Dial: fix goroutine leak on cancelled context

The goroutine performing the banner message exchange would block on the
error channel before returning, but the error channel was not drained if
the context is cancelled before the error is sent to the error channel.

Author: problame

Diff for: dial.go

@@ -186,6 +186,7 @@ func Dial(dialCtx context.Context, endpoint Endpoint) (*SSHConn , error) {
 
 	confErrChan := make(chan error)
 	go func() {
+		defer close(confErrChan)
 		var buf bytes.Buffer
 		if _, err := io.CopyN(&buf, cmd, int64(len(banner_msg))); err != nil {
 			confErrChan <- &SSHError{err, "read banner"}
@@ -208,13 +209,22 @@ func Dial(dialCtx context.Context, endpoint Endpoint) (*SSHConn , error) {
 			confErrChan <- &SSHError{err, "send begin message"}
 			return
 		}
-		close(confErrChan)
 	}()
 
 	select {
 	case <-dialCtx.Done():
+
 		commandCancel()
+		// cancelling will make one of the calls in above goroutine fail,
+		// and the goroutine will send the error to confErrChan
+		//
+		// ignore the error and return the cancellation cause
+
+		// draining always terminates because we know the channel is always closed
+		for _ = range confErrChan {}
+
 		return nil, dialCtx.Err()
+
 	case err := <-confErrChan:
 		if err != nil {
 			commandCancel()