fix(feed): format RSS build dates

lib/pleroma/web/feed/feed_view.ex

@@ -38,6 +38,20 @@ defmodule Pleroma.Web.Feed.FeedView do
     end
   end
 
+  def most_recent_update([], :rss), do: nil
+
+  def most_recent_update(activities, :rss) do
+    activities
+    |> Enum.map(fn activity ->
+      case Object.normalize(activity, fetch: false) do
+        %Object{updated_at: updated_at} -> updated_at
+        _ -> activity.updated_at
+      end
+    end)
+    |> Enum.max_by(&NaiveDateTime.to_erl/1)
+    |> to_rfc2822()
+  end
+
   def most_recent_update(activities, user, :atom) do
     (List.first(activities) || user).updated_at
     |> to_rfc3339()

lib/pleroma/web/templates/feed/feed/tag.rss.eex

@@ -12,7 +12,9 @@
     <webfeeds:logo><%= feed_logo() %></webfeeds:logo>
     <webfeeds:icon><%= feed_logo() %></webfeeds:icon>
     <webfeeds:accentColor>2b90d9</webfeeds:accentColor>
-    <lastBuildDate><%= most_recent_update(@activities) || to_rfc2822(NaiveDateTime.utc_now()) %></lastBuildDate>
+    <%= with last_build_date when not is_nil(last_build_date) <- most_recent_update(@activities, :rss) do %>
+      <lastBuildDate><%= last_build_date %></lastBuildDate>
+    <% end %>
     <generator><%= Pleroma.Application.named_version() %></generator>
     <%= for activity <- @activities do %>
     <%= render Phoenix.Controller.view_module(@conn), "_tag_activity.xml", Map.merge(assigns, prepare_activity(activity)) %>

test/pleroma/web/feed/tag_controller_test.exs

@@ -105,8 +105,10 @@ defmodule Pleroma.Web.Feed.TagControllerTest do
         }
       ])
 
+    edited_at = NaiveDateTime.add(object.updated_at, 60)
+
     object
-    |> Ecto.Changeset.change(data: object_data)
+    |> Ecto.Changeset.change(data: object_data, updated_at: edited_at)
     |> Pleroma.Repo.update()
 
     {:ok, activity2} = CommonAPI.post(user, %{status: "42 This is :moominmamma #PleromaArt"})
@@ -141,6 +143,9 @@ defmodule Pleroma.Web.Feed.TagControllerTest do
              FeedView.to_rfc2822(activity1.data["published"])
            ]
 
+    assert xpath(xml, ~x"//channel/lastBuildDate/text()"s) ==
+             FeedView.to_rfc2822(edited_at)
+
     assert xpath(xml, ~x"//channel/item/media:content/@url"sl) == [
              "https://peertube.moe/static/webseed/df5f464b-be8d-46fb-ad81-2d4c2d1630e3-480.mp4"
            ]
@@ -181,6 +186,16 @@ defmodule Pleroma.Web.Feed.TagControllerTest do
            ]
   end
 
+  test "does not fabricate lastBuildDate for an empty RSS feed", %{conn: conn} do
+    response =
+      conn
+      |> put_req_header("accept", "application/rss+xml")
+      |> get(tag_feed_path(conn, :feed, "empty.rss"))
+      |> response(200)
+
+    refute response =~ "<lastBuildDate>"
+  end
+
   describe "private instance" do
     setup do: clear_config([:instance, :public], false)