Use Lwt_{condition,mutex} instead of lwt tasks to serialize change_and_push

src/git_kv.ml

@@ -9,7 +9,9 @@ type t =
   ; branch : Git.Reference.t
   ; store : Store.t
   ; mutable committed : Digestif.SHA1.t option
-  ; mutable change_and_push_waiter : unit Lwt.t option
+  ; mutable change_and_push_running : bool
+  ; condition : unit Lwt_condition.t
+  ; mutex : Lwt_mutex.t
   ; mutable head : Store.hash option }
 
 let init_store () =
@@ -102,7 +104,7 @@ let connect ctx endpoint =
   init_store () >>= fun store ->
   let store = to_invalid store in
   let edn, branch = split_url endpoint in
-  let t = { ctx ; edn ; branch ; store ; committed= None; change_and_push_waiter= None; head= None } in
+  let t = { ctx ; edn ; branch ; store ; committed= None; change_and_push_running= false; condition= Lwt_condition.create (); mutex= Lwt_mutex.create (); head= None } in
   pull t >>= fun r ->
   let _r = to_invalid r in
   Lwt.return t
@@ -370,7 +372,7 @@ let of_octets ctx ~remote data =
        Result.fold ~ok:Fun.id ~error:(function `Msg msg -> failwith msg) >>= fun store ->
        analyze store data >>= fun head ->
        let edn, branch = split_url remote in
-       Lwt.return_ok { ctx ; edn ; branch ; store ; committed= None; change_and_push_waiter= None; head; })
+       Lwt.return_ok { ctx ; edn ; branch ; store ; committed= None; change_and_push_running= false; condition= Lwt_condition.create (); mutex= Lwt_mutex.create (); head; })
     (fun _exn ->
        Lwt.return_error (`Msg "Invalid PACK file"))
 
@@ -684,12 +686,13 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
          [change_and_push_waiter] before we wait on the existing
          [change_and_push_waiter] task without any yield point in between to
          ensure serializability. *)
-      let th, wk = Lwt.wait () in
-      let th' = t.change_and_push_waiter in
-      t.change_and_push_waiter <- Some th;
-      ( match th' with
-        | None -> Lwt.return_unit
-        | Some th -> th ) >>= fun () ->
+      let open Lwt.Syntax in
+      let* () = Lwt_mutex.with_lock t.mutex @@ fun () ->
+        let rec await () =
+          if t.change_and_push_running
+          then Lwt_condition.wait ~mutex:t.mutex t.condition >>= await
+          else begin t.change_and_push_running <- true; Lwt.return_unit end in
+        await () in
       ( let open Lwt_result.Infix in
         tree_root_hash_of_store t >>= fun tree_root_hash ->
         let t' = { t with committed = Some tree_root_hash } in
@@ -722,12 +725,9 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
       >|= Result.map_error
         (fun err -> `Msg (Fmt.str "error pushing %a" Store.pp_error err))
       >>= fun res ->
-      Lwt.wakeup_later wk () ;
-      (* (hannes) since some other task may have mutated the
-         change_and_push_waiter, we only reset it to None if there's a physical
-         equality between its value and our created task above. *)
-      (match t.change_and_push_waiter with
-       | Some th' -> if th' == th then t.change_and_push_waiter <- None
-       | None -> ());
+      let* () = Lwt_mutex.with_lock t.mutex @@ fun () ->
+        t.change_and_push_running <- false;
+        Lwt_condition.signal t.condition ();
+        Lwt.return_unit in
       Lwt.return res
 end