open Lwt.Infix

let argument_error = 64

module K = struct
  open Cmdliner

  let check =
    let doc = Arg.info ~doc:"Only check the cache" ["check"] in
    Arg.(value & flag doc)
  
  let verify_sha256 =
    let doc = Arg.info
      ~doc:"Verify the SHA256 checksums of the cache contents, and \
            re-build the other checksum caches."
      ["verify-sha256"]
    in
    Arg.(value & flag doc)
  
  let remote =
    let doc = Arg.info
      ~doc:"Remote repository url, use suffix #foo to specify a branch 'foo': \
            https://github.com/ocaml/opam-repository.git"
      ["remote"]
    in
    Arg.(value & opt string "https://github.com/ocaml/opam-repository.git#master" doc)
  
  let parallel_downloads =
    let doc = Arg.info
        ~doc:"Amount of parallel HTTP downloads"
        ["parallel-downloads"]
    in
    Arg.(value & opt int 20 doc)
  
  let hook_url =
    let doc = Arg.info
        ~doc:"URL to conduct an update of the git repository" ["hook-url"]
    in
    Arg.(value & opt string "update" doc)
  
  let port =
    let doc = Arg.info ~doc:"HTTP listen port." ["port"] in
    Arg.(value & opt int 80 doc)
  
  let sectors_cache =
    let doc = "Number of sectors reserved for each checksum cache (md5, sha512)." in
    let doc = Arg.info ~doc ["sectors-cache"] in
    Arg.(value & opt int64 Int64.(mul 4L 2048L) doc)
  
  let sectors_git =
    let doc = "Number of sectors reserved for git dump." in
    let doc = Arg.info ~doc ["sectors-git"] in
    Arg.(value & opt int64 Int64.(mul 40L (mul 2L 1024L)) doc)
  
  let ignore_local_git =
    let doc = "Ignore restoring locally saved git repository." in
    let doc = Arg.info ~doc ["ignore-local-git"] in
    Arg.(value & flag doc)

  type t =
    { check : bool
    ; verify_sha256 : bool
    ; remote : string
    ; parallel_downloads : int
    ; hook_url : string
    ; port : int
    ; sectors_cache : int64
    ; sectors_git : int64
    ; ignore_local_git : bool }

  let v check verify_sha256 remote parallel_downloads hook_url port
    sectors_cache sectors_git ignore_local_git =
    { check; verify_sha256; remote; parallel_downloads; hook_url; port
    ; sectors_cache; sectors_git; ignore_local_git }

  let setup =
    Term.(const v
          $ check
          $ verify_sha256
          $ remote
          $ parallel_downloads
          $ hook_url
          $ port
          $ sectors_cache
          $ sectors_git
          $ ignore_local_git)
end

module Make
  (BLOCK : Mirage_block.S)
  (Time : Mirage_time.S)
  (Pclock : Mirage_clock.PCLOCK)
  (Stack : Tcpip.Stack.V4V6)
  (_ : sig end)
  (HTTP : Http_mirage_client.S) = struct

  module Part = Mirage_block_partition.Make(BLOCK)
  module KV = Tar_mirage.Make_KV_RW(Pclock)(Part)
  module Cache = OneFFS.Make(Part)
  module Store = Git_kv.Make(Pclock)

  module SM = Map.Make(String)
  module SSet = Set.Make(String)

  let compare_hash = Archive_checksum.Hash.compare

  module HM = Archive_checksum.HM

  let hash_to_string = Archive_checksum.Hash.to_string

  let hash_of_string = Archive_checksum.Hash.of_string

  let hex_to_key h = Mirage_kv.Key.v (Ohex.encode h)

  let hex_of_string s =
    match Ohex.decode s with
    | d -> Ok d
    | exception Invalid_argument err -> Error (`Msg err)

  let hm_to_s hm =
    HM.fold (fun h v acc ->
        hash_to_string h ^ "=" ^ Ohex.encode v ^ "\n" ^ acc)
      hm ""

  module Git = struct
    let find_contents store =
      let rec go store path acc =
        Store.list store path >>= function
        | Error e ->
          Logs.err (fun m -> m "error %a while listing %a"
                       Store.pp_error e Mirage_kv.Key.pp path);
          Lwt.return acc
        | Ok steps ->
          Lwt_list.fold_left_s (fun acc (step, _) ->
              Store.exists store step >>= function
              | Error e ->
                Logs.err (fun m -> m "error %a for exists %a" Store.pp_error e
                             Mirage_kv.Key.pp step);
                Lwt.return acc
              | Ok None ->
                Logs.warn (fun m -> m "no typ for %a" Mirage_kv.Key.pp step);
                Lwt.return acc
              | Ok Some `Value -> Lwt.return (step :: acc)
              | Ok Some `Dictionary -> go store step acc) acc steps
      in
      go store Mirage_kv.Key.empty []

    let decode_digest filename str =
      let hex h s =
        match hex_of_string s with
        | Ok d -> Some (h, d)
        | Error `Msg msg ->
          Logs.warn (fun m -> m "%s invalid hex (%s) %s" filename msg s); None
      in
      match String.split_on_char '=' str with
      | [ data ] -> hex `MD5 data
      | [ "md5" ; data ] -> hex `MD5 data
      | [ "sha256" ; data ] -> hex `SHA256 data
      | [ "sha512" ; data ] -> hex `SHA512 data
      | [ hash ; _ ] -> Logs.warn (fun m -> m "%s unknown hash %s" filename hash); None
      | _ -> Logs.warn (fun m -> m "%s unexpected hash format %S" filename str); None

