terminal: Explain FreeBSD current_vt behavior

common/terminal.c

@@ -138,6 +138,31 @@ int terminal_current_vt(int fd) {
 		log_errorf("could not retrieve VT state: %s", strerror(errno));
 		return -1;
 	}
+
+	// The FreeBSD VT_GETACTIVE is implemented in the kernel as follows:
+	//
+	//	static int
+	//	vtterm_ioctl(struct terminal *tm, u_long cmd, caddr_t data,
+	// 		struct thread *td)
+	//	{
+	//		struct vt_window *vw = tm->tm_softc;
+	//		struct vt_device *vd = vw->vw_device;
+	//		...
+	//		switch (cmd) {
+	//		...
+	//		case VT_GETACTIVE:
+	//			*(int *)data = vd->vd_curwindow->vw_number + 1;
+	//			return (0);
+	//		...
+	//		}
+	//		...
+	//	}
+	//
+	// The side-effect here being that the returned VT number is one
+	// greater than the internal VT number, which is what is used for e.g.
+	// numbering the associated VT. To simplify things, we subtract one
+	// from the returned VT number before returning it.
+
 	if (vt < 1) {
 		log_errorf("invalid vt: %d", vt);
 		return -1;