fmt.Errorf("timeout creating topology client stream")
	}

	// Catch topology client stream error inside a goroutine.
	go func() {
		select {
		case err := <-started:
			hn.printErrf("topology client: %v", err)

		case <-hn.runCtx.Done():
		}
	}()

	return nil
}

// attachPubKey queries an unlocked node to retrieve its public key.
func (hn *HarnessNode) attachPubKey() error {
	// Obtain the lnid of this node for quick identification purposes.
	info := hn.RPC.GetInfo()
	hn.PubKeyStr = info.IdentityPubkey

	pubkey, err := hex.DecodeString(info.IdentityPubkey)
	if err != nil {
		return err
	}
	copy(hn.PubKey[:], pubkey)

	return nil
}

// cleanup cleans up all the temporary files created by the node's process.
func (hn *HarnessNode) cleanup() error {
	if hn.Cfg.backupDbDir != "" {
		err := os.RemoveAll(hn.Cfg.backupDbDir)
		if err != nil {
			return fmt.Errorf("unable to remove backup dir: %v",
				err)
		}
	}

	return os.RemoveAll(hn.Cfg.BaseDir)
}

// waitForProcessExit Launch a new goroutine which that bubbles up any
// potential fatal process errors to the goroutine running the tests.
func (hn *HarnessNode) waitForProcessExit() {
	errChan := make(chan error, 1)
	go func() {
		err := hn.cmd.Wait()
		errChan <- err
	}()

	select {
	case err := <-errChan:
		if err == nil {
			break
		}

		// If the process has already been canceled, we can exit early
		// as the logs have already been saved.
		if strings.Contains(err.Error(), "Wait was already called") {
			return
		}

		// Otherwise, we print the error, break the select and save
		// logs.
		hn.printErrf("wait process exit got err: %v", err)
		break

	case <-time.After(DefaultTimeout * 2):
		hn.printErrf("timeout waiting for process to exit")
	}

	// Make sure log file is closed and renamed if necessary.
	finalizeLogfile(hn)

	// Rename the etcd.log file if the node was running on embedded
	// etcd.
	finalizeEtcdLog(hn)
}

// Stop attempts to stop the active lnd process.
func (hn *HarnessNode) Stop() error {
	// Do nothing if the process is not running.
	if hn.runCtx == nil {
		hn.printErrf("found nil run context")
		return nil
	}

	// Stop the runCtx.
	hn.cancel()

	// Wait for lnd process to exit in the end.
	defer hn.waitForProcessExit()

	// If we ever reaches the state where `Watcher` is initialized, it
	// means the node has an authed connection and all its RPC clients are
	// ready for use. Thus we will try to stop it via the RPC.
	if hn.Watcher != nil {
		// Don't watch for error because sometimes the RPC connection
		// gets closed before a response is returned.
		req := lnrpc.StopRequest{}

		ctxt, cancel := context.WithCancel(context.Background())
		defer cancel()

		err := wait.NoError(func() error {
			_, err := hn.RPC.LN.StopDaemon(ctxt, &req)

			switch {
			case err == nil:
				return nil

			// Try again if a recovery/rescan is in progress.
			case strings.Contains(
				err.Error(), "recovery in progress",
			):
				return err

			default:
				return nil
			}
		}, DefaultTimeout)
		if err != nil {
			return err
		}

		// Wait for goroutines to be finished.
		done := make(chan struct{})
		go func() {
			hn.Watcher.wg.Wait()
			close(done)
		}()

		// If the goroutines fail to finish before timeout, we'll print
		// the error to console and continue.
		select {
		case <-time.After(DefaultTimeout):
			hn.printErrf("timeout on wait group")
		case <-done:
		}
	} else {
		// If the rpc clients are not initiated, we'd kill the process
		// manually.
		hn.printErrf("found nil RPC clients")
		if err := hn.kill(); err != nil {
			return fmt.Errorf("killing process got: %v", err)
		}
	}

	// Close any attempts at further grpc connections.
	if hn.conn != nil {
		err := status.Code(hn.conn.Close())
		switch err {
		case codes.OK:
			return nil

		// When the context is canceled above, we might get the
		// following error as the context is no longer active.
		case codes.Canceled:
			return nil

		case codes.Unknown:
			return fmt.Errorf("unknown error attempting to stop "+
				"grpc client: %v", err)

		default:
			return fmt.Errorf("error attempting to stop "+
				"grpc client: %v", err)
		}
	}

	return nil
}

// Shutdown stops the active lnd process and cleans up any temporary
// directories created along the way.
func (hn *HarnessNode) Shutdown() error {
	if err := hn.Stop(); err != nil {
		return err
	}
	if err := hn.cleanup(); err != nil {
		return err
	}
	return nil
}