    let extract_urls filename str =
      (* in an opam file, there may be:
         url { src: <string> checksum: [ STRING ] } <- list of hash
         url { src: <string> checksum: STRING } <- single hash
         url { archive: <string> checksum: STRING } <- MD5
      *)
      let open OpamParserTypes.FullPos in
      let opamfile = OpamParser.FullPos.string str filename in
      let unavailable =
        List.exists
          (function
            | { pelem = Variable ({ pelem = "available" ; _ },
                                  { pelem = (Bool false | List { pelem = [{ pelem = Bool false; _ }] ; _ }); _ })
              ; _ } -> true
            | _ -> false)
          opamfile.file_contents
      in
      if unavailable then
        (Logs.info (fun m -> m "%s is marked unavailable, skipping" filename);
         None)
      else
      let url_section =
        List.find_opt (function
            | { pelem = Section ({ section_kind = { pelem = "url" ; _ } ; _ }) ; _} -> true | _ -> false)
          opamfile.file_contents
      in
      match url_section with
      | Some { pelem = Section ({ section_items = { pelem = items ; _ }; _}) ; _ } ->
        begin
          let url =
            List.find_opt
              (function { pelem = Variable ({ pelem = "src" ; _ }, _); _ } -> true | _ -> false)
              items
          and archive =
            List.find_opt
              (function { pelem = Variable ({ pelem = "archive" ; _ }, _); _ } -> true | _ -> false)
              items
          and checksum =
            List.find_opt
              (function { pelem = Variable ({ pelem = "checksum" ; _ }, _); _ } -> true | _ -> false)
              items
          in
          let url =
            match url, archive with
            | Some { pelem = Variable (_, { pelem = String url ; _ }) ; _ }, None -> Some url
            | None, Some { pelem = Variable (_, { pelem = String url ; _ }); _ } -> Some url
            | _ ->
              Logs.warn (fun m -> m "%s neither src nor archive present" filename); None
          in
          let csum =
            match checksum with
            | Some { pelem = Variable (_, { pelem = List { pelem = csums ; _ } ; _ }); _ } ->
              let csums =
                List.fold_left (fun acc ->
                    function
                    | { pelem = String csum ; _ } ->
                      begin match decode_digest filename csum with
                        | None -> acc
                        | Some (h, v) ->
                          HM.update h (function
                              | None -> Some v
                              | Some v' when String.equal v v' -> None
                              | Some v' ->
                                Logs.warn (fun m -> m "for %s, hash %s, multiple keys are present: %s %s"
                                              (Option.value ~default:"NONE" url) (hash_to_string h) (Ohex.encode v) (Ohex.encode v'));
                                None)
                            acc
                      end
                    | _ -> acc) HM.empty csums
              in
              Some csums
            | Some { pelem = Variable (_, { pelem = String csum ; _ }) ; _ } ->
              begin match decode_digest filename csum with
                | None -> None
                | Some (h, v) -> Some (HM.singleton h v)
              end
            | _ ->
              Logs.warn (fun m -> m "couldn't decode checksum in %s" filename);
              None
          in
          match url, csum with
          | Some url, Some cs -> Some (url, cs)
          | _ -> None
        end
      | _ -> Logs.debug (fun m -> m "no url section for %s" filename); None

    let find_urls store =
      find_contents store >>= fun paths ->
      let opam_paths =
        List.filter (fun p -> Mirage_kv.Key.basename p = "opam") paths
      in
      Lwt_list.fold_left_s (fun acc path ->
          Store.get store path >|= function
          | Ok data ->
            (* TODO report parser errors *)
            (try
               let url_csums = extract_urls (Mirage_kv.Key.to_string path) data in
               Option.fold ~none:acc ~some:(fun (url, csums) ->
                   if HM.cardinal csums = 0 then
                     (Logs.warn (fun m -> m "no checksums for %s, ignoring" url); acc)
                   else
                     SM.update url (function
                         | None -> Some csums
                         | Some csums' ->
                           if HM.for_all (fun h v ->
                               match HM.find_opt h csums with
                               | None -> true | Some v' -> String.equal v v')
                               csums'
                           then
                             Some (HM.union (fun _h v _v' -> Some v) csums csums')
                           else begin
                             Logs.warn (fun m -> m "mismatching hashes for %s: %s vs %s"
                                           url (hm_to_s csums') (hm_to_s csums));
                             None
                           end) acc) url_csums
             with _ ->
               Logs.warn (fun m -> m "some error in %a, ignoring" Mirage_kv.Key.pp path);
               acc)
          | Error e -> Logs.warn (fun m -> m "Store.get: %a" Store.pp_error e); acc)
        SM.empty opam_paths
  end

  module Disk = struct
    type t = {
      mutable md5s : string SM.t ;
      mutable sha512s : string SM.t ;
      dev : KV.t ;
      dev_md5s : Cache.t ;
      dev_sha512s : Cache.t ;
    }

    let pending = Mirage_kv.Key.v "pending"

    let to_delete = Mirage_kv.Key.v "to-delete"

    let empty dev dev_md5s dev_sha512s = { md5s = SM.empty ; sha512s = SM.empty ; dev; dev_md5s; dev_sha512s }


    let marshal_sm (sm : string SM.t) =
      let version = char_of_int 1 in
      String.make 1 version ^ Marshal.to_string sm []

