base: safeguard entrypoint against double manage

This patch adds sanity checks to the RPC entrypoint that detect attempts
to manage or dissolve the same RPC object twice. This is not always a
bug. I.e., if RPC objects are implemented in the modern way where the
object manages/dissolves itself. As the generic framework code (in
particular root/component.h) cannot rely on this pattern, it has to
call manage/dissolve for session objects anyway. For modern session
objects, this double attempt would result in a serious error (double
insertion into the object pool's AVL tree).

Issue #2398

repos/base/include/root/component.h

@@ -169,7 +169,12 @@ class Genode::Root_component : public Rpc_object<Typed_root<SESSION_TYPE> >,
 				throw Root::Unavailable();
 			}
 
-			_ep->manage(s);
+			/*
+			 * Consider that the session-object constructor may already have
+			 * called 'manage'.
+			 */
+			if (!s->cap().valid())
+				_ep->manage(s);
 
 			aquire_guard.ack = true;
 			return *s;