Source file sc_rollup_inbox_repr.ml

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(**

   A Merkelized inbox represents a list of messages. This list
   is decomposed into sublists of messages, one for each non-empty Tezos
   level greater than the level of the Last Cemented Commitment (LCC).

   This module is designed to:

   1. provide a space-efficient representation for proofs of inbox
   inclusions (only for inboxes obtained at the end of block
   validation) ;

   2. offer an efficient function to add a new batch of messages in the
   inbox at the current level.

   To solve (1), we use a proof tree H which is implemented by a sparse
   merkelized skip list allowing for compact inclusion proofs (See
   {!skip_list_repr.ml}).

   To solve (2), we maintain a separate proof tree C witnessing the
   contents of messages of the current level.

   The protocol maintains the hashes of the head of H, and the root hash of C.

   The rollup node needs to maintain a full representation for C and a
   partial representation for H back to the level of the LCC.

*)
type error += Invalid_level_add_messages of Raw_level_repr.t

type error += Inbox_proof_error of string

type error += Tried_to_add_zero_messages

type error += Empty_upper_level of Raw_level_repr.t

let () =
  let open Data_encoding in
  register_error_kind
    `Permanent
    ~id:"sc_rollup_inbox.invalid_level_add_messages"
    ~title:"Internal error: Trying to add a message to an inbox from the past"
    ~description:
      "An inbox can only accept messages for its current level or for the next \
       levels."
    (obj1 (req "level" Raw_level_repr.encoding))
    (function Invalid_level_add_messages level -> Some level | _ -> None)
    (fun level -> Invalid_level_add_messages level) ;

  register_error_kind
    `Permanent
    ~id:"sc_rollup_inbox.inbox_proof_error"
    ~title:
      "Internal error: error occurred during proof production or validation"
    ~description:"An inbox proof error."
    ~pp:(fun ppf e -> Format.fprintf ppf "Inbox proof error: %s" e)
    (obj1 (req "error" string))
    (function Inbox_proof_error e -> Some e | _ -> None)
    (fun e -> Inbox_proof_error e) ;

  register_error_kind
    `Permanent
    ~id:"sc_rollup_inbox.add_zero_messages"
    ~title:"Internal error: trying to add zero messages"
    ~description:
      "Message adding functions must be called with a positive number of \
       messages"
    ~pp:(fun ppf _ -> Format.fprintf ppf "Tried to add zero messages")
    empty
    (function Tried_to_add_zero_messages -> Some () | _ -> None)
    (fun () -> Tried_to_add_zero_messages) ;

  register_error_kind
    `Permanent
    ~id:"sc_rollup_inbox.empty_upper_level"
    ~title:"Internal error: No payload found in a [Level_crossing] proof"
    ~description:
      "Failed to find any message in the [upper_level] of a [Level_crossing] \
       proof"
    (obj1 (req "upper_level" Raw_level_repr.encoding))
    (function Empty_upper_level upper_level -> Some upper_level | _ -> None)
    (fun upper_level -> Empty_upper_level upper_level)

module Int64_map = Map.Make (Int64)

(* 32 *)
let hash_prefix = "\003\250\174\238\208" (* scib1(55) *)

module Hash = struct
  let prefix = "scib1"

  let encoded_size = 55

  module H =
    Blake2B.Make
      (Base58)
      (struct
        let name = "inbox_hash"

        let title = "The hash of an inbox of a smart contract rollup"

        let b58check_prefix = hash_prefix

        (* defaults to 32 *)
        let size = None
      end)

  include H

  let () = Base58.check_encoded_prefix b58check_encoding prefix encoded_size

  let of_context_hash context_hash =
    Context_hash.to_bytes context_hash |> of_bytes_exn

  let to_context_hash hash = to_bytes hash |> Context_hash.of_bytes_exn

  include Path_encoding.Make_hex (H)
end

module Skip_list_parameters = struct
  let basis = 2
end

module Skip_list = Skip_list_repr.Make (Skip_list_parameters)

let hash_skip_list_cell cell =
  let current_level_hash = Skip_list.content cell in
  let back_pointers_hashes = Skip_list.back_pointers cell in
  Hash.to_bytes current_level_hash
  :: List.map Hash.to_bytes back_pointers_hashes
  |> Hash.hash_bytes

module V1 = struct
  type history_proof = (Hash.t, Hash.t) Skip_list.cell

  let equal_history_proof = Skip_list.equal Hash.equal Hash.equal

  let history_proof_encoding : history_proof Data_encoding.t =
    Skip_list.encoding Hash.encoding Hash.encoding

  let pp_history_proof fmt history =
    let history_hash = hash_skip_list_cell history in
    Format.fprintf
      fmt
      "@[hash : %a@;%a@]"
      Hash.pp
      history_hash
      (Skip_list.pp ~pp_content:Hash.pp ~pp_ptr:Hash.pp)
      history

  (** Construct an inbox [history] with a given [capacity]. If you
      are running a rollup node, [capacity] needs to be large enough to
      remember any levels for which you may need to produce proofs. *)
  module History =
    Bounded_history_repr.Make
      (struct
        let name = "inbox_history"
      end)
      (Hash)
      (struct
        type t = history_proof

        let pp = pp_history_proof

        let equal = equal_history_proof

        let encoding = history_proof_encoding
      end)

  (*

   At a given level, an inbox is composed of metadata of type [t] and
   [current_level], a [tree] representing the messages of the current level
   (held by the [Raw_context.t] in the protocol).