    let unmarshal_sm s =
      let version = int_of_char s.[0] in
      match version with
      | 1 -> Ok (Marshal.from_string s 1 : string SM.t)
      | _ -> Error ("Unsupported version " ^ string_of_int version)

    let update_caches t =
      Cache.write t.dev_md5s (marshal_sm t.md5s) >>= fun r ->
      (match r with
       | Ok () -> Logs.info (fun m -> m "Set 'md5s'")
       | Error e -> Logs.warn (fun m -> m "Failed to write 'md5s': %a" Cache.pp_write_error e));
      Cache.write t.dev_sha512s (marshal_sm t.sha512s) >>= fun r ->
      match r with
      | Ok () -> Logs.info (fun m -> m "Set 'sha512s'"); Lwt.return_unit
      | Error e ->
        Logs.warn (fun m -> m "Failed to write 'sha512s': %a" Cache.pp_write_error e);
        Lwt.return_unit

    let find_key t h key =
      assert (List.length (Mirage_kv.Key.segments key) = 1);
      match
        match h with
        | `MD5 ->
          Option.map Mirage_kv.Key.v (SM.find_opt (Mirage_kv.Key.basename key) t.md5s)
        | `SHA512 ->
          Option.map Mirage_kv.Key.v (SM.find_opt (Mirage_kv.Key.basename key) t.sha512s)
        | `SHA256 -> Some key
        | _ -> None
      with
      | None -> Error `Not_found
      | Some x -> Ok x

    let read_chunked t h v f a =
      match find_key t h v with
      | Error `Not_found ->
        Lwt.return (Error (`Not_found v))
      | Ok key ->
        KV.size t.dev key >>= function
        | Error e ->
          Logs.err (fun m -> m "error %a while reading %s %a"
                       KV.pp_error e (hash_to_string h) Mirage_kv.Key.pp v);
          Lwt.return (Error (`Not_found key))
        | Ok len ->
          let chunk_size = 4096 in
          let rec read_more a offset =
            if offset < len then
              KV.get_partial t.dev key ~offset ~length:chunk_size >>= function
              | Ok data ->
                f a data >>= fun a ->
                read_more a Optint.Int63.(add offset (of_int chunk_size))
              | Error e ->
                Logs.err (fun m -> m "error %a while reading %s %a"
                             KV.pp_error e (hash_to_string h) Mirage_kv.Key.pp v);
                Lwt.return (Error e)
            else
              Lwt.return (Ok a)
          in
          read_more a Optint.Int63.zero

    (*
    module HM_running = struct
      
      let empty h =
        let module H = (val Mirage_crypto.Hash.module_of h) in
        (* We need MD5, SHA256 and SHA512. [h] is likely one of the
           aforementioned and in that case we don't compute the same hash twice
        *)
        HM.empty
        |> HM.add `MD5 Mirage_crypto.Hash.MD5.empty
        |> HM.add `SHA256 Mirage_crypto.Hash.SHA256.empty
        |> HM.add `SHA512 Mirage_crypto.Hash.SHA512.empty
        |> HM.add h H.empty

      let feed t data =
        HM.map (fun h v ->
            let module H = (val Mirage_crypto.Hash.module_of h) in
            H.feed v data)
          t

      let get =
        HM.map (fun h v ->
            let module H = (val Mirage_crypto.Hash.module_of h) in
            H.get v)


    end
       *)

    let content_length_of_string s =
      match Int64.of_string s with
      | len when len >= 0L -> `Fixed len
      | _ | exception _ -> `Bad_response

    let body_length headers =
      match H2.Headers.get_multi headers "content-length" with
      | [] -> `Unknown
      | [ x ] ->  content_length_of_string x
      | hd :: tl ->
        (* if there are multiple content-length headers we require them all to be
         * exactly equal. *)
        if List.for_all (String.equal hd) tl then
          content_length_of_string hd
        else
          `Bad_response

    let body_length (response : Http_mirage_client.response) =
      if response.status <> `OK then
        `Bad_response
      else
        body_length response.headers

    let pending_key (hash, csum) =
      match hash with
      | `SHA512 ->
        (* We can't use hex because the filename would become too long for tar *)
        Mirage_kv.Key.(pending / hash_to_string hash / Base64.encode_string ~alphabet:Base64.uri_safe_alphabet ~pad:false csum)
      | _ ->
        Mirage_kv.Key.(pending / hash_to_string hash / Ohex.encode csum)

    let to_delete_key (hash, csum) =
      let rand = "random" in (* FIXME: generate random string *)
      let encoded_csum =
        match hash with
        | `SHA512 ->
          (* We can't use hex because the filename would become too long for tar *)
          Base64.encode_string ~alphabet:Base64.uri_safe_alphabet ~pad:false csum
        | _ ->
          Ohex.encode csum
      in
      Mirage_kv.Key.(to_delete / hash_to_string hash / (encoded_csum ^ "." ^ rand))

