Add "grouped sessions" which have independent name, options, current window and

so on but where the linked windows are synchronized (ie creating, killing
windows and so on are mirrored between the sessions). A grouped session may be
created by passing -t to new-session.

Had this around for a while, tested by a couple of people.

cmd-break-pane.c

@@ -78,6 +78,7 @@ cmd_break_pane_exec(struct cmd *self, struct cmd_ctx *ctx)
  		session_select(s, wl->idx);
 
 	server_redraw_session(s);
+	server_status_session_group(s);
 
 	return (0);
 }

cmd-choose-session.c

@@ -54,7 +54,9 @@ cmd_choose_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 	struct cmd_choose_session_data	*cdata;
 	struct winlink			*wl;
 	struct session			*s;
+	struct session_group		*sg;
 	u_int			 	 i, idx, cur;
+	char				 tmp[64];
 
 	if (ctx->curclient == NULL) {
 		ctx->error(ctx, "must be run interactively");
@@ -76,10 +78,18 @@ cmd_choose_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 			cur = idx;
 		idx++;
 
+		sg = session_group_find(s);
+		if (sg == NULL)
+			*tmp = '\0';
+		else {
+			idx = session_group_index(sg);
+			xsnprintf(tmp, sizeof tmp, " (group %u)", idx);
+		}
+
 		window_choose_add(wl->window->active, i,
-		    "%s: %u windows [%ux%u]%s", s->name,
+		    "%s: %u windows [%ux%u]%s%s", s->name,
 		    winlink_count(&s->windows), s->sx, s->sy,
-		    s->flags & SESSION_UNATTACHED ? "" : " (attached)");
+		    tmp, s->flags & SESSION_UNATTACHED ? "" : " (attached)");
 	}
 
 	cdata = xmalloc(sizeof *cdata);

cmd-choose-window.c

@@ -89,7 +89,7 @@ cmd_choose_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 			flag = '+';
 		else if (wm == s->curw)
 			flag = '*';
-		else if (wm == SLIST_FIRST(&s->lastw))
+		else if (wm == TAILQ_FIRST(&s->lastw))
 			flag = '-';
 
 		title = w->active->screen->title;

cmd-kill-session.c

@@ -53,7 +53,7 @@ cmd_kill_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 
 	for (i = 0; i < ARRAY_LENGTH(&clients); i++) {
 		c = ARRAY_ITEM(&clients, i);
-		if (c->session == s) {
+		if (c != NULL && c->session == s) {
 			c->session = NULL;
 			server_write_client(c, MSG_EXIT, NULL, 0);
 		}

cmd-link-window.c

@@ -43,19 +43,19 @@ int
 cmd_link_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 {
 	struct cmd_srcdst_data	*data = self->data;
-	struct session		*dst;
+	struct session		*src, *dst;
 	struct winlink		*wl;
 	char			*cause;
 	int			 idx, kflag, dflag;
 
-	if ((wl = cmd_find_window(ctx, data->src, NULL)) == NULL)
+	if ((wl = cmd_find_window(ctx, data->src, &src)) == NULL)
 		return (-1);
 	if ((idx = cmd_find_index(ctx, data->dst, &dst)) == -2)
 		return (-1);
 
 	kflag = data->chflags & CMD_CHFLAG('k');
 	dflag = data->chflags & CMD_CHFLAG('d');
-	if (server_link_window(wl, dst, idx, kflag, !dflag, &cause) != 0) {
+	if (server_link_window(src, wl, dst, idx, kflag, !dflag, &cause) != 0) {
 		ctx->error(ctx, "can't link window: %s", cause);
 		xfree(cause);
 		return (-1);

cmd-list-sessions.c

@@ -42,23 +42,32 @@ const struct cmd_entry cmd_list_sessions_entry = {
 int
 cmd_list_sessions_exec(unused struct cmd *self, struct cmd_ctx *ctx)
 {
-	struct session	*s;
+	struct session		*s;
-	char		*tim;
+	struct session_group	*sg;
-	u_int		 i;
+	char			*tim, tmp[64];
-	time_t		 t;
+	u_int			 i, idx;
+	time_t			 t;
 
 	for (i = 0; i < ARRAY_LENGTH(&sessions); i++) {
 		s = ARRAY_ITEM(&sessions, i);
 		if (s == NULL)
 			continue;
 
