Avoid having to scan the list just to get the number of items

Signed-off-by: Frediano Ziglio <[email protected]>

Author: freddy77

ocaml/forkexecd/test/fe_test.ml

@@ -79,13 +79,17 @@ let fd_list () =
   let path = "/proc/self/fd" in
   (* get rid of the fd used to read the directory *)
   Array.fold_right
-    (fun fd_num l ->
-      try (fd_num, Unix.readlink (Filename.concat path fd_num)) :: l
-      with _ -> l
+    (fun fd_num (l, n) ->
+      try
+        let pair = (fd_num, Unix.readlink (Filename.concat path fd_num)) in
+        (pair :: l, n + 1)
+      with _ -> (l, n)
     )
-    (Sys.readdir path) []
+    (Sys.readdir path) ([], 0)
 
-let fd_count () = List.length (fd_list ())
+let fd_count () =
+  let _, n = fd_list () in
+  n
 
 let irrelevant_strings = ["irrelevant"; "not"; "important"]
 
@@ -259,7 +263,7 @@ let slave = function
         List.filter (fun x -> not (List.mem x irrelevant_strings)) rest
       in
       (* Check that these fds are present *)
-      let pairs = fd_list () in
+      let pairs, _ = fd_list () in
       (* Filter any of stdin,stdout,stderr which have been mapped to /dev/null *)
       let filtered =
         List.filter