change_and_push may return an error, report it

app/mgit.ml

@@ -140,7 +140,7 @@ let repl store fd_in =
     | [ "quit"; ] -> Lwt.return ()
     | [ "fold"; ] ->
       Store.change_and_push store0 (fun store1 -> go store1)
-      >>= fun () -> go store0
+      >>= fun _ -> go store0
     | [ "save"; filename ] ->
       save store0 filename >|= ignore
       >>= fun _ -> if is_a_tty then Fmt.pr "\n%!" ; go store0

src/git_kv.ml

@@ -614,7 +614,9 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
 
   let change_and_push t f =
     let open Lwt.Infix in
-    if t.in_closure then Fmt.invalid_arg "Nested change_and_push" ;
+    if t.in_closure then
+      Lwt.return_error (`Msg "Nested change_and_push")
+    else
       (* XXX(dinosaure): serialize [batch]. If we do [Lwt.both (batch ..) (batch ..)], they
          can not run concurrently! The second will waiting the first to finish. *)
       ( match t.committed with
@@ -649,8 +651,9 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
                      >>? fun () ->
                      Store.shallow t.store hash >|= Result.ok) >>= fun () ->
           Lwt.return_ok res )
-    >|= Result.map_error (Fmt.to_to_string Store.pp_error)
+      >|= Result.map_error
-    >|= Result.get_ok >>= fun res ->
+        (fun err -> `Msg (Fmt.str "error pushing %a" Store.pp_error err))
+      >>= fun res ->
       Lwt.wakeup_later wk () ;
       t.committed <- None ;
       Lwt.return res

src/git_kv.mli

@@ -67,5 +67,5 @@ module Make (Pclock : Mirage_clock.PCLOCK) : sig
                             | `Reference_not_found of Git.Reference.t
                             | Mirage_kv.write_error ]
 
-  val change_and_push : t -> (t -> 'a Lwt.t) -> 'a Lwt.t
+  val change_and_push : t -> (t -> 'a Lwt.t) -> ('a, [> `Msg of string ]) result Lwt.t
 end