    let write_partial t (hash, csum) =
      (* XXX: we may be in trouble if different hash functions are used for the same archive *)
      let key = pending_key (hash, csum) in
      let ( >>>= ) = Lwt_result.bind in
      fun response r data ->
        Lwt.return r >>>= fun (digests, acc) ->
        let digests = Archive_checksum.update_digests digests data in
        match acc with
        | `Init ->
          begin match body_length response with
          | `Bad_response -> Lwt.return (Error `Bad_response)
          | `Fixed size ->
            KV.allocate t.dev key (Optint.Int63.of_int64 size)
            |> Lwt_result.map_error (fun e -> `Write_error e)
            >>>= fun () ->
            KV.set_partial t.dev key ~offset:Optint.Int63.zero data
            |> Lwt_result.map_error (fun e -> `Write_error e) >>>= fun () ->
            let len = String.length data in
            let offset = Optint.Int63.of_int len in
            Lwt.return_ok (digests, `Fixed_body (size, offset))
          | `Unknown ->
            Lwt.return_ok (digests, `Unknown data)
          end
        | `Fixed_body (size, offset) ->
          KV.set_partial t.dev key ~offset data
          |> Lwt_result.map_error (fun e -> `Write_error e) >>>= fun () ->
          let len = String.length data in
          let offset = Optint.Int63.(add offset (of_int len)) in
          Lwt.return_ok (digests, `Fixed_body (size, offset))
        | `Unknown body ->
          Lwt.return_ok (digests, `Unknown (body ^ data))

    let check_csums_digests csums digests =
      let csums' = Archive_checksum.digests_to_hm digests in
      let common_bindings = List.filter (fun (h, _) -> HM.mem h csums) (HM.bindings csums') in
      List.length common_bindings > 0 &&
      List.for_all
        (fun (h, csum) -> String.equal csum (HM.find h csums))
        common_bindings

    let finalize_write t (hash, csum) ~url (body : [ `Unknown of string | `Fixed_body of int64 * Optint.Int63.t | `Init ]) csums digests =
      let sizes_match, body_size_in_header =
        match body with
        | `Fixed_body (reported, actual) -> Optint.Int63.(equal (of_int64 reported) actual), true
        | `Unknown _ -> true, false
        | `Init -> assert false
      in
      let source = pending_key (hash, csum) in
      if check_csums_digests csums digests && sizes_match then
        let sha256 = Ohex.encode Digestif.SHA256.(to_raw_string (get digests.sha256))
        and md5 = Ohex.encode Digestif.MD5.(to_raw_string (get digests.md5))
        and sha512 = Ohex.encode Digestif.SHA512.(to_raw_string (get digests.sha512)) in
        let dest = Mirage_kv.Key.v sha256 in
        begin match body with
        | `Unknown body ->
          Logs.info (fun m -> m "downloaded %s, now writing" url);
          KV.set t.dev dest body
        | `Fixed_body (_reported_size, _actual_size) ->
          Logs.info (fun m -> m "downloaded %s" url);
          KV.rename t.dev ~source ~dest
        | `Init -> assert false
        end >|= function
        | Ok () ->
          t.md5s <- SM.add md5 sha256 t.md5s;
          t.sha512s <- SM.add sha512 sha256 t.sha512s
        | Error e ->
          Logs.err (fun m -> m "Write failure for %s: %a" url KV.pp_write_error e)
      else begin
        (if sizes_match then
           Logs.err (fun m -> m "Bad checksum %s: computed %s expected %s" url
                        (hash_to_string hash) (Ohex.encode csum))
         else match body with
           | `Fixed_body (reported, actual) ->
             Logs.err (fun m -> m "Size mismatch %s: received %a bytes expected %Lu bytes"
                          url Optint.Int63.pp actual reported)
           | `Unknown _ -> assert false
           | `Init -> assert false);
        if body_size_in_header then
          (* if the checksums mismatch we want to delete the file. We are only
             able to do so if it was the latest created file, so we expect and
             error. Ideally, we want to match for `Append_only or other errors *)
          KV.remove t.dev source >>= function
          | Ok () -> Lwt.return_unit
          | Error e ->
            (* we failed to delete the file so we mark it for deletion *)
            let dest = to_delete_key (hash, csum) in
            Logs.warn (fun m -> m "Failed to remove %a: %a. Moving it to %a"
                          Mirage_kv.Key.pp source KV.pp_write_error e Mirage_kv.Key.pp dest);
            KV.rename t.dev ~source ~dest >|= function
            | Ok () -> ()
            | Error e ->
              Logs.warn (fun m -> m "Error renaming file %a -> %a: %a"
                            Mirage_kv.Key.pp source Mirage_kv.Key.pp dest KV.pp_write_error e)
        else
          Lwt.return_unit
      end


