opam-mirror/mirage/unikernel.ml

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
    Mirage_runtime.register_arg 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
    Mirage_runtime.register_arg 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
    Mirage_runtime.register_arg
      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
    Mirage_runtime.register_arg 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
    Mirage_runtime.register_arg Arg.(value & opt string "update" doc)

  let port =
    let doc = Arg.info ~doc:"HTTP listen port." ["port"] in
    Mirage_runtime.register_arg Arg.(value & opt int 80 doc)

  let sectors_cache =
    let doc = "Number of sectors reserved for each checksum cache (md5, sha512). Only used with --initialize-disk." in
    let doc = Arg.info ~doc ["sectors-cache"] in
    Mirage_runtime.register_arg Arg.(value & opt int64 Int64.(mul 4L 2048L) doc)

  let sectors_git =
    let doc = "Number of sectors reserved for git dump. Only used with --initialize-disk" in
    let doc = Arg.info ~doc ["sectors-git"] in
    Mirage_runtime.register_arg Arg.(value & opt int64 Int64.(mul 40L (mul 2L 1024L)) doc)

  let sectors_swap =
    let doc = "Number of sectors reserved for swap. Only used with --initialize-disk" in
    let doc = Arg.info ~doc ["sectors-swap"] in
    Mirage_runtime.register_arg Arg.(value & opt int64 Int64.(mul 1024L 2048L) doc)

  let initialize_disk =
    let doc = "Initialize the disk with a partition table. THIS IS DESTRUCTIVE!" in
    let doc = Arg.info ~doc ["initialize-disk"] in
    Mirage_runtime.register_arg Arg.(value & flag doc)

  let ignore_local_git =
    let doc = "Ignore restoring locally saved git repository." in
    let doc = Arg.info ~doc ["ignore-local-git"] in
    Mirage_runtime.register_arg Arg.(value & flag doc)
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 = Partitions.Make(BLOCK)
  module KV = Tar_mirage.Make_KV_RW(Pclock)(Part)
  module Cache = OneFFS.Make(Part)
  module Swap = Swapfs.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 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 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 = Opam_file.extract_urls (Mirage_kv.Key.to_string path) data in
               List.fold_left (fun acc (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) 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

  let active_downloads = ref SM.empty

  let add_to_active url ts =
    active_downloads := SM.add url (ts, 0) !active_downloads

  let remove_active url =
    active_downloads := SM.remove url !active_downloads

  let active_add_bytes url written =
    match SM.find_opt url !active_downloads with
    | None -> ()
    | Some (ts, written') ->
      active_downloads := SM.add url (ts, written + written')
          !active_downloads

  let failed_downloads = ref SM.empty

  let add_failed url ts reason =
    remove_active url;
    failed_downloads := SM.add url (ts, reason) !failed_downloads

  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 ;
      dev_swap : Swap.t ;
    }

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

    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

    let init_write t csums =
      let quux, csums = Archive_checksum.init_write csums in
      let swap = Swap.empty t.dev_swap in
      quux, Ok (csums, swap)

    let write_partial t (hash, csum) url =
      (* XXX: we may be in trouble if different hash functions are used for the same archive *)
      let ( >>>= ) = Lwt_result.bind in
      fun response r data ->
        Lwt.return r >>>= fun (digests, swap) ->
        let digests = Archive_checksum.update_digests digests data in
        active_add_bytes url (String.length data);
        Swap.append swap data >|= function
        | Ok () -> Ok (digests, swap)
        | Error swap_err -> Error (`Swap swap_err)

    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 set_from_handle dev dest h =
      let size = Optint.Int63.of_int64 (Swap.size h) in
      KV.allocate dev dest size >>= fun r ->
      let rec loop offset =
        if offset = Swap.size h then
          Lwt.return_ok ()
        else
          let length = Int64.(to_int (min 4096L (sub (Swap.size h) offset))) in
          Swap.get_partial h ~offset ~length >>= fun r ->
          match r with
          | Error e -> Lwt.return (Error (`Swap e))
          | Ok data ->
            KV.set_partial dev dest ~offset:(Optint.Int63.of_int64 offset) data
            >>= fun r ->
            match r with
            | Error e -> Lwt.return (Error (`Write_error e))
            | Ok () ->
              loop Int64.(add offset (of_int length))
      in
      match r with
      | Ok () ->
        loop 0L
      | Error e ->
        Lwt.return (Error (`Write_error e))

