fix(notifications): handle unknown users when filtering tagged mentions

The access check from the previous commit crashed on tags of unknown
users in followers-only posts, so such posts failed to be received.
Also add tests for the check.

changelog.d/mention-notify-access.security

@@ -0,0 +1 @@
+Do not notify tagged users who cannot see the post

lib/pleroma/web/common_api/utils.ex

@@ -465,7 +465,7 @@ defmodule Pleroma.Web.CommonAPI.Utils do
 
       true ->
         author = User.get_cached_by_ap_id(actor)
-        to_followers = author.follower_address in recipients
+        to_followers = match?(%User{}, author) and author.follower_address in recipients
 
         Enum.filter(tagged_mentions, fn tagged ->
           cond do
@@ -473,8 +473,10 @@ defmodule Pleroma.Web.CommonAPI.Utils do
               true
 
             to_followers ->
-              tagged_user = User.get_cached_by_ap_id(tagged)
+              case User.get_cached_by_ap_id(tagged) do
-              User.following?(tagged_user, author)
+                %User{} = tagged_user -> User.following?(tagged_user, author)
+                _ -> false
+              end
 
             true ->
               false

test/pleroma/notification_test.exs

@@ -7,6 +7,8 @@ defmodule Pleroma.NotificationTest do
 
   import Pleroma.Factory
 
+  require Pleroma.Constants
+
   alias Pleroma.FollowingRelationship
   alias Pleroma.Notification
   alias Pleroma.Repo
@@ -611,6 +613,102 @@ defmodule Pleroma.NotificationTest do
       assert other_user in enabled_receivers
     end
 
+    defp incoming_note_tagging(author, to, tagged_ap_ids) do
+      tags = Enum.map(tagged_ap_ids, &%{"type" => "Mention", "href" => &1, "name" => &1})
+
+      {:ok, activity} =
+        Transmogrifier.handle_incoming(%{
+          "@context" => "https://www.w3.org/ns/activitystreams",
+          "type" => "Create",
+          "to" => to,
+          "actor" => author.ap_id,
+          "object" => %{
+            "type" => "Note",
+            "id" => Pleroma.Web.ActivityPub.Utils.generate_object_id(),
+            "to" => to,
+            "content" => "tagged",
+            "tag" => tags,
+            "attributedTo" => author.ap_id
+          }
+        })
+
+      activity
+    end
+
+    defp notified_ap_ids(activity),
+      do: activity |> Notification.get_notified_from_activity() |> Enum.map(& &1.ap_id)
+
+    test "it does not notify tagged users who cannot see a followers-only post" do
+      author = insert(:user)
+      tagged = insert(:user)
+
+      activity = incoming_note_tagging(author, [author.follower_address], [tagged.ap_id])
+
+      refute tagged.ap_id in notified_ap_ids(activity)
+    end
+
+    test "it notifies tagged followers of a followers-only post" do
+      author = insert(:user)
+      tagged = insert(:user)
+      {:ok, _, _} = Pleroma.User.follow(tagged, author)
+
+      activity = incoming_note_tagging(author, [author.follower_address], [tagged.ap_id])
+
+      assert tagged.ap_id in notified_ap_ids(activity)
+    end
+
+    test "it does not notify tagged users who are not addressed by a direct message" do
+      author = insert(:user)
+      addressed = insert(:user)
+      tagged = insert(:user)
+
+      activity = incoming_note_tagging(author, [addressed.ap_id], [tagged.ap_id])
+      receivers = notified_ap_ids(activity)
+
+      assert addressed.ap_id in receivers
+      refute tagged.ap_id in receivers
+    end
+
+    test "it notifies tagged users of a public post even if they are not addressed" do
+      author = insert(:user)
+      tagged = insert(:user)
+
+      activity =
+        incoming_note_tagging(author, [Pleroma.Constants.as_public()], [tagged.ap_id])
+
+      assert tagged.ap_id in notified_ap_ids(activity)
+    end
+
+    test "it does not notify tagged users with a pending follow request" do
+      author = insert(:user, is_locked: true)
+      tagged = insert(:user)
+      {:ok, _, _, _} = CommonAPI.follow(author, tagged)
+
+      activity = incoming_note_tagging(author, [author.follower_address], [tagged.ap_id])
+
+      refute tagged.ap_id in notified_ap_ids(activity)
+    end
+
+    test "it ignores tags of unknown users in a followers-only post" do
+      author = insert(:user)
+      tagged = insert(:user)
+
+      follower = insert(:user)
+      {:ok, _, _} = Pleroma.User.follow(follower, author)
+
+      activity =
+        incoming_note_tagging(author, [author.follower_address], [
+          "https://unknown.example/users/nobody",
+          tagged.ap_id,
+          follower.ap_id
+        ])
+
+      receivers = notified_ap_ids(activity)
+
+      refute tagged.ap_id in receivers
+      assert follower.ap_id in receivers
+    end
+
     test "it does not send notifications to users who are only cc in new messages" do
       user = insert(:user)
       other_user = insert(:user)