    (* on disk, we use a flat file system where the filename is the sha256 of the data *)
    let init ~verify_sha256 dev dev_md5s dev_sha512s =
      KV.list dev Mirage_kv.Key.empty >>= function
      | Error e -> Logs.err (fun m -> m "error %a listing kv" KV.pp_error e); assert false
      | Ok entries ->
        let t = empty dev dev_md5s dev_sha512s in
        Cache.read t.dev_md5s >>= fun r ->
        (match r with
         | Ok Some s ->
           if not verify_sha256 then
             Result.iter (fun md5s -> t.md5s <- md5s) (unmarshal_sm s)
         | Ok None -> Logs.debug (fun m -> m "No md5s cached")
         | Error e -> Logs.warn (fun m -> m "Error reading md5s cache: %a" Cache.pp_error e));
        Cache.read t.dev_sha512s >>= fun r ->
        (match r with
         | Ok Some s ->
           if not verify_sha256 then
             Result.iter (fun sha512s -> t.sha512s <- sha512s) (unmarshal_sm s)
         | Ok None -> Logs.debug (fun m -> m "No sha512s cached")
         | Error e -> Logs.warn (fun m -> m "Error reading sha512s cache: %a" Cache.pp_error e));
        let md5s = SSet.of_list (List.map snd (SM.bindings t.md5s))
        and sha512s = SSet.of_list (List.map snd (SM.bindings t.sha512s)) in
        let idx = ref 1 in
        (* XXX: should we do something about pending downloads?? *)
        let entries =
          List.filter (fun (p, _) ->
              not (Mirage_kv.Key.equal p pending || Mirage_kv.Key.equal p to_delete))
            entries
        in
        Lwt_list.iter_s (fun (path, typ) ->
            if !idx mod 10 = 0 then Gc.full_major () ;
            match typ with
            | `Dictionary ->
              Logs.warn (fun m -> m "unexpected dictionary at %a" Mirage_kv.Key.pp path);
              Lwt.return_unit
            | `Value ->
              let open Digestif in
              let sha256_final =
                if verify_sha256 then
                  let f s =
                    let digest = SHA256.(to_raw_string (get s)) in
                    if not (String.equal (Mirage_kv.Key.basename path) (Ohex.encode digest)) then
                      Logs.err (fun m -> m "corrupt SHA256 data for %a, \
                                            computed %s (should remove)"
                                   Mirage_kv.Key.pp path (Ohex.encode digest))
                  in
                  Some f
                else
                  None
              and md5_final =
                if not (SSet.mem (Mirage_kv.Key.basename path) md5s) then
                  let f s =
                    let digest = MD5.(to_raw_string (get s)) in
                    t.md5s <- SM.add (Ohex.encode digest) (Mirage_kv.Key.basename path) t.md5s
                  in
                  Some f
                else
                  None
              and sha512_final =
                if not (SSet.mem (Mirage_kv.Key.basename path) sha512s) then
                  let f s =
                    let digest = SHA512.(to_raw_string (get s)) in
                    t.sha512s <- SM.add (Ohex.encode digest) (Mirage_kv.Key.basename path) t.sha512s
                  in
                  Some f
                else
                  None
              in
              match sha256_final, md5_final, sha512_final with
              | None, None, None -> Lwt.return_unit
              | _ ->
                read_chunked t `SHA256 path
                  (fun (sha256, md5, sha512) data ->
                     Lwt.return
                       (Option.map (fun t -> SHA256.feed_string t data) sha256,
                        Option.map (fun t -> MD5.feed_string t data) md5,
                        Option.map (fun t -> SHA512.feed_string t data) sha512))
                  (Option.map (fun _ -> SHA256.empty) sha256_final,
                   Option.map (fun _ -> MD5.empty) md5_final,
                   Option.map (fun _ -> SHA512.empty) sha512_final) >|= function
                | Error e ->
                  Logs.err (fun m -> m "error %a of %a while computing digests"
                               KV.pp_error e Mirage_kv.Key.pp path)
                | Ok (sha256, md5, sha512) ->
                  Option.iter (fun f -> f (Option.get sha256)) sha256_final;
                  Option.iter (fun f -> f (Option.get md5)) md5_final;
                  Option.iter (fun f -> f (Option.get sha512)) sha512_final;
                  Logs.info (fun m -> m "added %a" Mirage_kv.Key.pp path))
          entries >>= fun () ->
        update_caches t >|= fun () ->
        t

    let exists t h v =
      match find_key t h v with
      | Error _ -> Lwt.return false
      | Ok x ->
        KV.exists t.dev x >|= function
        | Ok Some `Value -> true
        | Ok Some `Dictionary ->
          Logs.err (fun m -> m "unexpected dictionary for %s %a"
                       (hash_to_string h) Mirage_kv.Key.pp v);
          false
        | Ok None -> false
        | Error e ->
          Logs.err (fun m -> m "exists %s %a returned %a"
                       (hash_to_string h) Mirage_kv.Key.pp v KV.pp_error e);
          false

    let last_modified t h v =
      match find_key t h v with
      | Error _ as e -> Lwt.return e
      | Ok x ->
        KV.last_modified t.dev x >|= function
        | Ok data -> Ok data
        | Error e ->
          Logs.err (fun m -> m "error %a while last_modified %s %a"
                       KV.pp_error e (hash_to_string h) Mirage_kv.Key.pp v);
          Error `Not_found

    let size t h v =
      match find_key t h v with
      | Error _ as e -> Lwt.return e
      | Ok x ->
        KV.size t.dev x >|= function
        | Ok s -> Ok s
        | Error e ->
          Logs.err (fun m -> m "error %a while size %s %a"
                       KV.pp_error e (hash_to_string h) Mirage_kv.Key.pp v);
          Error `Not_found
    end

  module Tarball = struct
    module High : sig
      type t
      type 'a s = 'a Lwt.t

      external inj : 'a s -> ('a, t) Tar.io = "%identity"
      external prj : ('a, t) Tar.io -> 'a s = "%identity"
    end = struct
      type t
      type 'a s = 'a Lwt.t

      external inj : 'a -> 'b = "%identity"
      external prj : 'a -> 'b = "%identity"
    end

    let to_buffer buf t =
      let rec run : type a. (a, [> `Msg of string ] as 'err, High.t) Tar.t -> (a, 'err) result Lwt.t
        = function
        | Tar.Write str ->
          Buffer.add_string buf str;
          Lwt.return_ok ()
        | Tar.Read _ -> assert false
        | Tar.Really_read _ -> assert false
        | Tar.Seek _ -> assert false
        | Tar.Return value -> Lwt.return value
        | Tar.High value -> High.prj value
        | Tar.Bind (x, f) ->
            let open Lwt_result.Infix in
            run x >>= fun value -> run (f value) in
      run t

