[PATCH v4 2/4] tools/ocaml/xenstored: backport find_opt/update from 4.06

[Edwin Török <edvin.torok@xxxxxxxxxx>]

We are currently on OCaml 4.02 as minimum version.
To make the followup optimizations compile backport these functions from
OCaml 4.06.

This implementation is less efficient than the one in the 4.06 standard
library which has access to the internals of the Map.

Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
---
Changed since V3:
* repost after XSA to avoid conflicts
---
 tools/ocaml/xenstored/stdext.ml | 19 +++++++++++++++++++
 tools/ocaml/xenstored/trie.ml   |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/tools/ocaml/xenstored/stdext.ml b/tools/ocaml/xenstored/stdext.ml
index e1567c4dfa..0640602449 100644
--- a/tools/ocaml/xenstored/stdext.ml
+++ b/tools/ocaml/xenstored/stdext.ml
@@ -50,6 +50,25 @@ module Filename = struct
                cmd :: args |> List.map quote |> String.concat " "
 end
 
+module Map = struct
+module Make(Ord: Map.OrderedType) = struct
+
+include Map.Make(Ord)
+
+let find_opt k t = try Some (find k t) with Not_found -> None
+
+let update k f t =
+  let r = find_opt k t in
+  let r' = f r in
+  match r, r' with
+  | None, None -> t
+  | Some _, None -> remove k t
+  | Some r, Some r' when r == r' -> t
+  | _, Some r' -> add k r' t
+
+end
+end
+
 module String = struct include String
 
 let of_char c = String.make 1 c
diff --git a/tools/ocaml/xenstored/trie.ml b/tools/ocaml/xenstored/trie.ml
index dc42535092..f513f4e608 100644
--- a/tools/ocaml/xenstored/trie.ml
+++ b/tools/ocaml/xenstored/trie.ml
@@ -13,6 +13,8 @@
  * GNU Lesser General Public License for more details.
  *)
 
+open Stdext
+
 module Node =
 struct
        type ('a,'b) t =  {
-- 
2.29.2