// kill kills the lnd process.
func (hn *HarnessNode) kill() error {
	return hn.cmd.Process.Kill()
}

// printErrf prints an error to the console.
func (hn *HarnessNode) printErrf(format string, a ...interface{}) {
	fmt.Printf("itest error from [%s:%s]: %s\n", // nolint:forbidigo
		hn.Cfg.LogFilenamePrefix, hn.Cfg.Name,
		fmt.Sprintf(format, a...))
}

func postgresDatabaseDsn(dbName string) string {
	return fmt.Sprintf(postgresDsn, dbName)
}

// createTempPgDb creates a temp postgres database.
func createTempPgDb() (string, error) {
	// Create random database name.
	randBytes := make([]byte, 8)
	_, err := rand.Read(randBytes)
	if err != nil {
		return "", err
	}
	dbName := "itest_" + hex.EncodeToString(randBytes)

	// Create database.
	err = executePgQuery("CREATE DATABASE " + dbName)
	if err != nil {
		return "", err
	}

	return dbName, nil
}

// executePgQuery executes a SQL statement in a postgres db.
func executePgQuery(query string) error {
	pool, err := pgxpool.Connect(
		context.Background(),
		postgresDatabaseDsn("postgres"),
	)
	if err != nil {
		return fmt.Errorf("unable to connect to database: %v", err)
	}
	defer pool.Close()

	_, err = pool.Exec(context.Background(), query)
	return err
}

// renameFile is a helper to rename (log) files created during integration
// tests.
func renameFile(fromFileName, toFileName string) {
	err := os.Rename(fromFileName, toFileName)
	if err != nil {
		fmt.Printf("could not rename %s to %s: %v\n", // nolint:forbidigo
			fromFileName, toFileName, err)
	}
}

// getFinalizedLogFilePrefix returns the finalize log filename.
func getFinalizedLogFilePrefix(hn *HarnessNode) string {
	pubKeyHex := hex.EncodeToString(
		hn.PubKey[:logPubKeyBytes],
	)

	return fmt.Sprintf("%s/%d-%s-%s-%s",
		lntest.GetLogDir(), hn.Cfg.NodeID,
		hn.Cfg.LogFilenamePrefix,
		hn.Cfg.Name, pubKeyHex)
}

// finalizeLogfile makes sure the log file cleanup function is initialized,
// even if no log file is created.
func finalizeLogfile(hn *HarnessNode) {
	// Exit early if there's no log file.
	if hn.logFile == nil {
		return
	}

	hn.logFile.Close()

	// If logoutput flag is not set, return early.
	if !*lntest.LogOutput {
		return
	}

	newFileName := fmt.Sprintf("%v.log",
		getFinalizedLogFilePrefix(hn),
	)
	renameFile(hn.filename, newFileName)
}

// finalizeEtcdLog saves the etcd log files when test ends.
func finalizeEtcdLog(hn *HarnessNode) {
	// Exit early if this is not etcd backend.
	if hn.Cfg.DbBackend != lntest.BackendEtcd {
		return
	}

	etcdLogFileName := fmt.Sprintf("%s/etcd.log", hn.Cfg.LogDir)
	newEtcdLogFileName := fmt.Sprintf("%v-etcd.log",
		getFinalizedLogFilePrefix(hn),
	)

	renameFile(etcdLogFileName, newEtcdLogFileName)
}

// addLogFile creates log files used by this node.
func addLogFile(hn *HarnessNode) error {
	var fileName string

	dir := lntest.GetLogDir()
	fileName = fmt.Sprintf("%s/%d-%s-%s-%s.log", dir, hn.Cfg.NodeID,
		hn.Cfg.LogFilenamePrefix, hn.Cfg.Name,
		hex.EncodeToString(hn.PubKey[:logPubKeyBytes]))

	// If the node's PubKey is not yet initialized, create a temporary file
	// name. Later, after the PubKey has been initialized, the file can be
	// moved to its final name with the PubKey included.
	if bytes.Equal(hn.PubKey[:4], []byte{0, 0, 0, 0}) {
		fileName = fmt.Sprintf("%s/%d-%s-%s-tmp__.log", dir,
			hn.Cfg.NodeID, hn.Cfg.LogFilenamePrefix,
			hn.Cfg.Name)
	}

	// Create file if not exists, otherwise append.
	file, err := os.OpenFile(fileName,
		os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
	if err != nil {
		return err
	}

	// Pass node's stderr to both errb and the file.
	w := io.MultiWriter(hn.cmd.Stderr, file)
	hn.cmd.Stderr = w

	// Pass the node's stdout only to the file.
	hn.cmd.Stdout = file

	// Let the node keep a reference to this file, such that we can add to
	// it if necessary.
	hn.logFile = file

	hn.filename = fileName

	return nil
}