Incremental directory reading

[patricoferris]

When first providing readdir we shelved an incremental version. This PR starts the ball rolling on adding support for something like with_dir_entries. Opening early to get some feedback on how it should look, should Eio.Path.walk walk directory tree recursively or just a single directory, leaving it to the user to descend should they want to. We should perhaps have a type entry too.

I wrote a small benchmark:

open Eio.Std

let ( / ) = Eio.Path.( / )

let dirname i = Fmt.str "dir-%i" i
let filename i = Fmt.str "file-%i" i

let make_bench_dir ?(depth=1) ?(width=1000) dir =
  let rec loop dir depth =
    if depth <= 0 then ()
    else (
      Eio.Path.mkdir ~perm:0o700 (dir / dirname depth);
      Fiber.List.iter ~max_fibers:10 (fun fname ->
        Eio.Path.save ~create:(`If_missing 0o644) (dir / fname) "hello world"
      ) (List.init width filename);
      loop (dir / dirname depth) (depth - 1)
    )
  in
  loop dir depth

let read_dir_walk ~f dir =
  let rec aux acc path =
    match Eio.Path.kind ~follow:true path with
    | `Directory as k ->
       let acc2 = f acc (k, path) in
       let files = Eio.Path.read_dir path in
       List.fold_left (fun acc f -> aux acc (path / f)) acc2 files
    | k -> f acc (k, path)
    in
  let files = Eio.Path.read_dir dir in
  List.fold_left (fun acc f -> aux acc (dir / f)) [] files

let rec walk ~f acc dir =
  Eio.Path.walk dir @@ fun seq ->
  Seq.fold_left (fun acc -> function
    | (`Directory, path) ->
      let acc2 = f acc (`Directory, dir / path) in
      walk ~f acc2 (dir / path)
    | (kind, fname) -> f acc (kind, dir / fname)) acc seq

let time fn =
  let now = Mtime_clock.now_ns () in
  let v = fn () in
  let stop = Mtime_clock.now_ns () in 
  Eio.traceln "Took: %a" Fmt.uint64_ns_span (Int64.sub stop now);
  v

module Path = struct
  include Eio.Path
  type t = Eio.Fs.dir_ty Eio.Path.t 

  let compare a b = String.compare (Eio.Path.native_exn a) (Eio.Path.native_exn b)
end

module S = Set.Make (Path)

let () = 
  Eio_main.run @@ fun env ->
  let tmpdirf = Filename.temp_dir "eio-walk-" "-bench" in
  let tmpdir = env#fs / tmpdirf in
  make_bench_dir ~depth:15 ~width:20_000 tmpdir;
  let f acc (_, path) = path :: acc in
  let files2 = time (fun () -> walk ~f [] tmpdir) |> S.of_list in
  let files1 = time (fun () -> read_dir_walk ~f tmpdir) |> S.of_list in
  Eio.traceln "Files 1: %i\n" (S.cardinal files1);
  Eio.traceln "Files 2: %i\n" (S.cardinal files2);
  Eio.traceln "Same files? %b\n" (S.equal files1 files2);
  let _ : int = Sys.command (Fmt.str "rm -r %s" tmpdirf) in
  ()

It is far from perfect and we should take into account FS caching, but on my machine I'm getting the following.

For Eio_linux: 1.46s to 110ms
For Eio_posix: 2.02s to 1.58s

[avsm]

I like this a lot, but I'm just wondering about extending it to cover error handling and traversal strategies, as Bos does: https://erratique.ch/software/bos/doc/Bos/OS/Path/index.html#fold

With these optional args, you can control which files to skip and what to do with errors mid-traversal.