Check if the index is in use and fail before creating the child process,

rather than leaving a stray child on failure.

session.c

@@ -212,10 +212,16 @@ session_new(struct session *s,
     const char *name, const char *cmd, const char *cwd, int idx, char **cause)
 {
 	struct window	*w;
+	struct winlink	*wl;
 	struct environ	 env;
 	const char	*shell;
 	u_int		 hlimit;
 
+	if ((wl = winlink_add(&s->windows, idx)) == NULL) {
+		xasprintf(cause, "index in use: %d", idx);
+		return (NULL);
+	}
+
 	environ_init(&env);
 	environ_copy(&global_environ, &env);
 	environ_copy(&s->environ, &env);
@@ -229,15 +235,18 @@ session_new(struct session *s,
 	w = window_create(
 	    name, cmd, shell, cwd, &env, s->tio, s->sx, s->sy, hlimit, cause);
 	if (w == NULL) {
+		winlink_remove(&s->windows, wl);
 		environ_free(&env);
 		return (NULL);
 	}
+	winlink_set_window(wl, w);
 	environ_free(&env);
 
 	if (options_get_number(&s->options, "set-remain-on-exit"))
 		options_set_number(&w->options, "remain-on-exit", 1);
 
-	return (session_attach(s, w, idx, cause));
+	session_group_synchronize_from(s);
+	return (wl);
 }
 
 /* Attach a window to a session. */
@@ -246,8 +255,12 @@ session_attach(struct session *s, struct window *w, int idx, char **cause)
 {
 	struct winlink	*wl;
 
-	if ((wl = winlink_add(&s->windows, w, idx)) == NULL)
+	if ((wl = winlink_add(&s->windows, idx)) == NULL) {
 		xasprintf(cause, "index in use: %d", idx);
+		return (NULL);
+	}
+	winlink_set_window(wl, w);
+
 	session_group_synchronize_from(s);
 	return (wl);
 }
@@ -526,7 +539,8 @@ session_group_synchronize1(struct session *target, struct session *s)
 
 	/* Link all the windows from the target. */
 	RB_FOREACH(wl, winlinks, ww) {
-		wl2 = winlink_add(&s->windows, wl->window, wl->idx);
+		wl2 = winlink_add(&s->windows, wl->idx);
+		winlink_set_window(wl2, wl->window);
 		wl2->flags |= wl->flags & WINLINK_ALERTFLAGS;
 	}
 

tmux.h

@@ -1828,7 +1828,8 @@ struct winlink	*winlink_find_by_index(struct winlinks *, int);
 struct winlink	*winlink_find_by_window(struct winlinks *, struct window *);
 int		 winlink_next_index(struct winlinks *, int);
 u_int		 winlink_count(struct winlinks *);
-struct winlink	*winlink_add(struct winlinks *, struct window *, int);
+struct winlink	*winlink_add(struct winlinks *, int);
+void		 winlink_set_window(struct winlink *, struct window *);
 void		 winlink_remove(struct winlinks *, struct winlink *);
 struct winlink	*winlink_next(struct winlink *);
 struct winlink	*winlink_previous(struct winlink *);

window.c

@@ -123,7 +123,7 @@ winlink_count(struct winlinks *wwl)
 }
 
 struct winlink *
-winlink_add(struct winlinks *wwl, struct window *w, int idx)
+winlink_add(struct winlinks *wwl, int idx)
 {
 	struct winlink	*wl;
 
@@ -135,14 +135,18 @@ winlink_add(struct winlinks *wwl, struct window *w, int idx)
 
 	wl = xcalloc(1, sizeof *wl);
 	wl->idx = idx;
-	wl->window = w;
 	RB_INSERT(winlinks, wwl, wl);
 
-	w->references++;
-
 	return (wl);
 }
 
+void
+winlink_set_window(struct winlink *wl, struct window *w)
+{
+	wl->window = w;
+	w->references++;
+}
+
 void
 winlink_remove(struct winlinks *wwl, struct winlink *wl)
 {
@@ -153,12 +157,14 @@ winlink_remove(struct winlinks *wwl, struct winlink *wl)
 		xfree(wl->status_text);
 	xfree(wl);
 
+	if (w != NULL) {
 		if (w->references == 0)
 			fatal("bad reference count");
 		w->references--;
 		if (w->references == 0)
 			window_destroy(w);
 	}
+}
 
 struct winlink *
 winlink_next(struct winlink *wl)