    let once data =
      let closed = ref false in
      fun () -> if !closed
        then Tar.High (High.inj (Lwt.return_ok None))
        else begin closed := true; Tar.High (High.inj (Lwt.return_ok (Some  data))) end

    let entries_of_git ~mtime store repo =
      Git.find_contents store >>= fun paths ->
      let entries = Lwt_stream.of_list paths in
      let to_entry path =
        Store.get store path >|= function
        | Ok data ->
          let data =
            if Mirage_kv.Key.(equal path (v "repo"))
            then repo else data in
          let file_mode = 0o644
          and mod_time = Int64.of_int mtime
          and user_id = 0
          and group_id = 0
          and size = String.length data in
          let hdr = Tar.Header.make ~file_mode ~mod_time ~user_id ~group_id
            (Mirage_kv.Key.to_string path) (Int64.of_int size) in
          Some (None, hdr, once data)
        | Error _ -> None in
      let entries = Lwt_stream.filter_map_s to_entry entries in
      Lwt.return begin fun () -> Tar.High (High.inj (Lwt_stream.get entries >|= Result.ok)) end

    let of_git repo store =
      let now = Ptime.v (Pclock.now_d_ps ()) in
      let mtime = Option.value ~default:0 Ptime.(Span.to_int_s (to_span now)) in
      entries_of_git ~mtime store repo >>= fun entries ->
      let t = Tar.out entries in
      let t = Tar_gz.out_gzipped ~level:4 ~mtime:(Int32.of_int mtime) Gz.Unix t in
      let buf = Buffer.create 1024 in
      to_buffer buf t >|= function
      | Ok () -> Buffer.contents buf
      | Error (`Msg msg) -> failwith msg
  end

  module Serve = struct
    let ptime_to_http_date ptime =
      let (y, m, d), ((hh, mm, ss), _) = Ptime.to_date_time ptime
      and weekday = match Ptime.weekday ptime with
        | `Mon -> "Mon" | `Tue -> "Tue" | `Wed -> "Wed" | `Thu -> "Thu"
        | `Fri -> "Fri" | `Sat -> "Sat" | `Sun -> "Sun"
      and month =
        [| "Jan" ; "Feb" ; "Mar" ; "Apr" ; "May" ; "Jun" ;
           "Jul" ; "Aug" ; "Sep" ; "Oct" ; "Nov" ; "Dec" |]
    in
    let m' = Array.get month (pred m) in
    Printf.sprintf "%s, %02d %s %04d %02d:%02d:%02d GMT" weekday d m' y hh mm ss

    let commit_id git_kv =
      Store.digest git_kv Mirage_kv.Key.empty >|= fun r ->
      Result.fold r ~ok:Fun.id
        ~error:(fun e ->
            Logs.err (fun m -> m "%a" Store.pp_error e);
            exit 2)

    let repo remote commit =
      let upstream = List.hd (String.split_on_char '#' remote) in
      Fmt.str
        {|opam-version: "2.0"
upstream: "%s#%s"
archive-mirrors: "cache"
stamp: %S
|} upstream commit commit

    let modified git_kv =
      Store.last_modified git_kv Mirage_kv.Key.empty >|= fun r ->
      let v =
        Result.fold r
          ~ok:Fun.id
          ~error:(fun _ -> Ptime.v (Pclock.now_d_ps ()))
      in
      ptime_to_http_date v

    type t = {
      mutable commit_id : string ;
      mutable modified : string ;
      mutable repo : string ;
      mutable index : string ;
    }

    let create remote git_kv =
      commit_id git_kv >>= fun commit_id ->
      modified git_kv >>= fun modified ->
      let repo = repo remote commit_id in
      Tarball.of_git repo git_kv >|= fun index ->
      { commit_id ; modified ; repo ; index }

    let update_lock = Lwt_mutex.create ()

    let update_git ~remote t git_kv =
      Lwt_mutex.with_lock update_lock (fun () ->
          Logs.info (fun m -> m "pulling the git repository");
          Git_kv.pull git_kv >>= function
          | Error `Msg msg ->
            Logs.err (fun m -> m "error %s while updating git" msg);
            Lwt.return None
          | Ok [] ->
            Logs.info (fun m -> m "git changes are empty");
            Lwt.return (Some [])
          | Ok changes ->
            commit_id git_kv >>= fun commit_id ->
            modified git_kv >>= fun modified ->
            Logs.info (fun m -> m "git: %s" commit_id);
            let repo = repo remote commit_id in
            Tarball.of_git repo git_kv >|= fun index ->
            t.commit_id <- commit_id ;
            t.modified <- modified ;
            t.repo <- repo ;
            t.index <- index;
            Some changes)

    let not_modified request (modified, etag) =
      match Httpaf.Headers.get request.Httpaf.Request.headers "if-modified-since" with
      | Some ts -> String.equal ts modified
      | None -> match Httpaf.Headers.get request.Httpaf.Request.headers "if-none-match" with
        | Some etags -> List.mem etag (String.split_on_char ',' etags)
        | None -> false

    let not_found reqd path =
      let data = "Resource not found " ^ path in
      let headers = Httpaf.Headers.of_list
          [ "content-length", string_of_int (String.length data) ] in
      let resp = Httpaf.Response.create ~headers `Not_found in
      Httpaf.Reqd.respond_with_string reqd resp data

    let respond_with_empty reqd resp =
      let hdr =
        Httpaf.Headers.add_unless_exists resp.Httpaf.Response.headers
          "connection" "close"
      in
      let resp = { resp with Httpaf.Response.headers = hdr } in
      Httpaf.Reqd.respond_with_string reqd resp ""