   The metadata contains :
   - [rollup] : the address of the rollup ;
   - [level] : the inbox level ;
   - [message_counter] : the number of messages in the [level]'s inbox ;
     the number of messages that have not been consumed by a commitment cementing ;
   - [nb_messages_in_commitment_period] :
     the number of messages during the commitment period ;
   - [starting_level_of_current_commitment_period] :
     the level marking the beginning of the current commitment period ;
   - [current_level_hash] : the root hash of [current_level] ;
   - [old_levels_messages] : a witness of the inbox history.

   When new messages are appended to the current level inbox, the
   metadata stored in the context may be related to an older level.
   In that situation, an archival process is applied to the metadata.
   This process saves the [current_level_hash] in the
   [old_levels_messages] and empties [current_level]. It then
   initialises a new level tree for the new messages---note that any
   intermediate levels are simply skipped. See
   {!Make_hashing_scheme.archive_if_needed} for details.

  *)
  type t = {
    rollup : Sc_rollup_repr.t;
    level : Raw_level_repr.t;
    nb_messages_in_commitment_period : int64;
    starting_level_of_current_commitment_period : Raw_level_repr.t;
    message_counter : Z.t;
    (* Lazy to avoid hashing O(n^2) time in [add_messages] *)
    current_level_hash : unit -> Hash.t;
    old_levels_messages : history_proof;
  }

  let equal inbox1 inbox2 =
    (* To be robust to addition of fields in [t]. *)
    let {
      rollup;
      level;
      nb_messages_in_commitment_period;
      starting_level_of_current_commitment_period;
      message_counter;
      current_level_hash;
      old_levels_messages;
    } =
      inbox1
    in
    Sc_rollup_repr.Address.equal rollup inbox2.rollup
    && Raw_level_repr.equal level inbox2.level
    && Compare.Int64.(
         equal
           nb_messages_in_commitment_period
           inbox2.nb_messages_in_commitment_period)
    && Raw_level_repr.(
         equal
           starting_level_of_current_commitment_period
           inbox2.starting_level_of_current_commitment_period)
    && Z.equal message_counter inbox2.message_counter
    && Hash.equal (current_level_hash ()) (inbox2.current_level_hash ())
    && equal_history_proof old_levels_messages inbox2.old_levels_messages

  let pp fmt
      {
        rollup;
        level;
        nb_messages_in_commitment_period;
        starting_level_of_current_commitment_period;
        message_counter;
        current_level_hash;
        old_levels_messages;
      } =
    Format.fprintf
      fmt
      "@[<hov 2>{ rollup = %a@;\
       level = %a@;\
       current messages hash  = %a@;\
       nb_messages_in_commitment_period = %s@;\
       starting_level_of_current_commitment_period = %a@;\
       message_counter = %a@;\
       old_levels_messages = %a@;\
       }@]"
      Sc_rollup_repr.Address.pp
      rollup
      Raw_level_repr.pp
      level
      Hash.pp
      (current_level_hash ())
      (Int64.to_string nb_messages_in_commitment_period)
      Raw_level_repr.pp
      starting_level_of_current_commitment_period
      Z.pp_print
      message_counter
      pp_history_proof
      old_levels_messages

  let inbox_level inbox = inbox.level

  let old_levels_messages inbox = inbox.old_levels_messages

  let current_level_hash inbox = inbox.current_level_hash ()

  let old_levels_messages_encoding =
    Skip_list.encoding Hash.encoding Hash.encoding

  let encoding =
    Data_encoding.(
      conv
        (fun {
               rollup;
               message_counter;
               nb_messages_in_commitment_period;
               starting_level_of_current_commitment_period;
               level;
               current_level_hash;
               old_levels_messages;
             } ->
          ( rollup,
            message_counter,
            nb_messages_in_commitment_period,
            starting_level_of_current_commitment_period,
            level,
            current_level_hash (),
            old_levels_messages ))
        (fun ( rollup,
               message_counter,
               nb_messages_in_commitment_period,
               starting_level_of_current_commitment_period,
               level,
               current_level_hash,
               old_levels_messages ) ->
          {
            rollup;
            message_counter;
            nb_messages_in_commitment_period;
            starting_level_of_current_commitment_period;
            level;
            current_level_hash = (fun () -> current_level_hash);
            old_levels_messages;
          })
        (obj7
           (req "rollup" Sc_rollup_repr.encoding)
           (req "message_counter" n)
           (req "nb_messages_in_commitment_period" int64)
           (req
              "starting_level_of_current_commitment_period"
              Raw_level_repr.encoding)
           (req "level" Raw_level_repr.encoding)
           (req "current_level_hash" Hash.encoding)
           (req "old_levels_messages" old_levels_messages_encoding)))

