Add a format window_linked which is 1 if a window has been linked

multiple times, also remove the default space in window_flags and use a
conditional to add it in window-status-format (this means additional
flags can be added in the option without extra spaces). From Thomas Adam
with tweaks by me.

cmd-kill-window.c

@@ -49,20 +49,13 @@ cmd_kill_window_exec(struct cmd *self, struct cmd_q *cmdq)
 	struct winlink	*wl, *wl2, *wl3;
 	struct window	*w;
 	struct session	*s;
-	struct session_group	*sg;
-	u_int			 references;
 
 	if ((wl = cmd_find_window(cmdq, args_get(args, 't'), &s)) == NULL)
 		return (CMD_RETURN_ERROR);
 	w = wl->window;
 
 	if (self->entry == &cmd_unlink_window_entry) {
-		sg = session_group_find(s);
+		if (!args_has(self->args, 'k') && !session_is_linked(s, w)) {
-		if (sg != NULL)
-			references = session_group_count(sg);
-		else
-			references = 1;
-		if (!args_has(self->args, 'k') && w->references == references) {
 			cmdq_error(cmdq, "window only linked to one session");
 			return (CMD_RETURN_ERROR);
 		}

format.c

@@ -621,6 +621,7 @@ format_defaults_winlink(struct format_tree *ft, struct session *s,
 	    !!(wl->flags & WINLINK_SILENCE));
 	format_add(ft, "window_last_flag", "%d",
 	    !!(wl == TAILQ_FIRST(&s->lastw)));
+	format_add(ft, "window_linked", "%d", session_is_linked(s, wl->window));
 
 	free(flags);
 }

options-table.c

@@ -737,7 +737,7 @@ const struct options_table_entry window_options_table[] = {
 
 	{ .name = "window-status-current-format",
 	  .type = OPTIONS_TABLE_STRING,
-	  .default_str = "#I:#W#F"
+	  .default_str = "#I:#W#{?window_flags,#{window_flags}, }"
 	},
 
 	{ .name = "window-status-current-style",
@@ -753,7 +753,7 @@ const struct options_table_entry window_options_table[] = {
 
 	{ .name = "window-status-format",
 	  .type = OPTIONS_TABLE_STRING,
-	  .default_str = "#I:#W#F"
+	  .default_str = "#I:#W#{?window_flags,#{window_flags}, }"
 	},
 
 	{ .name = "window-status-last-attr",

session.c

@@ -337,6 +337,20 @@ session_has(struct session *s, struct window *w)
 	return (0);
 }
 
+/*
+ * Return 1 if a window is linked outside this session (not including session
+ * groups). The window must be in this session!
+ */
+int
+session_is_linked(struct session *s, struct window *w)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(s)) != NULL)
+		return (w->references != session_group_count(sg));
+	return (w->references != 1);
+}
+
 struct winlink *
 session_next_alert(struct winlink *wl)
 {

tmux.1

@@ -3345,6 +3345,7 @@ The following variables are available, where appropriate:
 .It Li "window_index" Ta "#I" Ta "Index of window"
 .It Li "window_last_flag" Ta "" Ta "1 if window is the last used"
 .It Li "window_layout" Ta "" Ta "Window layout description"
+.It Li "window_linked" Ta "" Ta "1 if window is linked across sessions"
 .It Li "window_name" Ta "#W" Ta "Name of window"
 .It Li "window_panes" Ta "" Ta "Number of panes in window"
 .It Li "window_silence_flag" Ta "" Ta "1 if window has silence alert"

tmux.h

@@ -2328,6 +2328,7 @@ struct winlink	*session_attach(struct session *, struct window *, int,
 		     char **);
 int		 session_detach(struct session *, struct winlink *);
 int		 session_has(struct session *, struct window *);
+int		 session_is_linked(struct session *, struct window *);
 int		 session_next(struct session *, int);
 int		 session_previous(struct session *, int);
 int		 session_select(struct session *, int);

window.c

@@ -643,7 +643,7 @@ window_destroy_panes(struct window *w)
 	}
 }
 
-/* Return list of printable window flag symbols. No flags is just a space. */
+/* Retuns the printable flags on a window, empty string if no flags set. */
 char *
 window_printable_flags(struct session *s, struct winlink *wl)
 {
@@ -663,8 +663,6 @@ window_printable_flags(struct session *s, struct winlink *wl)
 		flags[pos++] = '-';
 	if (wl->window->flags & WINDOW_ZOOMED)
 		flags[pos++] = 'Z';
-	if (pos == 0)
-		flags[pos++] = ' ';
 	flags[pos] = '\0';
 	return (xstrdup(flags));
 }