+		sg = session_group_find(s);
+		if (sg == NULL)
+			*tmp = '\0';
+		else {
+			idx = session_group_index(sg);
+			xsnprintf(tmp, sizeof tmp, " (group %u)", idx);
+		}
+
 		t = s->tv.tv_sec;
 		tim = ctime(&t);
 		*strchr(tim, '\n') = '\0';
 
-		ctx->print(ctx, "%s: %u windows (created %s) [%ux%u]%s",
+		ctx->print(ctx, "%s: %u windows (created %s) [%ux%u]%s%s",
 		    s->name, winlink_count(&s->windows), tim, s->sx, s->sy,
-		    s->flags & SESSION_UNATTACHED ? "" : " (attached)");
+		    tmp, s->flags & SESSION_UNATTACHED ? "" : " (attached)");
 	}
 
 	return (0);

cmd-move-window.c

@@ -55,7 +55,7 @@ cmd_move_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 
 	kflag = data->chflags & CMD_CHFLAG('k');
 	dflag = data->chflags & CMD_CHFLAG('d');
-	if (server_link_window(wl, dst, idx, kflag, !dflag, &cause) != 0) {
+	if (server_link_window(src, wl, dst, idx, kflag, !dflag, &cause) != 0) {
 		ctx->error(ctx, "can't move window: %s", cause);
 		xfree(cause);
 		return (-1);

cmd-new-session.c

@@ -34,6 +34,7 @@ void	cmd_new_session_init(struct cmd *, int);
 size_t	cmd_new_session_print(struct cmd *, char *, size_t);
 
 struct cmd_new_session_data {
+	char	*target;
 	char	*newname;
 	char	*winname;
 	char	*cmd;
@@ -42,7 +43,7 @@ struct cmd_new_session_data {
 
 const struct cmd_entry cmd_new_session_entry = {
 	"new-session", "new",
-	"[-d] [-n window-name] [-s session-name] [command]",
+	"[-d] [-n window-name] [-s session-name] [-t target-session] [command]",
 	CMD_STARTSERVER|CMD_CANTNEST|CMD_SENDENVIRON, 0,
 	cmd_new_session_init,
 	cmd_new_session_parse,
@@ -58,6 +59,7 @@ cmd_new_session_init(struct cmd *self, unused int arg)
 
 	self->data = data = xmalloc(sizeof *data);
 	data->flag_detached = 0;
+	data->target = NULL;
 	data->newname = NULL;
 	data->winname = NULL;
 	data->cmd = NULL;
@@ -72,7 +74,7 @@ cmd_new_session_parse(struct cmd *self, int argc, char **argv, char **cause)
 	self->entry->init(self, KEYC_NONE);
 	data = self->data;
 
-	while ((opt = getopt(argc, argv, "ds:n:")) != -1) {
+	while ((opt = getopt(argc, argv, "ds:t:n:")) != -1) {
 		switch (opt) {
 		case 'd':
 			data->flag_detached = 1;
@@ -81,6 +83,10 @@ cmd_new_session_parse(struct cmd *self, int argc, char **argv, char **cause)
 			if (data->newname == NULL)
 				data->newname = xstrdup(optarg);
 			break;
+		case 't':
+			if (data->target == NULL)
+				data->target = xstrdup(optarg);
+			break;
 		case 'n':
 			if (data->winname == NULL)
 				data->winname = xstrdup(optarg);
@@ -94,6 +100,9 @@ cmd_new_session_parse(struct cmd *self, int argc, char **argv, char **cause)
 	if (argc != 0 && argc != 1)
 		goto usage;
 