  let number_of_messages_during_commitment_period inbox =
    inbox.nb_messages_in_commitment_period

  let start_new_commitment_period inbox level =
    {
      inbox with
      starting_level_of_current_commitment_period = level;
      nb_messages_in_commitment_period = 0L;
    }

  let starting_level_of_current_commitment_period inbox =
    inbox.starting_level_of_current_commitment_period

  let refresh_commitment_period ~commitment_period ~level inbox =
    let start = starting_level_of_current_commitment_period inbox in
    let freshness = Raw_level_repr.diff level start in
    let open Int32 in
    let open Compare.Int32 in
    if freshness >= commitment_period then (
      let nb_periods =
        to_int ((mul (div freshness commitment_period)) commitment_period)
      in
      let new_starting_level = Raw_level_repr.(add start nb_periods) in
      assert (Raw_level_repr.(new_starting_level <= level)) ;
      assert (
        rem (Raw_level_repr.diff new_starting_level start) commitment_period
        = 0l) ;
      start_new_commitment_period inbox new_starting_level)
    else inbox
end

type versioned = V1 of V1.t

let versioned_encoding =
  let open Data_encoding in
  union
    [
      case
        ~title:"V1"
        (Tag 0)
        V1.encoding
        (function V1 inbox -> Some inbox)
        (fun inbox -> V1 inbox);
    ]

include V1

let of_versioned = function V1 inbox -> inbox [@@inline]

let to_versioned inbox = V1 inbox [@@inline]

let key_of_message ix =
  ["message"; Data_encoding.Binary.to_string_exn Data_encoding.n ix]

let level_key = ["level"]

let number_of_messages_key = ["number_of_messages"]

type serialized_proof = bytes

let serialized_proof_encoding = Data_encoding.bytes

module type Merkelized_operations = sig
  type inbox_context

  type tree

  val hash_level_tree : tree -> Hash.t

  val new_level_tree : inbox_context -> Raw_level_repr.t -> tree Lwt.t

  val add_messages :
    inbox_context ->
    History.t ->
    t ->
    Raw_level_repr.t ->
    Sc_rollup_inbox_message_repr.serialized list ->
    tree option ->
    (tree * History.t * t) tzresult Lwt.t

  val add_messages_no_history :
    inbox_context ->
    t ->
    Raw_level_repr.t ->
    Sc_rollup_inbox_message_repr.serialized list ->
    tree option ->
    (tree * t) tzresult Lwt.t

  val get_message_payload :
    tree -> Z.t -> Sc_rollup_inbox_message_repr.serialized option Lwt.t

  val form_history_proof :
    inbox_context ->
    History.t ->
    t ->
    tree option ->
    (History.t * history_proof) tzresult Lwt.t

  val take_snapshot : t -> history_proof

  type inclusion_proof

  val inclusion_proof_encoding : inclusion_proof Data_encoding.t

  val pp_inclusion_proof : Format.formatter -> inclusion_proof -> unit

  val number_of_proof_steps : inclusion_proof -> int

  val verify_inclusion_proof :
    inclusion_proof -> history_proof -> history_proof -> bool

  type proof

  val pp_proof : Format.formatter -> proof -> unit

  val to_serialized_proof : proof -> serialized_proof

  val of_serialized_proof : serialized_proof -> proof option

  val verify_proof :
    Raw_level_repr.t * Z.t ->
    history_proof ->
    proof ->
    Sc_rollup_PVM_sig.inbox_message option tzresult Lwt.t

  val produce_proof :
    inbox_context ->
    History.t ->
    history_proof ->
    Raw_level_repr.t * Z.t ->
    (proof * Sc_rollup_PVM_sig.inbox_message option) tzresult Lwt.t

  val empty : inbox_context -> Sc_rollup_repr.t -> Raw_level_repr.t -> t Lwt.t

  module Internal_for_tests : sig
    val eq_tree : tree -> tree -> bool

    val produce_inclusion_proof :
      History.t ->
      history_proof ->
      history_proof ->
      inclusion_proof option tzresult

    val serialized_proof_of_string : string -> serialized_proof
  end
end

module type P = sig
  module Tree : Context.TREE with type key = string list and type value = bytes

