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open Error_monad

(** Persistent data manager.

    Every data read/write operation is protected by a mutex preventing
    concurrent data-races. *)

(** A data structure that represents files via their paths and encodings. *)
type 'a file = private {
  encoding : 'a Data_encoding.t;
  eq : 'a -> 'a -> bool;
  path : string;
  json : bool;
}

type 'a eq := 'a -> 'a -> bool

(** The type for the (persistent) data of a file. *)
type 'a t

(** This error is returned when the requested data is not found. *)
type error += Missing_stored_data of string

(** [make_file ?(json=false) ~filepath encoding] represents a file
    located at [filepath]. The content of this value is encoded using
    [encoding]. By default, the content is encoded in binary content
    except if [json=true].
    [eq] is used internally to test whether the
    contents of the file should be updated when calling {!write}
    or {!update_with}.

    {b Warning} It is the caller responsability to ensure that the
    base directory of the [filepath] exists; otherwise, reading and
    writing will fail. *)
val make_file :
  ?json:bool -> filepath:string -> 'a Data_encoding.t -> 'a eq -> 'a file

(** [get data] accesses the data (cached). *)
val get : 'a t -> 'a Lwt.t

(** [write data value] overwrites the previous [data] with the new
    [value]. The cache is only updated if the write succeeds. If
    the write fails, it is up to the user to retry. *)
val write : 'a t -> 'a -> unit tzresult Lwt.t

(** [write_file encoded_file value] raw writes the [encoded_file] with
    the [value].

    {b Warning} this function should not be used in a normal context
    as it aims to overwrite the target without avoiding data
    races. Favour the usage of [write]. *)
val write_file : 'a file -> 'a -> unit tzresult Lwt.t

(** [update_with data f] {b atomically} updates [data] with the result
    of the application of [f]. Concurrent accesses to the data will
    block until the value is updated.

    The cache is only updated if the write succeeds. If
    the write fails, it is up to the user to retry.

    {b Warning} Calling read/write in [f] will result in a deadlock. *)
val update_with : 'a t -> ('a -> 'a Lwt.t) -> unit tzresult Lwt.t

(** [load encoded_file] loads and decode a data from an
    [encoded_file]. *)
val load : 'a file -> 'a t tzresult Lwt.t

(** [init encoded_file eq ~initial_data] creates or load an on-disk
    data. If the file already exists, then the data is read from the
    file. Otherwise, [initial_data] is used. *)
val init : 'a file -> initial_data:'a -> 'a t tzresult Lwt.t