tools/oxenstored: Reject invalid watch paths early

Watches on invalid paths were accepted, but they would never trigger.  The
client also got no notification that its watch is bad and would never trigger.

Found again by the structured fuzzer, due to an error on live update reload:
the invalid watch paths would get rejected during live update and the list of
watches would be different pre/post live update.

The testcase is watch on `//`, which is an invalid path.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
(cherry picked from commit dc8caf214f)

tools/ocaml/xenstored/connection.ml

@@ -158,18 +158,17 @@ let get_children_watches con path =
 let is_dom0 con =
 	Perms.Connection.is_dom0 (get_perm con)
 
-let add_watch con path token =
+let add_watch con (path, apath) token =
 	if !Quota.activate && !Define.maxwatch > 0 &&
 	   not (is_dom0 con) && con.nb_watches > !Define.maxwatch then
 		raise Quota.Limit_reached;
-	let apath = get_watch_path con path in
 	let l = get_watches con apath in
 	if List.exists (fun w -> w.token = token) l then
 		raise Define.Already_exist;
 	let watch = watch_create ~con ~token ~path in
 	Hashtbl.replace con.watches apath (watch :: l);
 	con.nb_watches <- con.nb_watches + 1;
-	apath, watch
+	watch
 
 let del_watch con path token =
 	let apath = get_watch_path con path in

tools/ocaml/xenstored/connections.ml

@@ -114,8 +114,10 @@ let key_of_path path =
 	"" :: Store.Path.to_string_list path
 
 let add_watch cons con path token =
-	let apath, watch = Connection.add_watch con path token in
+	let apath = Connection.get_watch_path con path in
+	(* fail on invalid paths early by calling key_of_str before adding watch *)
 	let key = key_of_str apath in
+	let watch = Connection.add_watch con (path, apath) token in
 	let watches =
  		if Trie.mem cons.watches key
  		then Trie.find cons.watches key