  type t = Tree.t

  type tree = Tree.tree

  val commit_tree : Tree.t -> string list -> Tree.tree -> unit Lwt.t

  val lookup_tree : Tree.t -> Hash.t -> tree option Lwt.t

  type proof

  val proof_encoding : proof Data_encoding.t

  val proof_before : proof -> Hash.t

  val verify_proof :
    proof -> (tree -> (tree * 'a) Lwt.t) -> (tree * 'a) option Lwt.t

  val produce_proof :
    Tree.t -> tree -> (tree -> (tree * 'a) Lwt.t) -> (proof * 'a) option Lwt.t
end

module Make_hashing_scheme (P : P) :
  Merkelized_operations with type tree = P.tree and type inbox_context = P.t =
struct
  module Tree = P.Tree

  type inbox_context = P.t

  type tree = P.tree

  let hash_level_tree level_tree = Hash.of_context_hash (Tree.hash level_tree)

  let set_level tree level =
    let level_bytes =
      Data_encoding.Binary.to_bytes_exn Raw_level_repr.encoding level
    in
    Tree.add tree level_key level_bytes

  let find_level tree =
    let open Lwt_syntax in
    let+ level_bytes = Tree.(find tree level_key) in
    Option.bind
      level_bytes
      (Data_encoding.Binary.of_bytes_opt Raw_level_repr.encoding)

  let set_number_of_messages tree number_of_messages =
    let number_of_messages_bytes =
      Data_encoding.Binary.to_bytes_exn Data_encoding.n number_of_messages
    in
    Tree.add tree number_of_messages_key number_of_messages_bytes

  (** Initialise the merkle tree for a new level in the inbox. We have
      to include the [level] in this structure so that it cannot be
      forged by a malicious rollup node. *)
  let new_level_tree ctxt level =
    let open Lwt_syntax in
    let tree = Tree.empty ctxt in
    let* tree = set_number_of_messages tree Z.zero in
    set_level tree level

  let add_message inbox payload level_tree =
    let open Lwt_tzresult_syntax in
    let message_index = inbox.message_counter in
    let message_counter = Z.succ message_index in
    let*! level_tree =
      Tree.add
        level_tree
        (key_of_message message_index)
        (Bytes.of_string
           (payload : Sc_rollup_inbox_message_repr.serialized :> string))
    in
    let*! level_tree = set_number_of_messages level_tree message_counter in
    let nb_messages_in_commitment_period =
      Int64.succ inbox.nb_messages_in_commitment_period
    in
    let inbox =
      {
        starting_level_of_current_commitment_period =
          inbox.starting_level_of_current_commitment_period;
        current_level_hash = inbox.current_level_hash;
        rollup = inbox.rollup;
        level = inbox.level;
        old_levels_messages = inbox.old_levels_messages;
        message_counter;
        nb_messages_in_commitment_period;
      }
    in
    return (level_tree, inbox)

  let get_message_payload level_tree message_index =
    let open Lwt_syntax in
    let key = key_of_message message_index in
    let* bytes = Tree.(find level_tree key) in
    return
    @@ Option.map
         (fun bs ->
           Sc_rollup_inbox_message_repr.unsafe_of_string (Bytes.to_string bs))
         bytes

  (** [no_history] creates an empty history with [capacity] set to
      zero---this makes the [remember] function a no-op. We want this
      behaviour in the protocol because we don't want to store
      previous levels of the inbox. *)
  let no_history = History.empty ~capacity:0L

  let take_snapshot inbox =
    let prev_cell = inbox.old_levels_messages in
    let prev_cell_ptr = hash_skip_list_cell prev_cell in
    Skip_list.next ~prev_cell ~prev_cell_ptr (current_level_hash inbox)

  let key_of_level level =
    let level_bytes =
      Data_encoding.Binary.to_bytes_exn Raw_level_repr.encoding level
    in
    Bytes.to_string level_bytes

  let commit_tree ctxt tree inbox_level =
    let key = [key_of_level inbox_level] in
    P.commit_tree ctxt key tree

  let form_history_proof ctxt history inbox level_tree =
    let open Lwt_tzresult_syntax in
    let*! () =
      let*! tree =
        match level_tree with
        | Some tree -> Lwt.return tree
        | None -> new_level_tree ctxt inbox.level
      in
      commit_tree ctxt tree inbox.level
    in
    let prev_cell = inbox.old_levels_messages in
    let prev_cell_ptr = hash_skip_list_cell prev_cell in
    let*? history = History.remember prev_cell_ptr prev_cell history in
    let cell =
      Skip_list.next ~prev_cell ~prev_cell_ptr (current_level_hash inbox)
    in
    return (history, cell)

  (** [archive_if_needed ctxt history inbox new_level level_tree]
      is responsible for ensuring that the {!add_messages} function
      below has a correctly set-up [level_tree] to which to add the
      messages. If [new_level] is a higher level than the current inbox,
      we create a new inbox level tree at that level in which to start
      adding messages, and archive the earlier levels depending on the
      [history] parameter's [capacity]. If [level_tree] is [None] (this
      happens when the inbox is first created) we similarly create a new
      empty level tree with the right [level] key.

