Distributed_db: fix concurency issue

Concurent request of the same ressource might insert multiple 'Pending
request' in the request tracking table. Resulting, only one of them
will ever be satisfied and some worker might be stuck for ever. We
avoid this be removing any cooperation between lookup and insertion in
the table.

src/node/shell/distributed_db_functors.ml

@@ -151,14 +151,23 @@ end = struct
     match Memory_table.find s.memory k with
     | exception Not_found -> begin
         Disk_table.read_opt s.disk k >>= function
+        | Some v -> Lwt.return v
         | None ->
+            match Memory_table.find s.memory k with
+            | exception Not_found -> begin
                 let waiter, wakener = Lwt.wait () in
                 Memory_table.add s.memory k (Pending (wakener, param)) ;
                 Scheduler.request s.scheduler peer k ;
                 waiter
-        | Some v -> Lwt.return v
               end
-    | Pending (w, _) -> Lwt.waiter_of_wakener w
+            | Pending (w, _) ->
+                Scheduler.request s.scheduler peer k ;
+                Lwt.waiter_of_wakener w
+            | Found v -> Lwt.return v
+      end
+    | Pending (w, _) ->
+        Scheduler.request s.scheduler peer k ;
+        Lwt.waiter_of_wakener w
     | Found v -> Lwt.return v
 
   let prefetch s ?peer k param = Lwt.ignore_result (fetch s ?peer k param)