+	if (data->target != NULL && (argc == 1 || data->winname != NULL))
+		goto usage;
+
 	if (argc == 1)
 		data->cmd = xstrdup(argv[0]);
 
@@ -110,7 +119,7 @@ int
 cmd_new_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 {
 	struct cmd_new_session_data	*data = self->data;
-	struct session			*s;
+	struct session			*s, *groupwith;
 	struct window			*w;
 	struct environ			 env;
 	struct termios			 tio, *tiop;
@@ -124,6 +133,11 @@ cmd_new_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 		return (-1);
 	}
 
+	groupwith = NULL;
+	if (data->target != NULL &&
+	    (groupwith = cmd_find_session(ctx, data->target)) == NULL)
+		return (-1);
+
 	/*
 	 * There are three cases:
 	 *
@@ -204,7 +218,9 @@ cmd_new_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 		sy = 1;
 
 	/* Figure out the command for the new window. */
-	if (data->cmd != NULL)
+	if (data->target != NULL)
+		cmd = NULL;
+	else if (data->cmd != NULL)
 		cmd = data->cmd;
 	else
 		cmd = options_get_string(&global_s_options, "default-command");
@@ -227,7 +243,7 @@ cmd_new_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 	environ_free(&env);
 
 	/* Set the initial window name if one given. */
-	if (data->winname != NULL) {
+	if (cmd != NULL && data->winname != NULL) {
 		w = s->curw->window;
 
 		xfree(w->name);
@@ -236,6 +252,16 @@ cmd_new_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 		options_set_number(&w->options, "automatic-rename", 0);
 	}
 
+	/*
+	 * If a target session is given, this is to be part of a session group,
+	 * so add it to the group and synchronize.
+	 */
+	if (groupwith != NULL) {
+		session_group_add(groupwith, s);
+		session_group_synchronize_to(s);
+		session_select(s, RB_ROOT(&s->windows)->idx);
+	}
+
 	/*
 	 * Set the client to the new session. If a command client exists, it is
 	 * taking this session and needs to get MSG_READY and stay around.

cmd-new-window.c

@@ -164,9 +164,9 @@ cmd_new_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 	}
 	if (!data->flag_detached) {
 		session_select(s, wl->idx);
-		server_redraw_session(s);
+		server_redraw_session_group(s);
-	} else
+	} else	
-		server_status_session(s);
+		server_status_session_group(s);
 
 	return (0);
 }

cmd-rename-window.c

@@ -53,7 +53,7 @@ cmd_rename_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 	wl->window->name = xstrdup(data->arg);
 	options_set_number(&wl->window->options, "automatic-rename", 0);
 
-	server_status_session(s);
+	server_status_window(wl->window);
 
 	return (0);
 }

cmd-swap-window.c

@@ -44,6 +44,7 @@ cmd_swap_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 {
 	struct cmd_srcdst_data	*data = self->data;
 	struct session		*src, *dst;
+	struct session_group	*sg_src, *sg_dst;
 	struct winlink		*wl_src, *wl_dst;
 	struct window		*w;
 
@@ -52,6 +53,14 @@ cmd_swap_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 	if ((wl_dst = cmd_find_window(ctx, data->dst, &dst)) == NULL)
 		return (-1);
 
+	sg_src = session_group_find(src);
+	sg_dst = session_group_find(dst);
+	if (src != dst &&
+	    sg_src != NULL && sg_dst != NULL && sg_src == sg_dst) {
+		ctx->error(ctx, "can't move window, sessions are grouped");
+		return (-1);
+	}
+
 	if (wl_dst->window == wl_src->window)
 		return (0);
 