      This function and {!form_history_proof} are the only places we
      begin new level trees. *)
  let archive_if_needed ctxt history inbox new_level level_tree =
    let open Lwt_result_syntax in
    if Raw_level_repr.(inbox.level = new_level) then
      match level_tree with
      | Some tree -> return (history, inbox, tree)
      | None ->
          let*! tree = new_level_tree ctxt new_level in
          return (history, inbox, tree)
    else
      let* history, old_levels_messages =
        form_history_proof ctxt history inbox level_tree
      in
      let*! tree = new_level_tree ctxt new_level in
      let inbox =
        {
          starting_level_of_current_commitment_period =
            inbox.starting_level_of_current_commitment_period;
          current_level_hash = inbox.current_level_hash;
          rollup = inbox.rollup;
          nb_messages_in_commitment_period =
            inbox.nb_messages_in_commitment_period;
          old_levels_messages;
          level = new_level;
          message_counter = Z.zero;
        }
      in
      return (history, inbox, tree)

  let add_messages ctxt history inbox level payloads level_tree =
    let open Lwt_tzresult_syntax in
    let* () =
      fail_when
        (match payloads with [] -> true | _ -> false)
        Tried_to_add_zero_messages
    in
    let* () =
      fail_when
        Raw_level_repr.(level < inbox.level)
        (Invalid_level_add_messages level)
    in
    let* history, inbox, level_tree =
      archive_if_needed ctxt history inbox level level_tree
    in
    let* level_tree, inbox =
      List.fold_left_es
        (fun (level_tree, inbox) payload ->
          add_message inbox payload level_tree)
        (level_tree, inbox)
        payloads
    in
    let current_level_hash () = hash_level_tree level_tree in
    return (level_tree, history, {inbox with current_level_hash})

  let add_messages_no_history ctxt inbox level payloads level_tree =
    let open Lwt_tzresult_syntax in
    let+ level_tree, _, inbox =
      add_messages ctxt no_history inbox level payloads level_tree
    in
    (level_tree, inbox)

  (* An [inclusion_proof] is a path in the Merkelized skip list
     showing that a given inbox history is a prefix of another one.
     This path has a size logarithmic in the difference between the
     levels of the two inboxes.

     [Irmin.Proof.{tree_proof, stream_proof}] could not be reused here
     because there is no obvious encoding of sequences in these data
     structures with the same guarantee about the size of proofs. *)
  type inclusion_proof = history_proof list

  let inclusion_proof_encoding =
    let open Data_encoding in
    list history_proof_encoding

  let pp_inclusion_proof fmt proof =
    Format.pp_print_list pp_history_proof fmt proof

  let number_of_proof_steps proof = List.length proof

  let lift_ptr_path deref ptr_path =
    let rec aux accu = function
      | [] -> Some (List.rev accu)
      | x :: xs -> Option.bind (deref x) @@ fun c -> aux (c :: accu) xs
    in
    aux [] ptr_path

  let verify_inclusion_proof proof a b =
    let assoc = List.map (fun c -> (hash_skip_list_cell c, c)) proof in
    let path = List.split assoc |> fst in
    let deref =
      let open Hash.Map in
      let map = of_seq (List.to_seq assoc) in
      fun ptr -> find_opt ptr map
    in
    let cell_ptr = hash_skip_list_cell b in
    let target_ptr = hash_skip_list_cell a in
    Skip_list.valid_back_path
      ~equal_ptr:Hash.equal
      ~deref
      ~cell_ptr
      ~target_ptr
      path

  type proof =
    (* See the main docstring for this type (in the mli file) for
       definitions of the three proof parameters [starting_point],
       [message] and [snapshot]. In the below we deconstruct
       [starting_point] into [(l, n)] where [l] is a level and [n] is a
       message index.

       In a [Single_level] proof, [level] is the skip list cell for the
       level [l], [inc] is an inclusion proof of [level] into
       [snapshot] and [message_proof] is a tree proof showing that

         [exists level_tree .
              (hash_level_tree level_tree = level.content)
          AND (payload_and_level n level_tree = (_, (message, l)))]

       Note: in the case that [message] is [None] this shows that
       there's no value at the index [n]; in this case we also must
       check that [level] equals [snapshot] (otherwise, we'd need a
       [Level_crossing] proof instead. *)
    | Single_level of {
        level : history_proof;
        inc : inclusion_proof;
        message_proof : P.proof;
      }
    (* See the main docstring for this type (in the mli file) for
       definitions of the three proof parameters [starting_point],
       [message] and [snapshot]. In the below we deconstruct
       [starting_point] as [(l, n)] where [l] is a level and [n] is a
       message index.

