Enforce poll limits and add error handling for MastodonAPI's post endpoint

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -473,12 +473,6 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
       params
       |> Map.put("in_reply_to_status_id", params["in_reply_to_id"])
 
-    idempotency_key =
-      case get_req_header(conn, "idempotency-key") do
-        [key] -> key
-        _ -> Ecto.UUID.generate()
-      end
-
     scheduled_at = params["scheduled_at"]
 
     if scheduled_at && ScheduledActivity.far_enough?(scheduled_at) do
@@ -491,17 +485,40 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIController do
     else
       params = Map.drop(params, ["scheduled_at"])
 
-      {:ok, activity} =
-        Cachex.fetch!(:idempotency_cache, idempotency_key, fn _ ->
-          CommonAPI.post(user, params)
-        end)
+      case get_cached_status_or_post(conn, params) do
+        {:ignore, message} ->
+          conn
+          |> put_status(401)
+          |> json(%{error: message})
 
-      conn
-      |> put_view(StatusView)
-      |> try_render("status.json", %{activity: activity, for: user, as: :activity})
+        {:error, message} ->
+          conn
+          |> put_status(401)
+          |> json(%{error: message})
+
+        {_, activity} ->
+          conn
+          |> put_view(StatusView)
+          |> try_render("status.json", %{activity: activity, for: user, as: :activity})
+      end
     end
   end
 
+  defp get_cached_status_or_post(%{assigns: %{user: user}} = conn, params) do
+    idempotency_key =
+      case get_req_header(conn, "idempotency-key") do
+        [key] -> key
+        _ -> Ecto.UUID.generate()
+      end
+
+    Cachex.fetch(:idempotency_cache, idempotency_key, fn _ ->
+      case CommonAPI.post(user, params) do
+        {:ok, activity} -> activity
+        {:error, message} -> {:ignore, message}
+      end
+    end)
+  end
+
   def delete_status(%{assigns: %{user: user}} = conn, %{"id" => id}) do
     with {:ok, %Activity{}} <- CommonAPI.delete(id, user) do
       json(conn, %{})