prevent nil cmd panic in stop if cmd never started, with a started chan

dcrdtest/node.go

@@ -177,14 +177,15 @@ func (n *nodeConfig) String() string {
 type node struct {
 	config *nodeConfig
 
-	cmd     *exec.Cmd
-	cmdDone chan error // Sent error and closed after cmdErr is assigned
-	cmdErr  error      // Must only be read after receiving on cmdDone
-	pidFile string
-	stderr  io.ReadCloser
-	stdout  io.ReadCloser
-	wg      sync.WaitGroup
-	pid     int
+	cmd        *exec.Cmd
+	cmdStarted chan struct{} // Closed after cmd is stared and assigned
+	cmdDone    chan error    // Sent error and closed after cmdErr is assigned
+	cmdErr     error         // Must only be read after receiving on cmdDone
+	pidFile    string
+	stderr     io.ReadCloser
+	stdout     io.ReadCloser
+	wg         sync.WaitGroup
+	pid        int
 
 	// Locally bound addresses for the subsystems.
 	p2pAddr string
@@ -204,9 +205,10 @@ func (n *node) logf(format string, args ...interface{}) {
 // as the base for the log and data directories for dcrd.
 func newNode(config *nodeConfig, dataDir string) (*node, error) {
 	return &node{
-		config:  config,
-		dataDir: dataDir,
-		cmdDone: make(chan error, 1),
+		config:     config,
+		dataDir:    dataDir,
+		cmdStarted: make(chan struct{}),
+		cmdDone:    make(chan error, 1),
 	}, nil
 }
 
@@ -312,6 +314,7 @@ func (n *node) start(ctx context.Context) error {
 		return fmt.Errorf("%w: %v", errDcrdCmdExec, err)
 	}
 	n.cmd = cmd
+	close(n.cmdStarted)
 	n.pid = n.cmd.Process.Pid
 
 	// Unblock pipes now that pid is available.
@@ -365,6 +368,11 @@ func (n *node) stop() error {
 	log.Tracef("stop %p", n.cmd)
 	defer log.Tracef("stop done")
 
+	if _, ok := <-n.cmdStarted; !ok {
+		// has not been started (yet, or ever)
+		return nil
+	}
+
 	select {
 	case <-n.cmdDone:
 		// already stopped
@@ -398,8 +406,7 @@ func (n *node) stop() error {
 	// Wait for command to exit.
 	log.Tracef("stop cmd.Wait")
 	<-n.cmdDone
-	err = n.cmdErr
-	if err != nil {
+	if err := n.cmdErr; err != nil {
 		log.Debugf("stop cmd.Wait error: %v", err)
 	}