[PATCH v4 2/4] Map: 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>
---
Changes since V3:
* this patch is new in V4
---
 tools/ocaml/xenstored/stdext.ml | 21 +++++++++++++++++++++
 tools/ocaml/xenstored/trie.ml   |  2 ++
 2 files changed, 23 insertions(+)

diff --git a/tools/ocaml/xenstored/stdext.ml b/tools/ocaml/xenstored/stdext.ml
index 4f2f3a2c8c..5bebe2aa27 100644
--- a/tools/ocaml/xenstored/stdext.ml
+++ b/tools/ocaml/xenstored/stdext.ml
@@ -44,6 +44,27 @@ let default d v =
 let maybe f v =
        match v with None -> () | Some x -> f x
 
+module Map = struct
+module Make(Ord: Map.OrderedType) = struct
+
+include Map.Make(Ord)
+
+let find_opt k t =
+       (* avoid raising exceptions, they can be expensive *)
+       if mem k t then Some (find k t) else 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 =  {
-- 
2.25.1