Avoids calling Unix.readlink twice

In case of getting the list of files we called Unix.readlink twice for every file
descriptor.
In case we wanted just the count we computed the list anyway.
Factor out code to avoid calling function twice and do not compute the list
is not needed.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>

ocaml/forkexecd/test/fe_test.ml

@@ -75,18 +75,21 @@ let shuffle x =
   done ;
   Array.to_list arr
 
-let fd_list () =
+let fds_fold f init =
   let path = "/proc/self/fd" in
-  List.filter (* get rid of the fd used to read the directory *)
-    (fun x ->
+  (* get rid of the fd used to read the directory *)
+  Array.fold_right
+    (fun fd_num acc ->
       try
-        ignore (Unix.readlink (Filename.concat path x)) ;
-        true
-      with _ -> false
+        let link = Unix.readlink (Filename.concat path fd_num) in
+        f fd_num link acc
+      with _ -> acc
     )
-    (Array.to_list (Sys.readdir path))
+    (Sys.readdir path) init
 
-let fd_count () = List.length (fd_list ())
+let fd_list () = fds_fold (fun fd_num link l -> (fd_num, link) :: l) []
+
+let fd_count () = fds_fold (fun _ _ n -> n + 1) 0
 
 let irrelevant_strings = ["irrelevant"; "not"; "important"]
 
@@ -260,11 +263,7 @@ let slave = function
         List.filter (fun x -> not (List.mem x irrelevant_strings)) rest
       in
       (* Check that these fds are present *)
-      let path = "/proc/self/fd" in
-      let raw = fd_list () in
-      let pairs =
-        List.map (fun x -> (x, Unix.readlink (Filename.concat path x))) raw
-      in
+      let pairs = fd_list () in
       (* Filter any of stdin,stdout,stderr which have been mapped to /dev/null *)
       let filtered =
         List.filter