Add ForceMentionsInContentPolicy

Even though latest PleromaFE supports displaying these properly, mobile
apps still exist, so I think we should offer a workaround to those who
want it.

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

@@ -0,0 +1,80 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.ForceMentionsInContent do
+  alias Pleroma.Formatter
+  alias Pleroma.User
+
+  @behaviour Pleroma.Web.ActivityPub.MRF.Policy
+
+  defp do_extract({:a, attrs, _}, acc) do
+    if Enum.find(attrs, fn {name, value} ->
+         name == "class" && value in ["mention", "u-url mention", "mention u-url"]
+       end) do
+      href = Enum.find(attrs, fn {name, _} -> name == "href" end) |> elem(1)
+      acc ++ [href]
+    else
+      acc
+    end
+  end
+
+  defp do_extract({_, _, children}, acc) do
+    do_extract(children, acc)
+  end
+
+  defp do_extract(nodes, acc) when is_list(nodes) do
+    Enum.reduce(nodes, acc, fn node, acc -> do_extract(node, acc) end)
+  end
+
+  defp do_extract(_, acc), do: acc
+
+  defp extract_mention_uris_from_content(content) do
+    {:ok, tree} = :fast_html.decode(content, format: [:html_atoms])
+    do_extract(tree, [])
+  end
+
+  @impl true
+  def filter(%{"type" => "Create", "object" => %{"type" => "Note", "tag" => tag}} = object) do
+    # image-only posts from pleroma apparently reach this MRF without the content field
+    content = object["object"]["content"] || ""
+
+    mention_users =
+      tag
+      |> Enum.filter(fn tag -> tag["type"] == "Mention" end)
+      |> Enum.map(& &1["href"])
+      |> Enum.reject(&is_nil/1)
+      |> Enum.map(fn ap_id_or_uri ->
+        case User.get_or_fetch_by_ap_id(ap_id_or_uri) do
+          {:ok, user} -> {ap_id_or_uri, user}
+          _ -> {ap_id_or_uri, User.get_by_uri(ap_id_or_uri)}
+        end
+      end)
+      |> Enum.reject(fn {_, user} -> user == nil end)
+      |> Enum.into(%{})
+
+    explicitly_mentioned_uris = extract_mention_uris_from_content(content)
+
+    added_mentions =
+      Enum.reduce(mention_users, "", fn {uri, user}, acc ->
+        unless uri in explicitly_mentioned_uris do
+          acc <> Formatter.mention_from_user(user)
+        else
+          acc
+        end
+      end)
+
+    content =
+      if added_mentions != "",
+        do: added_mentions <> " " <> content,
+        else: content
+
+    {:ok, put_in(object["object"]["content"], content)}
+  end
+
+  @impl true
+  def filter(object), do: {:ok, object}
+
+  @impl true
+  def describe, do: {:ok, %{}}
+end