Add no-detach-on-destroy client option (useful for control mode

clients). From laur dot aliste at gmail dot com, GitHub issue 4242.

server-client.c

@@ -3683,6 +3683,8 @@ server_client_set_flags(struct client *c, const char *flags)
 			flag = CLIENT_IGNORESIZE;
 		else if (strcmp(next, "active-pane") == 0)
 			flag = CLIENT_ACTIVEPANE;
+		else if (strcmp(next, "no-detach-on-destroy") == 0)
+			flag = CLIENT_NO_DETACH_ON_DESTROY;
 		if (flag == 0)
 			continue;
 
@@ -3716,6 +3718,8 @@ server_client_get_flags(struct client *c)
 		strlcat(s, "control-mode,", sizeof s);
 	if (c->flags & CLIENT_IGNORESIZE)
 		strlcat(s, "ignore-size,", sizeof s);
+	if (c->flags & CLIENT_NO_DETACH_ON_DESTROY)
+		strlcat(s, "no-detach-on-destroy,", sizeof s);
 	if (c->flags & CLIENT_CONTROL_NOOUTPUT)
 		strlcat(s, "no-output,", sizeof s);
 	if (c->flags & CLIENT_CONTROL_WAITEXIT)

server-fn.c

@@ -425,7 +425,7 @@ void
 server_destroy_session(struct session *s)
 {
 	struct client	*c;
-	struct session	*s_new = NULL;
+	struct session	*s_new = NULL, *cs_new, *use_s;
 	int		 detach_on_destroy;
 
 	detach_on_destroy = options_get_number(s->options, "detach-on-destroy");
@@ -437,15 +437,26 @@ server_destroy_session(struct session *s)
 		s_new = session_previous_session(s);
 	else if (detach_on_destroy == 4)
 		s_new = session_next_session(s);
-	if (s_new == s)
-		s_new = NULL;
+
+	/*
+	 * If no suitable new session was found above, then look for any
+	 * session as an alternative in case a client needs it.
+	 */
+	if (s_new == NULL &&
+	    (detach_on_destroy == 1 || detach_on_destroy == 2))
+		cs_new = server_find_session(s, server_newer_session);
+
 	TAILQ_FOREACH(c, &clients, entry) {
 		if (c->session != s)
 			continue;
+		use_s = s_new;
+		if (use_s == NULL && (c->flags & CLIENT_NO_DETACH_ON_DESTROY))
+			use_s = cs_new;
+
 		c->session = NULL;
 		c->last_session = NULL;
-		server_client_set_session(c, s_new);
-		if (s_new == NULL)
+		server_client_set_session(c, use_s);
+		if (use_s == NULL)
 			c->flags |= CLIENT_EXIT;
 	}
 	recalculate_sizes();

tmux.1

@@ -1066,6 +1066,9 @@ The flags are:
 the client has an independent active pane
 .It ignore-size
 the client does not affect the size of other clients
+.It no-detach-on-destroy
+do not detach the client when the session it is attached to is destroyed if
+there are any other sessions
 .It no-output
 the client does not receive pane output in control mode
 .It pause-after=seconds

tmux.h

@@ -1924,6 +1924,7 @@ struct client {
 #define CLIENT_BRACKETPASTING 0x1000000000ULL
 #define CLIENT_ASSUMEPASTING 0x2000000000ULL
 #define CLIENT_REDRAWSCROLLBARS 0x4000000000ULL
+#define CLIENT_NO_DETACH_ON_DESTROY 0x8000000000ULL
 #define CLIENT_ALLREDRAWFLAGS		\
 	(CLIENT_REDRAWWINDOW|		\
 	 CLIENT_REDRAWSTATUS|		\