feat(state): Store session-token links using Irmin instead of JSON file

Replace the JSON-based storage of session-token-date entries with an Irmin Git-backed key-value store.

Sessions are no longer stored in 'sessions.json' but in an Irmin Git repository ('session_store.git' by default).

Credits: this patch reuses code from oauth-moodle-dev by the following authors.
Co-authored-by: Louis Tariot <77079482+Plictox@users.noreply.github.com>

Author: MNassimM

src/state/dune

@@ -40,5 +40,5 @@
  (name learnocaml_store)
  (wrapped false)
  (modules Learnocaml_store)
- (libraries cryptokit lwt_utils learnocaml_api)
+ (libraries cryptokit lwt_utils learnocaml_api irmin irmin-git irmin-git.unix)
 )

src/state/learnocaml_store.ml

@@ -302,50 +302,50 @@ end
 module Session = struct
 
   include Session
+  open Lwt.Syntax
 
-  let file = "sessions.json"
+  module Store = Irmin_git_unix.FS.KV(Irmin.Contents.Json_value)
+  module Info = Irmin_git_unix.Info(Store.Info)
 
-  let enc =
-    let open Json_encoding in
-    list (obj3
-      (req "session" Session.enc)
-      (req "token" Token.enc)
-      (req "last_connection" float))
-
-  let path dir = Filename.concat dir file
-
-  let load dir =
-    let p = path dir in
-    Lwt_unix.file_exists dir >>= fun dir_exists ->
-    (if not dir_exists then Lwt_unix.mkdir dir 0o700 else Lwt.return_unit) >>= fun () ->
-    Lwt_unix.file_exists p >>= function
-    | false ->
-        Printf.printf "No session file, creating empty list\n%!";
-        Lwt.return []
-    | true ->
-        Printf.printf "Loading sessions from: %s\n%!" p;
-        get_from_file enc p
+  let repo_path = ref "./session_store.git"
 
-  let save dir table =
-    write_to_file enc table (path dir)
+  let config () = Irmin_git.config ~bare:true !repo_path
 
-  let get_user_token session =
-    load !data_dir >>= fun table ->
-    match List.find_opt (fun (s, _, _) -> s = session) table with
-    | Some (_, token, _) -> Lwt.return_some token
-    | None -> Lwt.return_none
+  type entry = {
+      session : Session.t;
+      token : Token.t;
+      last_connection : float;
+    }
+
+  let enc =
+    let open Json_encoding in
+    conv
+      (fun {session; token; last_connection} -> (session, token, last_connection))
+      (fun (session, token, last_connection) -> {session; token; last_connection})
+      (obj3
+         (req "session" Session.enc)
+         (req "token" Token.enc)
+         (req "last_connection" float))
 
   let set_session session token =
     let now = Unix.gettimeofday () in
-    load !data_dir >>= fun table ->
-    let table = (session, token, now) :: table in
-    save !data_dir table
+    let* repo = Store.Repo.v (config ()) in
+    let* t = Store.main repo in
+    Store.set_exn t ~info:(Info.v "Set session/token") [Session.to_string session] (Json_encoding.construct enc {session; token; last_connection = now})
+
+  let get_user_token session =
+    let* repo = Store.Repo.v (config ()) in
+    let* t = Store.main repo in
+    Store.find t [Session.to_string session] >|= function
+    | Some value ->
+       let entry = Json_encoding.destruct enc value in
+       Some entry.token
+    | None -> None
 
   let gen_session () =
     let len = 32 in
     Cryptokit.Random.string Cryptokit.Random.secure_rng len
-  |> Cryptokit.transform_string @@ Cryptokit.Hexa.encode ()
-
+    |> Cryptokit.transform_string @@ Cryptokit.Hexa.encode ()
 end
 
 module Token = struct

src/state/learnocaml_store.mli

@@ -116,26 +116,21 @@ module Session: sig
 
   include module type of struct include Session end
 
-  val file : string
-
-  val enc : (t * Token.t * float) list Json_encoding.encoding
-
-  val path : string -> string
-
-  (** Loads the session table from disk. *)
-  val load : string -> (t * Token.t * float) list Lwt.t
-
-  (** Saves the given session table to disk *)
-  val save : string -> (t * Token.t * float) list -> unit Lwt.t
+  type entry = {
+      session : Session.t;
+      token : Token.t;
+      last_connection : float;
+    }
+  val enc : entry Json_encoding.encoding
 
   (** Retrieves the token associated with the given session. *)
   val get_user_token : t -> Token.t option Lwt.t
 
   (** Associates a token to a session. *)
   val set_session : t -> Token.t -> unit Lwt.t
 
-   (** Generates a fresh session identifier *) 
-  val gen_session : unit -> t
+   (** Generates a fresh session identifier *)
+  val gen_session : unit -> Session.t
 end