@@ -64,9 +73,12 @@ cmd_swap_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 		if (src != dst)
 			session_select(src, wl_src->idx);
 	}
-	server_redraw_session(src);
+	session_group_synchronize_from(src);
-	if (src != dst)
+	server_redraw_session_group(src);
-		server_redraw_session(dst);
+	if (src != dst) {
+		session_group_synchronize_from(dst);
+		server_redraw_session_group(dst);
+	}
 	recalculate_sizes();
 
 	return (0);

cmd-unlink-window.c

@@ -42,16 +42,28 @@ cmd_unlink_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 {
 	struct cmd_target_data	*data = self->data;
 	struct winlink		*wl;
-	struct session		*s;
+	struct window		*w;
+	struct session		*s, *s2;
+	struct session_group	*sg;
+	u_int			 references;
 
 	if ((wl = cmd_find_window(ctx, data->target, &s)) == NULL)
 		return (-1);
+	w = wl->window;
 
-	if (!(data->chflags & CMD_CHFLAG('k')) && wl->window->references == 1) {
+	sg = session_group_find(s);
+	if (sg != NULL) {
+		references = 0;
+		TAILQ_FOREACH(s2, &sg->sessions, gentry)
+			references++;
+	} else
+		references = 1;
+
+	if (!(data->chflags & CMD_CHFLAG('k')) && w->references == references) {
 		ctx->error(ctx, "window is only linked to one session");
 		return (-1);
 	}
-
+	
 	server_unlink_window(s, wl);
 	recalculate_sizes();
 

server-fn.c

@@ -104,6 +104,19 @@ server_redraw_session(struct session *s)
 	}
 }
 
+void
+server_redraw_session_group(struct session *s)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(s)) == NULL)
+		server_redraw_session(s);
+	else {
+		TAILQ_FOREACH(s, &sg->sessions, gentry)
+			server_redraw_session(s);
+	}
+}
+
 void
 server_status_session(struct session *s)
 {
@@ -119,6 +132,19 @@ server_status_session(struct session *s)
 	}
 }
 
+void
+server_status_session_group(struct session *s)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(s)) == NULL)
+		server_status_session(s);
+	else {
+		TAILQ_FOREACH(s, &sg->sessions, gentry)
+			server_status_session(s);
+	}
+}
+
 void
 server_redraw_window(struct window *w)
 {
@@ -220,18 +246,27 @@ server_kill_window(struct window *w)
 			continue;
 		
 		if (session_detach(s, wl))
-			server_destroy_session(s);
+			server_destroy_session_group(s);
-		else
+		else {
 			server_redraw_session(s);
+			server_status_session_group(s);
+		}
 	}
 }
 
 int
-server_link_window(
+server_link_window(struct session *src, struct winlink *srcwl,
-    struct winlink *srcwl, struct session *dst, int dstidx,
+    struct session *dst, int dstidx, int killflag, int selectflag, char **cause)
-    int killflag, int selectflag, char **cause)
 {
-	struct winlink	*dstwl;
+	struct winlink		*dstwl;
+	struct session_group	*srcsg, *dstsg;
+
+	srcsg = session_group_find(src);
+	dstsg = session_group_find(dst);
+	if (src != dst && srcsg != NULL && dstsg != NULL && srcsg == dstsg) {
+		xasprintf(cause, "sessions are grouped");
+		return (-1);
+	}
 
 	dstwl = NULL;
 	if (dstidx != -1)
@@ -260,12 +295,9 @@ server_link_window(
 	if (dstwl == NULL)
 		return (-1);
 
