Fix kissattach 'Device or resource busy'

Sometimes kissattach had an issue using the pseudo terminal on some versions
of Linux: 'Error setting line discipline: TIOCSETD: Device or resource busy'.

This fix resolves the issue by not reading from the pty's master fd, until
kissattach has opened and configured the slave. This is implemented using
select() to wait for data before reading from the master fd.

Author: martinhpedersen

kiss.c

@@ -884,25 +884,28 @@ static int kiss_get (/* MYFDTYPE fd*/ void )
 #else		/* Linux/Cygwin version */
 
 	int n = 0;
+	fd_set fd_in;
+	int rc;
 
 	while ( n == 0 ) {
+	  /* Reading from master fd of the pty before the client has connected leads to trouble with kissattach. */
+	  /* Use select to check if the slave has sent any data before trying to read from it. */
+	  FD_ZERO(&fd_in);
+	  rc = select(pt_master_fd + 1, &fd_in, NULL, &fd_in, NULL);
 
-	  n = read(pt_master_fd, &ch, (size_t)1);
+	  if (rc == 0)
+	  {
+	    continue;
+	  }
 
-	  if (n != 1) {
+	  if (rc == MYFDERROR
+	      || (n = read(pt_master_fd, &ch, (size_t)1)) != 1)
+	  {
 
 	    text_color_set(DW_COLOR_ERROR);
 	    dw_printf ("\nError receiving kiss message from client application.  Closing %s.\n\n", pt_slave_name);
 	    perror ("");
 
-	    /* Message added between 1.1 beta test and final version 1.1 */
-
-	    /* TODO: Determine root cause and find proper solution. */
-
-	    dw_printf ("This is a known problem that sometimes shows up when using with kissattach.\n");
-	    dw_printf ("There are a couple work-arounds described in the Dire Wolf User Guide\n");
-	    dw_printf ("and the Raspberry Pi APRS documents.\n");
-
 	    close (pt_master_fd);
 
 	    pt_master_fd = MYFDERROR;