MastoAPI: fix & test giving MRF reject reasons

lib/pleroma/web/mastodon_api/controllers/status_controller.ex

@@ -172,6 +172,11 @@ def create(%{assigns: %{user: user}, body_params: %{status: _} = params} = conn,
         with_direct_conversation_id: true
       )
     else
+      {:error, {:reject, message}} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: message})
+
       {:error, message} ->
         conn
         |> put_status(:unprocessable_entity)

test/web/mastodon_api/controllers/status_controller_test.exs

@@ -22,6 +22,8 @@ defmodule Pleroma.Web.MastodonAPI.StatusControllerTest do
   setup do: clear_config([:instance, :federating])
   setup do: clear_config([:instance, :allow_relay])
   setup do: clear_config([:rich_media, :enabled])
+  setup do: clear_config([:mrf, :policies])
+  setup do: clear_config([:mrf_keyword, :reject])
 
   describe "posting statuses" do
     setup do: oauth_access(["write:statuses"])
@@ -157,6 +159,17 @@ test "it fails to create a status if `expires_in` is less or equal than an hour"
                |> json_response_and_validate_schema(422)
     end
 
+    test "Get MRF reason when posting a status is rejected by one", %{conn: conn} do
+      Pleroma.Config.put([:mrf_keyword, :reject], ["GNO"])
+      Pleroma.Config.put([:mrf, :policies], [Pleroma.Web.ActivityPub.MRF.KeywordPolicy])
+
+      assert %{"error" => "[KeywordPolicy] Matches with rejected keyword"} =
+               conn
+               |> put_req_header("content-type", "application/json")
+               |> post("api/v1/statuses", %{"status" => "GNO/Linux"})
+               |> json_response_and_validate_schema(422)
+    end
+
     test "posting an undefined status with an attachment", %{user: user, conn: conn} do
       file = %Plug.Upload{
         content_type: "image/jpg",