-	if (!selectflag)
+	if (selectflag)
-		server_status_session(dst);
-	else {
 		session_select(dst, dstwl->idx);
-		server_redraw_session(dst);
+	server_redraw_session_group(dst);
-	}
 
 	return (0);
 }
@@ -274,9 +306,24 @@ void
 server_unlink_window(struct session *s, struct winlink *wl)
 {
 	if (session_detach(s, wl))
-		server_destroy_session(s);
+		server_destroy_session_group(s);
 	else
-		server_redraw_session(s);
+		server_redraw_session_group(s);
+}
+
+void
+server_destroy_session_group(struct session *s)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(s)) == NULL)
+		server_destroy_session(s);
+	else {
+		TAILQ_FOREACH(s, &sg->sessions, gentry)
+			server_destroy_session(s);
+		TAILQ_REMOVE(&session_groups, sg, entry);
+		xfree(sg);
+	}
 }
 
 void

server.c

@@ -247,6 +247,7 @@ server_start(char *path)
 	ARRAY_INIT(&dead_clients);
 	ARRAY_INIT(&sessions);
 	ARRAY_INIT(&dead_sessions);
+	TAILQ_INIT(&session_groups);
 	mode_key_init_trees();
 	key_bindings_init();
 	utf8_build();
@@ -1243,10 +1244,11 @@ server_check_window(struct window *w)
 			if (wl->window != w)
 				continue;
 			if (session_detach(s, wl)) {
-				server_destroy_session(s);
+				server_destroy_session_group(s);
 				break;
 			}
 			server_redraw_session(s);
+			server_status_session_group(s);
 			goto restart;
 		}
 	}

session.c

@@ -30,6 +30,7 @@
 /* Global session list. */
 struct sessions	sessions;
 struct sessions dead_sessions;
+struct session_groups session_groups;
 
 struct winlink *session_next_activity(struct session *, struct winlink *);
 struct winlink *session_previous_activity(struct session *, struct winlink *);
@@ -131,7 +132,7 @@ session_create(const char *name, const char *cmd, const char *cwd,
 		fatal("gettimeofday failed");
 
 	s->curw = NULL;
-	SLIST_INIT(&s->lastw);
+	TAILQ_INIT(&s->lastw);
 	RB_INIT(&s->windows);
 	SLIST_INIT(&s->alerts);
 
@@ -164,11 +165,14 @@ session_create(const char *name, const char *cmd, const char *cwd,
 		s->name = xstrdup(name);
 	else
 		xasprintf(&s->name, "%u", i);
-	if (session_new(s, NULL, cmd, cwd, idx, cause) == NULL) {
+	
-		session_destroy(s);
+	if (cmd != NULL) {
-		return (NULL);
+		if (session_new(s, NULL, cmd, cwd, idx, cause) == NULL) {
+			session_destroy(s);
+			return (NULL);
+		}
+		session_select(s, RB_ROOT(&s->windows)->idx);
 	}
-	session_select(s, RB_ROOT(&s->windows)->idx);
 
 	log_debug("session %s created", s->name);
 
@@ -192,13 +196,14 @@ session_destroy(struct session *s)
 	if (s->tio != NULL)
 		xfree(s->tio);
 
+	session_group_remove(s);
 	session_alert_cancel(s, NULL);
 	environ_free(&s->environ);
 	options_free(&s->options);
 	paste_free_stack(&s->buffers);
 
-	while (!SLIST_EMPTY(&s->lastw))
+	while (!TAILQ_EMPTY(&s->lastw))
-		winlink_stack_remove(&s->lastw, SLIST_FIRST(&s->lastw));
+		winlink_stack_remove(&s->lastw, TAILQ_FIRST(&s->lastw));
 	while (!RB_EMPTY(&s->windows))
 		winlink_remove(&s->windows, RB_ROOT(&s->windows));
 
@@ -268,6 +273,7 @@ session_attach(struct session *s, struct window *w, int idx, char **cause)
 
 	if ((wl = winlink_add(&s->windows, w, idx)) == NULL)
 		xasprintf(cause, "index in use: %d", idx);
+	session_group_synchronize_from(s);
 	return (wl);
 }
 