       In a [Level_crossing] proof, [lower] is the skip list cell for
       the level [l] and [upper] must be the skip list cell that comes
       immediately after it in [snapshot]. If the inbox has been
       constructed correctly using the functions in this module that
       will be the next non-empty level in the inbox.

       [inc] is an inclusion proof of [upper] into [snapshot].
       [upper_level] is the level of [upper].

       The tree proof [lower_message_proof] shows the following:

         [exists level_tree .
               (hash_level_tree level_tree = lower.content)
           AND (payload_and_level n level_tree = (_, (None, l)))]

       in other words, there is no message at index [n] in
       level [l]. This means that level has been fully read.

       The tree proof [upper_message_proof] shows the following:

         [exists level_tree .
               (hash_level_tree level_tree = upper.content)
           AND (payload_and_level 0 level_tree = (_, (message, upper_level)))]

       in other words, if we look in the next non-empty level the
       message at index zero is [message]. *)
    | Level_crossing of {
        lower : history_proof;
        upper : history_proof;
        inc : inclusion_proof;
        lower_message_proof : P.proof;
        upper_message_proof : P.proof;
        upper_level : Raw_level_repr.t;
      }

  let pp_proof fmt proof =
    match proof with
    | Single_level {level; _} ->
        let hash = Skip_list.content level in
        Format.fprintf fmt "Single_level inbox proof at %a" Hash.pp hash
    | Level_crossing {lower; upper; upper_level; _} ->
        let lower_hash = Skip_list.content lower in
        let upper_hash = Skip_list.content upper in
        Format.fprintf
          fmt
          "Level_crossing inbox proof between %a and %a (upper_level %a)"
          Hash.pp
          lower_hash
          Hash.pp
          upper_hash
          Raw_level_repr.pp
          upper_level

  let proof_encoding =
    let open Data_encoding in
    union
      ~tag_size:`Uint8
      [
        case
          ~title:"Single_level"
          (Tag 0)
          (obj3
             (req "level" history_proof_encoding)
             (req "inclusion_proof" inclusion_proof_encoding)
             (req "message_proof" P.proof_encoding))
          (function
            | Single_level {level; inc; message_proof} ->
                Some (level, inc, message_proof)
            | _ -> None)
          (fun (level, inc, message_proof) ->
            Single_level {level; inc; message_proof});
        case
          ~title:"Level_crossing"
          (Tag 1)
          (obj6
             (req "lower" history_proof_encoding)
             (req "upper" history_proof_encoding)
             (req "inclusion_proof" inclusion_proof_encoding)
             (req "lower_message_proof" P.proof_encoding)
             (req "upper_message_proof" P.proof_encoding)
             (req "upper_level" Raw_level_repr.encoding))
          (function
            | Level_crossing
                {
                  lower;
                  upper;
                  inc;
                  lower_message_proof;
                  upper_message_proof;
                  upper_level;
                } ->
                Some
                  ( lower,
                    upper,
                    inc,
                    lower_message_proof,
                    upper_message_proof,
                    upper_level )
            | _ -> None)
          (fun ( lower,
                 upper,
                 inc,
                 lower_message_proof,
                 upper_message_proof,
                 upper_level ) ->
            Level_crossing
              {
                lower;
                upper;
                inc;
                lower_message_proof;
                upper_message_proof;
                upper_level;
              });
      ]

  let of_serialized_proof = Data_encoding.Binary.of_bytes_opt proof_encoding

  let to_serialized_proof = Data_encoding.Binary.to_bytes_exn proof_encoding

  let proof_error reason =
    let open Lwt_tzresult_syntax in
    fail (Inbox_proof_error reason)

  let check p reason = unless p (fun () -> proof_error reason)

  (** Utility function that checks the inclusion proof [inc] for any
      inbox proof.

      In the case of a [Single_level] proof this is just an inclusion
      proof between [level] and the inbox snapshot targeted the proof.

      In the case of a [Level_crossing] proof [inc] must be an inclusion
      proof between [upper] and the inbox snapshot. In this case we must
      additionally check that [lower] is the immediate predecessor of
      [upper] in the inbox skip list. NB: there may be many 'inbox
      levels' apart, but if the intervening levels are empty they will
      be immediate neighbours in the skip list because it misses empty
      levels out. *)
  let check_inclusions proof snapshot =
    check
      (match proof with
      | Single_level {inc; level; _} ->
          verify_inclusion_proof inc level snapshot
      | Level_crossing {inc; lower; upper; _} -> (
          let prev_cell = Skip_list.back_pointer upper 0 in
          match prev_cell with
          | None -> false
          | Some p ->
              verify_inclusion_proof inc upper snapshot
              && Hash.equal p (hash_skip_list_cell lower)))
      "invalid inclusions"

  (** To construct or verify a tree proof we need a function of type

      [tree -> (tree, result) Lwt.t]

      where [result] is some data extracted from the tree that we care
      about proving. [payload_and_level n] is such a function, used for
      checking both the inbox level specified inside the tree and the
      message at a particular index, [n].