    (* From the OPAM manual, all we need:
       /repo -- repository configuration file
       /cache -- cached archives
       /index.tar.gz -- archive containing the whole repository contents
    *)
    (* may include "announce: [ string { filter } ... ]" *)
    (* use Key_gen.remote for browse & upstream *)

    (* for repo and index.tar.gz:
        if Last_modified.not_modified request then
          let resp = Httpaf.Response.create `Not_modified in
          respond_with_empty reqd resp
        else *)
    let dispatch t store hook_url update _flow _conn reqd =
      let request = Httpaf.Reqd.request reqd in
      Logs.info (fun f -> f "requested %s" request.Httpaf.Request.target);
      match String.split_on_char '/' request.Httpaf.Request.target with
      | [ ""; x ] when String.equal x hook_url ->
        Lwt.async update;
        let data = "Update in progress" in
        let mime_type = "text/plain" in
        let headers = [
          "content-type", mime_type ;
          "etag", t.commit_id ;
          "last-modified", t.modified ;
          "content-length", string_of_int (String.length data) ;
        ] in
        let headers = Httpaf.Headers.of_list headers in
        let resp = Httpaf.Response.create ~headers `OK in
        Httpaf.Reqd.respond_with_string reqd resp data
      | [ ""; "repo" ] ->
        if not_modified request (t.modified, t.commit_id) then
          let resp = Httpaf.Response.create `Not_modified in
          respond_with_empty reqd resp
        else
          let data = t.repo in
          let mime_type = "text/plain" in
          let headers = [
            "content-type", mime_type ;
            "etag", t.commit_id ;
            "last-modified", t.modified ;
            "content-length", string_of_int (String.length data) ;
          ] in
          let headers = Httpaf.Headers.of_list headers in
          let resp = Httpaf.Response.create ~headers `OK in
          Httpaf.Reqd.respond_with_string reqd resp data
      | [ ""; "index.tar.gz" ] ->
        (* deliver prepared tarball *)
        if not_modified request (t.modified, t.commit_id) then
          let resp = Httpaf.Response.create `Not_modified in
          respond_with_empty reqd resp
        else
          let data = t.index in
          let mime_type = "application/octet-stream" in
          let headers = [
            "content-type", mime_type ;
            "etag", t.commit_id ;
            "last-modified", t.modified ;
            "content-length", string_of_int (String.length data) ;
          ] in
          let headers = Httpaf.Headers.of_list headers in
          let resp = Httpaf.Response.create ~headers `OK in
          Httpaf.Reqd.respond_with_string reqd resp data
      | "" :: "cache" :: hash_algo :: _ :: hash :: [] ->
        (* `<hash-algo>/<first-2-hash-characters>/<hash>` *)
        begin
          match hash_of_string hash_algo with
          | Error `Msg msg ->
            Logs.warn (fun m -> m "error decoding hash algo: %s" msg);
            not_found reqd request.Httpaf.Request.target
          | Ok h ->
            let hash = Mirage_kv.Key.v hash in
            Lwt.async (fun () ->
                (Disk.last_modified store h hash >|= function
                  | Error _ ->
                    Logs.warn (fun m -> m "error retrieving last modified");
                    t.modified
                  | Ok v -> ptime_to_http_date v) >>= fun last_modified ->
                if not_modified request (last_modified, Mirage_kv.Key.basename hash) then
                  let resp = Httpaf.Response.create `Not_modified in
                  respond_with_empty reqd resp;
                  Lwt.return_unit
                else
                  Disk.size store h hash >>= function
                  | Error _ ->
                    Logs.warn (fun m -> m "error retrieving size");
                    not_found reqd request.Httpaf.Request.target;
                    Lwt.return_unit
                  | Ok size ->
                    let size = Optint.Int63.to_string size in
                    let mime_type = "application/octet-stream" in
                    let headers = [
                      "content-type", mime_type ;
                      "etag", Mirage_kv.Key.basename hash ;
                      "last-modified", last_modified ;
                      "content-length", size ;
                    ]
                    in
                    let headers = Httpaf.Headers.of_list headers in
                    let resp = Httpaf.Response.create ~headers `OK in
                    let body = Httpaf.Reqd.respond_with_streaming reqd resp in
                    Disk.read_chunked store h hash (fun () chunk ->
                        let wait, wakeup = Lwt.task () in
                        (* FIXME: catch exception when body is closed *)
                        Httpaf.Body.write_string body chunk;
                        Httpaf.Body.flush body (Lwt.wakeup wakeup);
                        wait) () >|= fun _ ->
                    Httpaf.Body.close_writer body)
        end
      | _ ->
        Logs.warn (fun m -> m "unknown request %s" request.Httpaf.Request.target);
        not_found reqd request.Httpaf.Request.target