@@ -282,6 +288,7 @@ session_detach(struct session *s, struct winlink *wl)
 	session_alert_cancel(s, wl);
 	winlink_stack_remove(&s->lastw, wl);
 	winlink_remove(&s->windows, wl);
+	session_group_synchronize_from(s);
 	if (RB_EMPTY(&s->windows)) {
 		session_destroy(s);
 		return (1);
@@ -408,7 +415,7 @@ session_last(struct session *s)
 {
 	struct winlink	*wl;
 
-	wl = SLIST_FIRST(&s->lastw);
+	wl = TAILQ_FIRST(&s->lastw);
 	if (wl == NULL)
 		return (-1);
 	if (wl == s->curw)
@@ -420,3 +427,169 @@ session_last(struct session *s)
 	session_alert_cancel(s, wl);
 	return (0);
 }
+
+/* Find the session group containing a session. */
+struct session_group *
+session_group_find(struct session *target)
+{
+	struct session_group	*sg;
+	struct session		*s;
+
+	TAILQ_FOREACH(sg, &session_groups, entry) {
+		TAILQ_FOREACH(s, &sg->sessions, gentry) {
+			if (s == target)
+				return (sg);
+		}
+	}
+	return (NULL);
+}
+
+/* Find session group index. */
+u_int
+session_group_index(struct session_group *sg)
+{
+	struct session_group   *sg2;
+	u_int			i;
+
+	i = 0;
+	TAILQ_FOREACH(sg2, &session_groups, entry) {
+		if (sg == sg2)
+			return (i);
+		i++;
+	}
+
+	fatalx("session group not found");
+}
+
+/*
+ * Add a session to the session group containing target, creating it if
+ * necessary. 
+ */
+void
+session_group_add(struct session *target, struct session *s)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(target)) == NULL) {
+		sg = xmalloc(sizeof *sg);
+		TAILQ_INSERT_TAIL(&session_groups, sg, entry);
+		TAILQ_INIT(&sg->sessions);
+		TAILQ_INSERT_TAIL(&sg->sessions, target, gentry);
+	}
+	TAILQ_INSERT_TAIL(&sg->sessions, s, gentry);
+}
+
+/* Remove a session from its group and destroy the group if empty. */
+void
+session_group_remove(struct session *s)
+{
+	struct session_group	*sg;
+
+	if ((sg = session_group_find(s)) == NULL)
+		return;
+	TAILQ_REMOVE(&sg->sessions, s, gentry);
+	if (TAILQ_NEXT(TAILQ_FIRST(&sg->sessions), gentry) == NULL)
+		TAILQ_REMOVE(&sg->sessions, TAILQ_FIRST(&sg->sessions), gentry);
+	if (TAILQ_EMPTY(&sg->sessions)) {
+		TAILQ_REMOVE(&session_groups, sg, entry);
+		xfree(sg);
+	}
+}
+
+/* Synchronize a session to its session group. */
+void
+session_group_synchronize_to(struct session *s)
+{
+	struct session_group	*sg;
+	struct session		*target;
+
+	if ((sg = session_group_find(s)) == NULL)
+		return;
+
+	target = NULL;
+	TAILQ_FOREACH(target, &sg->sessions, gentry) {
+		if (target != s)
+			break;
+	}
+	session_group_synchronize1(target, s);
+}
+
+/* Synchronize a session group to a session. */
+void
+session_group_synchronize_from(struct session *target)
+{
+	struct session_group	*sg;
+	struct session		*s;
+
+	if ((sg = session_group_find(target)) == NULL)
+		return;
+
+	TAILQ_FOREACH(s, &sg->sessions, gentry) {
+		if (s != target)
+			session_group_synchronize1(target, s);
+	}
+}
+
+/*
+ * Synchronize a session with a target session. This means destroying all
+ * winlinks then recreating them, then updating the current window, last window
+ * stack and alerts.
+ */
+void
+session_group_synchronize1(struct session *target, struct session *s)
+{
+	struct winlinks		 old_windows, *ww;
+	struct winlink_stack	 old_lastw;
+	struct winlink		*wl, *wl2;
+	struct session_alert	*sa;
+
+	/* Don't do anything if the session is empty (it'll be destroyed). */
+	ww = &target->windows;
+	if (RB_EMPTY(ww))
+		return;
+
+	/* If the current window has vanished, move to the next now. */
+	if (s->curw != NULL) {
+		while (winlink_find_by_index(ww, s->curw->idx) == NULL)
+			session_next(s, 0);
+	}
+
+	/* Save the old pointer and reset it. */
+	memcpy(&old_windows, &s->windows, sizeof old_windows);
+	RB_INIT(&s->windows);
+
+	/* Link all the windows from the target. */
+	RB_FOREACH(wl, winlinks, ww)
+		winlink_add(&s->windows, wl->window, wl->idx);
+
+	/* Fix up the current window. */
+	if (s->curw != NULL)
+		s->curw = winlink_find_by_index(&s->windows, s->curw->idx);
+	else
+		s->curw = winlink_find_by_index(&s->windows, target->curw->idx);
+
+	/* Fix up the last window stack. */
+	memcpy(&old_lastw, &s->lastw, sizeof old_lastw);
+	TAILQ_INIT(&s->lastw);
+	TAILQ_FOREACH(wl, &old_lastw, sentry) {
+		wl2 = winlink_find_by_index(&s->windows, wl->idx);
+		if (wl2 != NULL)
+			TAILQ_INSERT_TAIL(&s->lastw, wl2, sentry);
+	}
+
+	/* And update the alerts list. */
+	SLIST_FOREACH(sa, &s->alerts, entry) {
+		wl = winlink_find_by_index(&s->windows, sa->wl->idx);
+		if (wl == NULL)
+			session_alert_cancel(s, sa->wl);
+		else
+			sa->wl = wl;
+	}
+
+	/* Then free the old winlinks list. */
+	while (!RB_EMPTY(&old_windows)) {
+		wl = RB_ROOT(&old_windows);
+		RB_REMOVE(winlinks, &old_windows, wl);
+		xfree(wl);
+	}
+}

