MRF NoEmptyPolicy: Deny posts from local users if there is no content or only mentions.

Helps prevent accidental button mashes from submitting incomplete posts

lib/pleroma/web/activity_pub/mrf/no_empty_policy.ex

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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.NoEmptyPolicy do
+  @moduledoc "Filter local activities which have no content"
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  alias Pleroma.Web
+
+  @impl true
+  def filter(%{"actor" => actor} = object) do
+    with true <- is_local?(actor),
+         true <- is_note?(object),
+         false <- has_attachment?(object),
+         true <- only_mentions?(object) do
+      {:reject, "[NoEmptyPolicy]"}
+    else
+      _ ->
+        {:ok, object}
+    end
+  end
+
+  def filter(object), do: {:ok, object}
+
+  defp is_local?(actor) do
+    if actor |> String.starts_with?("#{Web.base_url()}") do
+      true
+    else
+      false
+    end
+  end
+
+  defp has_attachment?(%{
+         "type" => "Create",
+         "object" => %{"type" => "Note", "attachment" => attachments}
+       })
+       when length(attachments) > 0,
+       do: true
+
+  defp has_attachment?(_), do: false
+
+  defp only_mentions?(%{"type" => "Create", "object" => %{"type" => "Note", "source" => source}}) do
+    non_mentions =
+      source |> String.split() |> Enum.filter(&(not String.starts_with?(&1, "@"))) |> length
+
+    if non_mentions > 0 do
+      false
+    else
+      true
+    end
+  end
+
+  defp only_mentions?(_), do: false
+
+  defp is_note?(%{"type" => "Create", "object" => %{"type" => "Note"}}), do: true
+  defp is_note?(_), do: false
+
+  @impl true
+  def describe, do: {:ok, %{}}
+end