  end

  let bad_archives = SSet.of_list Bad.archives

  let download_archives parallel_downloads disk http_client store =
    (* FIXME: handle resuming partial downloads *)
    Git.find_urls store >>= fun urls ->
    let urls = SM.filter (fun k _ -> not (SSet.mem k bad_archives)) urls in
    let pool = Lwt_pool.create parallel_downloads (Fun.const Lwt.return_unit) in
    let idx = ref 0 in
    Lwt_list.iter_p (fun (url, csums) ->
        Lwt_pool.use pool @@ fun () ->
        (* FIXME: check pending and to-delete *)
        HM.fold (fun h v r ->
            r >>= function
            | true -> Disk.exists disk h (hex_to_key v)
            | false -> Lwt.return false)
          csums (Lwt.return true) >>= function
        | true ->
          Logs.debug (fun m -> m "ignoring %s (already present)" url);
          Lwt.return_unit
        | false ->
          incr idx;
          if !idx mod 10 = 0 then Gc.full_major () ;
          Logs.info (fun m -> m "downloading %s" url);
          let quux, body_init = Archive_checksum.init_write csums in
          Http_mirage_client.request http_client url (Disk.write_partial disk quux) body_init >>= function
          | Ok (resp, r) ->
            begin match r with
              | Error `Bad_response ->
                Logs.warn (fun m -> m "%s: %a (reason %s)"
                              url H2.Status.pp_hum resp.status resp.reason);
                Lwt.return_unit
              | Error `Write_error e ->
                Logs.err (fun m -> m "%s: write error %a"
                              url KV.pp_write_error e);
                Lwt.return_unit
              | Ok (digests, body) ->
                Disk.finalize_write disk quux ~url body csums digests
            end
          | _ -> Lwt.return_unit)
      (SM.bindings urls) >>= fun () ->
    Disk.update_caches disk >|= fun () ->
    Logs.info (fun m -> m "downloading of %d urls done" (SM.cardinal urls))

  let dump_git git_dump git_kv =
    Git_kv.to_octets git_kv >>= fun data ->
    Cache.write git_dump data >|= function
    | Ok () ->
      Logs.info (fun m -> m "dumped git %d bytes" (String.length data))
    | Error e ->
      Logs.warn (fun m -> m "failed to dump git: %a" Cache.pp_write_error e)

  let restore_git ~remote git_dump git_ctx =
    Cache.read git_dump >>= function
    | Ok None -> Lwt.return (Error ())
    | Error e ->
      Logs.warn (fun m -> m "failed to read git state: %a" Cache.pp_error e);
      Lwt.return (Error ())
    | Ok Some data ->
      Git_kv.of_octets git_ctx ~remote data >|= function
      | Ok git_kv -> Ok git_kv
      | Error `Msg msg ->
        Logs.err (fun m -> m "error restoring git state: %s" msg);
        Error ()

  module Paf = Paf_mirage.Make(Stack.TCP)

  let start block _time _pclock stack git_ctx http_ctx
    { K.check; verify_sha256; remote; parallel_downloads; hook_url
    ; port; sectors_cache; sectors_git; ignore_local_git } =
    BLOCK.get_info block >>= fun info ->
    let git_start =
      let cache_size = Int64.(mul 2L sectors_cache) in
      Int64.(sub info.size_sectors (add cache_size sectors_git))
    in
    Part.connect git_start block >>= fun (kv, rest) ->
    let git_dump, rest = Part.subpartition sectors_git rest in
    let md5s, sha512s = Part.subpartition sectors_cache rest in
    KV.connect kv >>= fun kv ->
    Cache.connect md5s >>= fun md5s ->
    Cache.connect sha512s >>= fun sha512s ->
    Cache.connect git_dump >>= fun git_dump ->
    Logs.info (fun m -> m "Available bytes in tar storage: %Ld" (KV.free kv));
    Disk.init ~verify_sha256 kv md5s sha512s >>= fun disk ->
    if check then
      Lwt.return_unit
    else
      begin
        Logs.info (fun m -> m "Initializing git state. This may take a while...");
        (if ignore_local_git then
           Lwt.return (Error ())
         else
           restore_git ~remote git_dump git_ctx) >>= function
        | Ok git_kv -> Lwt.return git_kv
        | Error () ->
          Git_kv.connect git_ctx remote >>= fun git_kv ->
          dump_git git_dump git_kv >|= fun () ->
          git_kv
      end >>= fun git_kv ->
      Logs.info (fun m -> m "Done initializing git state!");
      Serve.commit_id git_kv >>= fun commit_id ->
      Logs.info (fun m -> m "git: %s" commit_id);
      Serve.create remote git_kv >>= fun serve ->
      Paf.init ~port (Stack.tcp stack) >>= fun t ->
      let update () =
        Serve.update_git ~remote serve git_kv >>= function
        | None | Some [] -> Lwt.return_unit
        | Some _changes ->
          dump_git git_dump git_kv >>= fun () ->
          download_archives parallel_downloads disk http_ctx git_kv
      in
      let service =
        Paf.http_service
          ~error_handler:(fun _ ?request:_ _ _ -> ())
          (Serve.dispatch serve disk hook_url update)
      in
      let `Initialized th = Paf.serve service t in
      Logs.info (fun f -> f "listening on %d/HTTP" port);
      Lwt.async (fun () ->
          let rec go () =
            Time.sleep_ns (Duration.of_hour 1) >>= fun () ->
            update () >>= fun () ->
            go ()
          in
          go ());
      download_archives parallel_downloads disk http_ctx git_kv >>= fun () ->
      (th >|= fun _v -> ())
end