status.c

@@ -516,7 +516,7 @@ status_print(struct session *s, struct winlink *wl, struct grid_cell *gc)
 		gc->attr = attr;
 
 	flag = ' ';
- 	if (wl == SLIST_FIRST(&s->lastw))
+ 	if (wl == TAILQ_FIRST(&s->lastw))
 		flag = '-';
 	if (wl == s->curw) {
 		fg = options_get_number(oo, "window-status-current-fg");

tmux.1

@@ -409,6 +409,7 @@ Lock all clients attached to
 .Op Fl d
 .Op Fl n Ar window-name
 .Op Fl s Ar session-name
+.Op Fl t Ar target-session
 .Op Ar command
 .Xc
 .D1 (alias: Ic new )
@@ -425,6 +426,26 @@ are the name of and command to execute in the initial window.
 If run from a terminal, any
 .Xr termios 4
 special characters are saved and used for new windows in the new session.
+.Pp
+If
+.Fl t
+is given, the new session is
+.Em grouped
+with
+.Ar target-session .
+This means they share the same set of windows - all windows from
+.Ar target-session
+are linked to the new session and any subsequent new windows or windows being
+closed are applied to both sessions.
+The current and previous window and any session options remain independent and
+either session may be killed without affecting the other.
+Giving
+.Fl n
+or
+.Ar command
+are invalid if
+.Fl t
+is used.
 .It Ic refresh-client Op Fl t Ar target-client
 .D1 (alias: Ic refresh )
 Refresh the current client if bound to a key, or a single client if one is given

tmux.h

@@ -739,10 +739,10 @@ struct winlink {
 	struct window	*window;
 
