Distributed_db: handle cancelation of call to fetch.

We properly count the number of waiters and cleanup the table when the
count drops to zero.

src/node/shell/distributed_db_functors.ml

@@ -107,6 +107,7 @@ end = struct
 
   and status =
     | Pending of { wakener : value Lwt.u ;
+                   mutable waiters : int ;
                    param : param }
     | Found of value
 
@@ -149,6 +150,21 @@ end = struct
     | Found v -> return v
     | Pending _ -> fail (Missing_data k)
 
+  let wrap s k t =
+    let t = Lwt.protected t in
+    Lwt.on_cancel t begin fun () ->
+      match Memory_table.find s.memory k with
+      | exception Not_found -> ()
+      | Found _ -> ()
+      | Pending data ->
+          data.waiters <- data.waiters - 1 ;
+          if data.waiters = 0 then begin
+            Memory_table.remove s.memory k ;
+            Scheduler.notify_cancelation s.scheduler k ;
+          end
+    end ;
+    t
+
   let fetch s ?peer k param =
     match Memory_table.find s.memory k with
     | exception Not_found -> begin
@@ -158,18 +174,21 @@ end = struct
             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 }) ;
+                Memory_table.add s.memory k
+                  (Pending { wakener ; waiters =  1 ; param }) ;
                 Scheduler.request s.scheduler peer k ;
-                waiter
+                wrap s k waiter
               end
-            | Pending { wakener = w ; _ } ->
+            | Pending data ->
                 Scheduler.request s.scheduler peer k ;
-                Lwt.waiter_of_wakener w
+                data.waiters <- data.waiters + 1 ;
+                wrap s k (Lwt.waiter_of_wakener data.wakener)
             | Found v -> Lwt.return v
       end
-    | Pending { wakener = w ; _ } ->
+    | Pending data ->
         Scheduler.request s.scheduler peer k ;
-        Lwt.waiter_of_wakener w
+        data.waiters <- data.waiters + 1 ;
+        wrap s k (Lwt.waiter_of_wakener data.wakener)
     | Found v -> Lwt.return v
 
   let prefetch s ?peer k param = Lwt.ignore_result (fetch s ?peer k param)