Perform account hashing in finalize_hashes call

Defer computation of account hashes to the moment the hashes of the mask
will actually be accessed.

This is useful if the same account gets overwritten a few times in the
same block.

src/lib/merkle_mask/masking_merkle_tree.ml

@@ -81,13 +81,13 @@ module Make (Inputs : Inputs_intf.S) = struct
   
       Manual definitions of sexp are needed to avoid warning 4 in autogenerated code
     *)
-  type unhashed_account_t = Hash.t * Location.t
+  type unhashed_account_t = Account.t option * Location.t
 
   let sexp_of_unhashed_account_t =
-    Tuple2.sexp_of_t Hash.sexp_of_t Location.sexp_of_t
+    Tuple2.sexp_of_t (Option.sexp_of_t Account.sexp_of_t) Location.sexp_of_t
 
   let unhashed_account_t_of_sexp =
-    Tuple2.t_of_sexp Hash.t_of_sexp Location.t_of_sexp
+    Tuple2.t_of_sexp (Option.t_of_sexp Account.t_of_sexp) Location.t_of_sexp
 
   type t =
     { uuid : Uuid.Stable.V1.t
@@ -291,7 +291,10 @@ module Make (Inputs : Inputs_intf.S) = struct
       Fn.compose snd @@ List.fold_map ~init ~f
 
     let compute_merge_hashes :
-           (Hash.t * Addr.t * [ `Left of Hash.t | `Right of Hash.t ] list) list
+           ( Account.t option
+           * Addr.t
+           * [ `Left of Hash.t | `Right of Hash.t ] list )
+           list
         -> (Addr.t * Hash.t) list =
       let process_pair height = function
         | (lh, laddr, `Left _ :: lpath), (rh, _, `Right _ :: _rpath) ->
@@ -340,7 +343,10 @@ module Make (Inputs : Inputs_intf.S) = struct
         | _ ->
             impl acc' (height + 1) (converge height task)
       in
-      impl [] 0
+      let hash_account =
+        Option.value_map ~default:Hash.empty_account ~f:Hash.hash_account
+      in
+      Fn.compose (impl [] 0) (List.map ~f:(Tuple3.map_fst ~f:hash_account))
 
     let finalize_hashes_do t unhashed_accounts =
       let merkle_path_batch =
@@ -355,7 +361,7 @@ module Make (Inputs : Inputs_intf.S) = struct
       in
       (* let _task = *)
       List.map2_exn
-        ~f:(fun (h, loc) p -> (h, Location.to_path_exn loc, p))
+        ~f:(fun (a, loc) p -> (a, Location.to_path_exn loc, p))
         unhashed_accounts merkle_path_batch
       |> List.stable_sort ~compare:(fun (_, a, _) (_, b, _) ->
              Addr.compare a b )
@@ -608,9 +614,7 @@ module Make (Inputs : Inputs_intf.S) = struct
                 t.current_location <- Some prev_loc
             | None ->
                 t.current_location <- None ) ;
-      (* update hashes *)
-      let account_hash = Hash.empty_account in
-      t.unhashed_accounts <- (account_hash, location) :: t.unhashed_accounts
+      t.unhashed_accounts <- (None, location) :: t.unhashed_accounts
 
     let set_account_unsafe t location account =
       assert_is_attached t ;
@@ -626,9 +630,7 @@ module Make (Inputs : Inputs_intf.S) = struct
     let set t location account =
       assert_is_attached t ;
       set_account_unsafe t location account ;
-      (* Update merkle path. *)
-      let account_hash = Hash.hash_account account in
-      t.unhashed_accounts <- (account_hash, location) :: t.unhashed_accounts
+      t.unhashed_accounts <- (Some account, location) :: t.unhashed_accounts
 
     (* if the mask's parent sets an account, we can prune an entry in the mask
        if the account in the parent is the same in the mask