 	RB_ENTRY(winlink) entry;
-	SLIST_ENTRY(winlink) sentry;
+	TAILQ_ENTRY(winlink) sentry;
 };
 RB_HEAD(winlinks, winlink);
-SLIST_HEAD(winlink_stack, winlink);
+TAILQ_HEAD(winlink_stack, winlink);
 
 /* Layout direction. */
 enum layout_type {
@@ -797,6 +797,13 @@ struct session_alert {
 	SLIST_ENTRY(session_alert) entry;
 };
 
+struct session_group {
+	TAILQ_HEAD(, session) sessions;
+
+	TAILQ_ENTRY(session_group) entry;
+};
+TAILQ_HEAD(session_groups, session_group);
+
 struct session {
 	char		*name;
 	struct timeval	 tv;
@@ -824,6 +831,8 @@ struct session {
 	struct environ	 environ;
 
 	int		 references;
+
+	TAILQ_ENTRY(session) gentry;
 };
 ARRAY_DECL(sessions, struct session *);
 
@@ -1456,7 +1465,9 @@ void	 server_write_session(
 void	 server_redraw_client(struct client *);
 void	 server_status_client(struct client *);
 void	 server_redraw_session(struct session *);
+void	 server_redraw_session_group(struct session *);
 void	 server_status_session(struct session *);
+void	 server_status_session_group(struct session *);
 void	 server_redraw_window(struct window *);
 void	 server_status_window(struct window *);
 void	 server_lock(void);
@@ -1464,9 +1475,10 @@ void	 server_lock_session(struct session *);
 void	 server_lock_client(struct client *);
 int	 server_unlock(const char *);
 void	 server_kill_window(struct window *);
-int	 server_link_window(
+int	 server_link_window(struct session *,
 	     struct winlink *, struct session *, int, int, int, char **);
 void	 server_unlink_window(struct session *, struct winlink *);
+void	 server_destroy_session_group(struct session *);
 void	 server_destroy_session(struct session *);
 void	 server_set_identify(struct client *);
 void	 server_clear_identify(struct client *);
@@ -1719,6 +1731,7 @@ char 		*default_window_name(struct window *);
 /* session.c */
 extern struct sessions sessions;
 extern struct sessions dead_sessions;
+extern struct session_groups session_groups;
 void	 session_alert_add(struct session *, struct window *, int);
 void	 session_alert_cancel(struct session *, struct winlink *);
 int	 session_alert_has(struct session *, struct winlink *, int);
@@ -1739,6 +1752,13 @@ int		 session_next(struct session *, int);
 int		 session_previous(struct session *, int);
 int		 session_select(struct session *, int);
 int		 session_last(struct session *);
+struct session_group *session_group_find(struct session *);
+u_int		 session_group_index(struct session_group *);
+void		 session_group_add(struct session *, struct session *);
+void		 session_group_remove(struct session *);
+void		 session_group_synchronize_to(struct session *);
+void		 session_group_synchronize_from(struct session *);
+void		 session_group_synchronize1(struct session *, struct session *);
 
 /* utf8.c */
 void	utf8_build(void);

window.c

@@ -174,7 +174,7 @@ winlink_stack_push(struct winlink_stack *stack, struct winlink *wl)
 		return;
 
 	winlink_stack_remove(stack, wl);
-	SLIST_INSERT_HEAD(stack, wl, sentry);
+	TAILQ_INSERT_HEAD(stack, wl, sentry);
 }
 
 void
@@ -184,10 +184,10 @@ winlink_stack_remove(struct winlink_stack *stack, struct winlink *wl)
 
 	if (wl == NULL)
 		return;
-
+	
-	SLIST_FOREACH(wl2, stack, sentry) {
+	TAILQ_FOREACH(wl2, stack, sentry) {
 		if (wl2 == wl) {
-			SLIST_REMOVE(stack, wl, winlink, sentry);
+			TAILQ_REMOVE(stack, wl, sentry);
 			return;
 		}
 	}