Similarly, for sessions use a callback to free rather than checking

every loop.

notify.c

@@ -123,7 +123,7 @@ notify_drain(void)
 		if (ne->client != NULL)
 			server_client_unref(ne->client);
 		if (ne->session != NULL)
-			ne->session->references--;
+			session_unref(ne->session);
 		if (ne->window != NULL)
 			window_remove_ref(ne->window);
 

server.c

@@ -58,7 +58,6 @@ int	server_create_socket(void);
 void	server_loop(void);
 int	server_should_shutdown(void);
 void	server_send_shutdown(void);
-void	server_clean_dead(void);
 void	server_accept_callback(int, short, void *);
 void	server_signal_callback(int, short, void *);
 void	server_child_signal(void);
@@ -204,7 +203,6 @@ server_start(int lockfd, char *lockfile)
 	RB_INIT(&all_window_panes);
 	TAILQ_INIT(&clients);
 	RB_INIT(&sessions);
-	RB_INIT(&dead_sessions);
 	TAILQ_INIT(&session_groups);
 	mode_key_init_trees();
 	key_bindings_init();
@@ -264,8 +262,6 @@ server_loop(void)
 
 		server_window_loop();
 		server_client_loop();
-
-		server_clean_dead();
 	}
 }
 
@@ -317,21 +313,6 @@ server_send_shutdown(void)
 		session_destroy(s);
 }
 
-/* Free dead, unreferenced clients and sessions. */
-void
-server_clean_dead(void)
-{
-	struct session	*s, *s1;
-
-	RB_FOREACH_SAFE(s, sessions, &dead_sessions, s1) {
-		if (s->references != 0)
-			continue;
-		RB_REMOVE(sessions, &dead_sessions, s);
-		free(s->name);
-		free(s);
-	}
-}
-
 /* Update socket execute permissions based on whether sessions are attached. */
 void
 server_update_socket(void)

session.c

@@ -27,12 +27,12 @@
 
 #include "tmux.h"
 
-/* Global session list. */
 struct sessions	sessions;
-struct sessions dead_sessions;
 u_int		next_session_id;
 struct session_groups session_groups;
 
+void	session_free(int, short, void *);
+
 struct winlink *session_next_alert(struct winlink *);
 struct winlink *session_previous_alert(struct winlink *);
 
@@ -109,7 +109,7 @@ session_create(const char *name, int argc, char **argv, const char *path,
 	struct winlink	*wl;
 
 	s = xmalloc(sizeof *s);
-	s->references = 0;
+	s->references = 1;
 	s->flags = 0;
 
 	if (gettimeofday(&s->creation_time, NULL) != 0)
@@ -164,6 +164,29 @@ session_create(const char *name, int argc, char **argv, const char *path,
 	return (s);
 }
 
+/* Remove a reference from a session. */
+void
+session_unref(struct session *s)
+{
+	log_debug("session %s has %d references", s->name, s->references);
+
+	s->references--;
+	if (s->references == 0)
+		event_once(-1, EV_TIMEOUT, session_free, s, NULL);
+}
+
+/* Free session. */
+void
+session_free(unused int fd, unused short events, void *arg)
+{
+	struct session	*s = arg;
+
+	log_debug("sesson %s freed (%d references)", s->name, s->references);
+
+	if (s->references == 0)
+		free(s);
+}
+
 /* Destroy a session. */
 void
 session_destroy(struct session *s)
@@ -191,7 +214,7 @@ session_destroy(struct session *s)
 
 	close(s->cwd);
 
-	RB_INSERT(sessions, &dead_sessions, s);
+	session_unref(s);
 }
 
 /* Check a session name is valid: not empty and no colons or periods. */

tmux.h

@@ -2259,7 +2259,6 @@ void	control_notify_session_close(struct session *);
 
 /* session.c */
 extern struct sessions sessions;
-extern struct sessions dead_sessions;
 extern struct session_groups session_groups;
 int	session_cmp(struct session *, struct session *);
 RB_PROTOTYPE(sessions, session, entry, session_cmp);
@@ -2271,6 +2270,7 @@ struct session	*session_create(const char *, int, char **, const char *,
 		     int, struct environ *, struct termios *, int, u_int,
 		     u_int, char **);
 void		 session_destroy(struct session *);
+void		 session_unref(struct session *);
 int		 session_check_name(const char *);
 void		 session_update_activity(struct session *);
 struct session	*session_next_session(struct session *);

window-choose.c

@@ -209,11 +209,11 @@ window_choose_data_create(int type, struct client *c, struct session *s)
 void
 window_choose_data_free(struct window_choose_data *wcd)
 {
-	wcd->start_client->references--;
+	server_client_unref(wcd->start_client);
-	wcd->start_session->references--;
+	session_unref(wcd->start_session);
 
 	if (wcd->tree_session != NULL)
-		wcd->tree_session->references--;
+		session_unref(wcd->tree_session);
 
 	free(wcd->ft_template);
 	format_free(wcd->ft);