      For this function, the [result] is

      [(payload, level) : string option * Raw_level_repr.t option]

      where [payload] is [None] if there was no message at the index.
      The [level] part of the result will only be [None] if the [tree]
      is not in the correct format for an inbox level. This should not
      happen if the [tree] was correctly initialised with
      [new_level_tree]. *)
  let payload_and_level n tree =
    let open Lwt_syntax in
    let* payload = get_message_payload tree n in
    let* level = find_level tree in
    return (tree, (payload, level))

  (** Utility function that handles all the verification needed for a
      particular message proof at a particular level. It calls
      [P.verify_proof], but also checks the proof has the correct
      [P.proof_before] hash and the [level] stored inside the tree is
      the expected one. *)
  let check_message_proof message_proof level_hash (l, n) label =
    let open Lwt_tzresult_syntax in
    let* () =
      check
        (Hash.equal level_hash (P.proof_before message_proof))
        (Format.sprintf "message_proof (%s) does not match history" label)
    in
    let*! result = P.verify_proof message_proof (payload_and_level n) in
    match result with
    | None -> proof_error (Format.sprintf "message_proof is invalid (%s)" label)
    | Some (_, (_, None)) ->
        proof_error
          (Format.sprintf "badly encoded level in message_proof (%s)" label)
    | Some (_, (payload_opt, Some proof_level)) ->
        let* () =
          check
            (Raw_level_repr.equal proof_level l)
            (Format.sprintf "incorrect level in message_proof (%s)" label)
        in
        return payload_opt

  let verify_proof (l, n) snapshot proof =
    assert (Z.(geq n zero)) ;
    let open Lwt_tzresult_syntax in
    let* () = check_inclusions proof snapshot in
    match proof with
    | Single_level p -> (
        let level_hash = Skip_list.content p.level in
        let* payload_opt =
          check_message_proof p.message_proof level_hash (l, n) "single level"
        in
        match payload_opt with
        | None ->
            if equal_history_proof snapshot p.level then return None
            else proof_error "payload is None but proof.level not top level"
        | Some payload ->
            return
            @@ Some
                 Sc_rollup_PVM_sig.
                   {inbox_level = l; message_counter = n; payload})
    | Level_crossing p -> (
        let lower_level_hash = Skip_list.content p.lower in
        let* should_be_none =
          check_message_proof
            p.lower_message_proof
            lower_level_hash
            (l, n)
            "lower"
        in
        let* () =
          match should_be_none with
          | None -> return ()
          | Some _ -> proof_error "more messages to read in lower level"
        in
        let upper_level_hash = Skip_list.content p.upper in
        let* payload_opt =
          check_message_proof
            p.upper_message_proof
            upper_level_hash
            (p.upper_level, Z.zero)
            "upper"
        in
        match payload_opt with
        | None ->
            (* [check_inclusions] checks at least two important properties:
               1. [p.lower_level] is different from [p.upper_level]
               2. [p.upper_level] is included in the snapshot

               If [p.upper_level] is included in the snapshot, the level was
               created by the protocol. If the protocol created a level tree
               at [p.upper_level] it *must* contain at least one message.
               So, if [p.upper_level] exists, at the index [Z.zero] (fetched
               here), a payload *must* exist.

               This code is then dead as long as we store only the nonempty
               inboxes.
            *)
            fail (Empty_upper_level p.upper_level)
        | Some payload ->
            return
            @@ Some
                 Sc_rollup_PVM_sig.
                   {
                     inbox_level = p.upper_level;
                     message_counter = Z.zero;
                     payload;
                   })

  (** Utility function; we convert all our calls to be consistent with
      [Lwt_tzresult_syntax]. *)
  let option_to_result e lwt_opt =
    let open Lwt_syntax in
    let* opt = lwt_opt in
    match opt with None -> proof_error e | Some x -> return (ok x)

