test: serialize and reserialize RPC error messages

We demonstrate a bug with how error messages are serialized. Currently
there are two "Error:" prefixes after serializing and deserializing.
This is because the prefix hasn't been stripped in the initial
serialization. We should strip the prefix since the severity information
is recorded elsewhere in the diagnostic data.

Signed-off-by: Ali Caglayan <alizter@gmail.com>

src/dune_engine/build_system.ml

@@ -92,6 +92,12 @@ module Error = struct
     | Diagnostic d -> `Diagnostic d.diagnostic
   ;;
 
+  let make ~id ~description ?dir ?promotion () =
+    match description with
+    | `Exn exn -> Exn { id; exn }
+    | `Diagnostic diagnostic -> Diagnostic { id; diagnostic; dir; promotion }
+  ;;
+
   module Set : sig
     type error := t
 

src/dune_engine/build_system.mli

@@ -88,6 +88,7 @@ module Error : sig
 
     module Map : Map.S with type key = t
 
+    val gen : unit -> t
     val compare : t -> t -> Ordering.t
     val to_int : t -> int
     val to_dyn : t -> Dyn.t
@@ -108,6 +109,14 @@ module Error : sig
 
   val promotion : t -> Diff_promotion.Annot.t option
 
+  val make
+    :  id:Id.t
+    -> description:[ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
+    -> ?dir:Path.t
+    -> ?promotion:Diff_promotion.Annot.t
+    -> unit
+    -> t
+
   module Event : sig
     type nonrec t =
       | Add of t

src/dune_rpc_impl/dune_rpc_impl.ml

@@ -5,6 +5,7 @@ module For_handlers = For_handlers
 module Private = Dune_rpc_client.Private
 module Watch_mode_config = Watch_mode_config
 module Where = Dune_rpc_client.Where
+module Diagnostics = Diagnostics
 
 module Poll_active =
   Dune_rpc_private.Registry.Poll

test/expect-tests/dune_rpc/dune

@@ -5,10 +5,12 @@
  (libraries
   ocaml_config
   dune_util
+  dune_console
   dune_rpc_private
   dune_rpc_server
   dune_rpc_impl
   dune_rpc_client
+  dune_engine
   stdune
   test_scheduler
   csexp

test/expect-tests/dune_rpc/dune_rpc_tests.ml

@@ -925,3 +925,19 @@ let%expect_test "print digests for all public RPCs" =
       Request: Sexp
       Response: 33528f248084297d123a6ebd4c3ddee0 |}]
 ;;
+
+let%expect_test "serialize and then derialize error messages" =
+  let id = Dune_engine.Build_system.Error.Id.gen () in
+  let main = User_error.make [ Pp.verbatim "Oh no!" ] in
+  Dune_console.print_user_message main;
+  let description =
+    `Diagnostic (Dune_engine.Compound_user_error.make ~main ~related:[])
+  in
+  Dune_engine.Build_system.Error.make ~id ~description ()
+  |> Dune_rpc_impl.Diagnostics.diagnostic_of_error
+  |> Dune_rpc_private.Diagnostic.to_user_message
+  |> Dune_console.print_user_message;
+  [%expect {|
+    Error: Oh no!
+    Error: Error: Oh no! |}]
+;;