    let finalize_write t (hash, csum) ~url swap csums digests =
      if check_csums_digests csums digests 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
        Logs.info (fun m -> m "downloaded %s, now writing" url);
        let temp = Mirage_kv.Key.(v "pending" // dest) in
        Lwt_result.bind
          (Lwt.finalize (fun () -> set_from_handle t.dev temp swap)
             (fun () -> Swap.free swap))
          (fun () -> KV.rename t.dev ~source:temp ~dest
                     |> Lwt_result.map_error (fun e -> `Write_error e))
        >|= function
        | Ok () ->
          remove_active url;
          t.md5s <- SM.add md5 sha256 t.md5s;
          t.sha512s <- SM.add sha512 sha256 t.sha512s
        | Error e ->
          let pp_error ppf = function
            | `Write_error e -> KV.pp_write_error ppf e
            | `Swap e -> Swap.pp_error ppf e
          in
          Logs.err (fun m -> m "Write failure for %s: %a" url pp_error e);
          add_failed url (Ptime.v (Pclock.now_d_ps ()))
            (Fmt.str "Write failure for %s: %a" url pp_error e)
      else begin
        add_failed url (Ptime.v (Pclock.now_d_ps ()))
          (Fmt.str "Bad checksum %s:%s: computed %s expected %s" url
             (hash_to_string hash)
             (Ohex.encode (Archive_checksum.get digests hash))
             (Ohex.encode csum));
        Logs.err (fun m -> m "Bad checksum %s:%s: computed %s expected %s" url
                     (hash_to_string hash)
                     (Ohex.encode (Archive_checksum.get digests hash))
                     (Ohex.encode csum));
        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 dev_swap =
      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 dev_swap 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
        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 (Some Tar.Header.Ustar, 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 ~level:Ustar 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 status disk =
      (* report status:
         - archive size (can we easily measure?) and number of "good" elements
         - list of current downloads
         - list of failed downloads
      *)
      let archive_stats =
        Fmt.str "<ul><li>%u validated archives on disk</li><li>%Lu bytes free</li></ul>"
          (SM.cardinal disk.Disk.md5s)
          (KV.free disk.Disk.dev)
      in
      let active_downloads =
        let header = "<h2>Active downloads</h2><ul>" in
        let content =
          SM.fold (fun url (ts, bytes_written) acc ->
              ("<li>" ^ Ptime.to_rfc3339 ?tz_offset_s:None ts ^ ": " ^ url ^ " " ^ string_of_int bytes_written ^ " bytes written to disk</li>")
              ^ acc)
            !active_downloads ""
        in
        header ^ content ^ "</ul>"
      and failed_downloads =
        let header = "<h2>Failed downloads</h2><ul>" in
        let content =
          SM.fold (fun url (ts, reason) acc ->
              ("<li>" ^ Ptime.to_rfc3339 ?tz_offset_s:None ts ^ ": " ^ url ^ " " ^ reason ^ "</li>")
              ^ acc)
            !failed_downloads ""
        in
        header ^ content ^ "</ul>"
      in
      "<html><head><title>Opam-mirror status page</title></head><body><h1>Opam mirror status</h1><div>"
      ^ String.concat "</div><div>" [ archive_stats ; active_downloads ; failed_downloads ]
        ^ "</div></body></html>"

    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
      | [ ""; x ] when String.equal x "status" ->
        let data = status store in
        let mime_type = "text/html" in
        let headers = [
          "content-type", mime_type ;
          "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 download_archives parallel_downloads disk http_client store =
    (* FIXME: handle resuming partial downloads *)
    Git.find_urls store >>= fun urls ->
    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 () ->
        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 = Disk.init_write disk csums in
          add_to_active url (Ptime.v (Pclock.now_d_ps ()));
          Http_mirage_client.request http_client url (Disk.write_partial disk quux url) 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);
                add_failed url (Ptime.v (Pclock.now_d_ps ()))
                  (Fmt.str "%a %s" 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);
                add_failed url (Ptime.v (Pclock.now_d_ps ()))
                  (Fmt.str "write error: %a" KV.pp_write_error e);
                Lwt.return_unit
              | Error `Swap e ->
                Logs.err (fun m -> m "%s: swap error %a"
                             url
                             Swap.pp_error e);
                add_failed url (Ptime.v (Pclock.now_d_ps ()))
                  (Fmt.str "swap error: %a" Swap.pp_error e);
                Lwt.return_unit
              | Ok (digests, body) ->
                Disk.finalize_write disk quux ~url body csums digests
            end
          | Error me ->
            add_failed url (Ptime.v (Pclock.now_d_ps ()))
              (Fmt.str "mimic error: %a" Mimic.pp_error me);
            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_mirror { Part.tar; swap; git_dump; md5s; sha512s } stack git_ctx http_ctx =
    KV.connect tar >>= fun kv ->
    Cache.connect git_dump >>= fun git_dump ->
    Cache.connect md5s >>= fun md5s ->
    Cache.connect sha512s >>= fun sha512s ->
    Swap.connect swap >>= fun swap ->
    Logs.info (fun m -> m "Available bytes in tar storage: %Ld" (KV.free kv));
    Disk.init ~verify_sha256:(K.verify_sha256 ()) kv md5s sha512s swap >>= fun disk ->
    let remote = K.remote () in
    if K.check () then
      Lwt.return_unit
    else
      begin
        Logs.info (fun m -> m "Initializing git state. This may take a while...");
        (if K.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:(K.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 (K.parallel_downloads ()) disk http_ctx git_kv
      in
      let service =
        Paf.http_service
          ~error_handler:(fun _ ?request:_ _ _ -> ())
          (Serve.dispatch serve disk (K.hook_url ()) update)
      in
      let `Initialized th = Paf.serve service t in
      Logs.info (fun f -> f "listening on %d/HTTP" (K.port ()));
      Lwt.async (fun () ->
          let rec go () =
            Time.sleep_ns (Duration.of_hour 1) >>= fun () ->
            update () >>= fun () ->
            go ()
          in
          go ());
      download_archives (K.parallel_downloads ()) disk http_ctx git_kv >>= fun () ->
      (th >|= fun _v -> ())

  let start block _time _pclock stack git_ctx http_ctx =
    let initialize_disk = K.initialize_disk ()
    and sectors_cache = K.sectors_cache ()
    and sectors_git = K.sectors_git ()
    and sectors_swap = K.sectors_swap () in
    if initialize_disk then
      Part.format block ~sectors_cache ~sectors_git ~sectors_swap >>= function
      | Ok () ->
        Logs.app (fun m -> m "Successfully initialized the disk! You may restart now without --initialize-disk.");
        Lwt.return_unit
      | Error `Msg e ->
        Logs.err (fun m -> m "Error formatting disk: %s" e);
        exit Mirage_runtime.argument_error
      | Error `Block e ->
        Logs.err (fun m -> m "Error formatting disk: %a" BLOCK.pp_write_error e);
        exit 2
    else
      Part.connect block >>= fun parts ->
      start_mirror parts stack git_ctx http_ctx
end