Dji Tello Halt does not terminate all the related goroutines and may …

…wait forever when it is called multiple times

Halt method waits forever when at least one of the goroutines is blocked by its Read method. To avoid this, I make Halt method close the connections before writing to doneCh.

platforms/dji/tello/driver.go

@@ -334,18 +334,23 @@ func (d *Driver) Start() error {
 // Halt stops the driver.
 func (d *Driver) Halt() (err error) {
 	// send a landing command when we disconnect, and give it 500ms to be received before we shutdown
-	d.Land()
-	readerCount := atomic.LoadInt32(&d.doneChReaderCount)
-	for i := 0; i < int(readerCount); i++ {
-		d.doneCh <- struct{}{}
+	if d.cmdConn != nil {
+		d.Land()
 	}
-
 	time.Sleep(500 * time.Millisecond)
 
-	d.cmdConn.Close()
+	if d.cmdConn != nil {
+		d.cmdConn.Close()
+	}
+
 	if d.videoConn != nil {
 		d.videoConn.Close()
 	}
+	readerCount := atomic.LoadInt32(&d.doneChReaderCount)
+	for i := 0; i < int(readerCount); i++ {
+		d.doneCh <- struct{}{}
+	}
+
 	return
 }