xapi-idl: Refactor out find_index and add it to Listext

The function is implemented using foldleft using a weird form, use a recursive
form instead.

Unfortunately the function was introduced in OCaml 5.1, so it had to be moved
to Listext so it can be reused.

Signed-off-by: Pau Ruiz Safont <[email protected]>

ocaml/libs/xapi-stdext/lib/xapi-stdext-std/listext.ml

@@ -208,4 +208,15 @@ module List = struct
   let find_minimum compare =
     let min a b = if compare a b <= 0 then a else b in
     function [] -> None | x :: xs -> Some (List.fold_left min x xs)
+
+  let find_index f l =
+    let rec loop i = function
+      | [] ->
+          None
+      | x :: _ when f x ->
+          Some i
+      | _ :: xs ->
+          loop (i + 1) xs
+    in
+    loop 0 l
 end

ocaml/libs/xapi-stdext/lib/xapi-stdext-std/listext.mli

@@ -60,6 +60,13 @@ module List : sig
       the sort order of [cmp], or [None] if the list is empty. When two ore
       more elements match the lowest value, the left-most is returned. *)
 
+  val find_index : ('a -> bool) -> 'a list -> int option
+  (** [find_index f l] returns the position of the first element in [l] that
+      satisfies [f x]. If there is no such element, returns [None].
+
+      When using OCaml compilers 5.1 or later, please use the standard library
+      instead. *)
+
   (** {1 Using indices to manipulate lists} *)
 
   val chop : int -> 'a list -> 'a list * 'a list

ocaml/xapi-idl/xen/device_number.ml

@@ -1,3 +1,5 @@
+module Listext = Xapi_stdext_std.Listext.List
+
 type bus_type = Xen | Scsi | Floppy | Ide [@@deriving rpcty]
 
 type t = bus_type * int * int [@@deriving rpcty]
@@ -76,11 +78,11 @@ let of_xenstore_int x =
         (Floppy, (x lsr 4) land ((1 lsl 4) - 1), x land ((1 lsl 4) - 1))
     | n ->
         let idx =
-          snd
-            (List.fold_left
-               (fun (i, res) e -> (i + 1, if e = n then i else res))
-               (0, -1) deprecated_ide_table
-            )
+          match Listext.find_index (Int.equal n) deprecated_ide_table with
+          | Some idx ->
+              idx
+          | None ->
+              failwith (Printf.sprintf "Unknown device number: %d" x)
         in
         let disk = ((x lsr 6) land ((1 lsl 2) - 1)) + (idx * 2) in
         let partition = x land ((1 lsl 6) - 1) in

ocaml/xapi-idl/xen/dune

@@ -11,6 +11,7 @@
    sexplib0
    threads
    xapi-idl
+   xapi-stdext-std
  )
  (wrapped false)
  (preprocess (pps ppx_deriving_rpc ppx_sexp_conv)))