base: handle cancelation of session creation

This patch improves the handling of the corner case where a client
vanishes while a session request is in flight (CREATE_REQUESTED but
not yet AVAILABLE). This corner case could be sporadically observed with
the init_loop test on base-linux.

In the original version, the session would eventually be delivered but
never picked up by anyone. Such a stale session still uses resources that
should better be released. In the new version, the parent checks for the
liveliness of the client whenever a session is delivered. If there is no
client of the session, a close request is immediately issued to the
server. The session state must be preserved until the close requests has
been answered.

repos/base/include/base/session_state.h

@@ -236,6 +236,8 @@ class Genode::Session_state : public Parent::Client, public Parent::Server
 		 */
 		Session::Label client_label() const { return label_from_args(_args.string()); }
 
+		bool client_exists() const { return _id_at_client.constructed(); }
+
 		/**
 		 * Return label presented to the server along with the session request
 		 */

repos/base/src/lib/base/child.cc

@@ -644,6 +644,20 @@ void Child::deliver_session_cap(Server::Id id, Session_capability cap)
 				return;
 			}
 
+			/*
+			 * If the client vanished during the session creation, the
+			 * session-close state change must be reflected to the server
+			 * as soon as the session becomes available. This enables the
+			 * server to wind down the session. If we just discarded the
+			 * session, the server's ID space would become inconsistent
+			 * with ours.
+			 */
+			if (!session.client_exists()) {
+				session.phase = Session_state::CLOSE_REQUESTED;
+				session.service().initiate_request(session);
+				return;
+			}
+
 			session.cap   = cap;
 			session.phase = Session_state::AVAILABLE;