  let produce_proof ctxt history inbox (l, n) =
    let open Lwt_tzresult_syntax in
    let deref ptr = History.find ptr history in
    let compare hash =
      let*! tree = P.lookup_tree ctxt hash in
      match tree with
      | None -> Lwt.return (-1)
      | Some tree -> (
          let open Lwt_syntax in
          let+ level = find_level tree in
          match level with
          | None -> -1
          | Some level -> Raw_level_repr.compare level l)
    in
    let*! result = Skip_list.search ~deref ~compare ~cell:inbox in
    let* inc, level =
      match result with
      | Skip_list.{rev_path; last_cell = Found level} ->
          return (List.rev rev_path, level)
      | {last_cell = Nearest _; _}
      | {last_cell = No_exact_or_lower_ptr; _}
      | {last_cell = Deref_returned_none; _} ->
          (* We are only interested to the result where [search] than a
             path to the cell we were looking for. All the other cases
             should be considered as an error. *)
          proof_error
            (Format.asprintf
               "Skip_list.search failed to find a valid path: %a"
               (Skip_list.pp_search_result ~pp_cell:pp_history_proof)
               result)
    in
    let* level_tree =
      option_to_result
        "could not find level_tree in the inbox_context"
        (P.lookup_tree ctxt (Skip_list.content level))
    in
    let* message_proof, (payload_opt, _) =
      option_to_result
        "failed to produce message proof for level_tree"
        (P.produce_proof ctxt level_tree (payload_and_level n))
    in
    match payload_opt with
    | Some payload ->
        return
          ( Single_level {level; inc; message_proof},
            Some
              Sc_rollup_PVM_sig.{inbox_level = l; message_counter = n; payload}
          )
    | None -> (
        if equal_history_proof inbox level then
          return (Single_level {level; inc; message_proof}, None)
        else
          let target_index = Skip_list.index level + 1 in
          let cell_ptr = hash_skip_list_cell inbox in
          let*? history = History.remember cell_ptr inbox history in
          let deref ptr = History.find ptr history in
          let* inc =
            option_to_result
              "failed to find path to upper level"
              (Lwt.return
                 (Skip_list.back_path ~deref ~cell_ptr ~target_index
                 |> Option.map (lift_ptr_path deref)
                 |> Option.join))
          in
          let* upper =
            option_to_result
              "back_path returned empty list"
              (Lwt.return (List.last_opt inc))
          in
          let* upper_level_tree =
            option_to_result
              "could not find upper_level_tree in the inbox_context"
              (P.lookup_tree ctxt (Skip_list.content upper))
          in
          let* upper_message_proof, (payload_opt, upper_level_opt) =
            option_to_result
              "failed to produce message proof for upper_level_tree"
              (P.produce_proof ctxt upper_level_tree (payload_and_level Z.zero))
          in
          let* upper_level =
            option_to_result
              "upper_level_tree was misformed---could not find level"
              (Lwt.return upper_level_opt)
          in
          match payload_opt with
          | None ->
              proof_error "if upper_level_tree exists, the payload must exist"
          | Some payload ->
              let input_given =
                Some
                  Sc_rollup_PVM_sig.
                    {
                      inbox_level = upper_level;
                      message_counter = Z.zero;
                      payload;
                    }
              in
              return
                ( Level_crossing
                    {
                      lower = level;
                      upper;
                      inc;
                      lower_message_proof = message_proof;
                      upper_message_proof;
                      upper_level;
                    },
                  input_given ))

  let empty context rollup level =
    let open Lwt_syntax in
    assert (Raw_level_repr.(level <> Raw_level_repr.root)) ;
    let pre_genesis_level = Raw_level_repr.root in
    let* initial_level = new_level_tree context pre_genesis_level in
    let* () = commit_tree context initial_level pre_genesis_level in
    let initial_hash = hash_level_tree initial_level in
    return
      {
        rollup;
        level;
        message_counter = Z.zero;
        nb_messages_in_commitment_period = 0L;
        starting_level_of_current_commitment_period = level;
        current_level_hash = (fun () -> initial_hash);
        old_levels_messages = Skip_list.genesis initial_hash;
      }

  module Internal_for_tests = struct
    let eq_tree = Tree.equal

    let produce_inclusion_proof history a b =
      let open Tzresult_syntax in
      let cell_ptr = hash_skip_list_cell b in
      let target_index = Skip_list.index a in
      let* history = History.remember cell_ptr b history in
      let deref ptr = History.find ptr history in
      Skip_list.back_path ~deref ~cell_ptr ~target_index
      |> Option.map (lift_ptr_path deref)
      |> Option.join |> return

    let serialized_proof_of_string x = Bytes.of_string x
  end
end

include (
  Make_hashing_scheme (struct
    module Tree = struct
      include Context.Tree

      type t = Context.t

      type tree = Context.tree

      type value = bytes

      type key = string list
    end

    type t = Context.t

    type tree = Context.tree

    let commit_tree _ctxt _key _tree =
      (* This is a no-op in the protocol inbox implementation *)
      Lwt.return ()

    let lookup_tree _ctxt _hash =
      (* We cannot find the tree without a full inbox_context *)
      Lwt.return None

    type proof = Context.Proof.tree Context.Proof.t

    let proof_encoding = Context.Proof_encoding.V1.Tree32.tree_proof_encoding

    let proof_before proof =
      match proof.Context.Proof.before with
      | `Value hash | `Node hash -> Hash.of_context_hash hash

    let verify_proof p f =
      Lwt.map Result.to_option (Context.verify_tree_proof p f)

    let produce_proof _ _ _ =
      (* We cannot produce a proof without full inbox_context *)
      Lwt.return None
  end) :
    Merkelized_operations
      with type tree = Context.tree
       and type inbox_context = Context.t)

type inbox = t