mrf/object_age: fix handling of non-public objects

Current logic unconditionally adds public adressing to "cc"
and follower adressing to "to" after attempting to strip it
from the other one. This creates serious problems:

First the bug prompting this investigation and fix,
unconditional addition creates duplicates when adressing
URIs already were in their intended final field; e.g.
this is prominently the case for all "unlisted" posts.
Since List.delete only removes the first occurence,
this then broke follower-adress stripping later on
making the policy ineffective.

It’s also just not safe in general wrt to non-public adressing:
e.g. pre-existing duplicates didn’t get fully stripped,
bespoke adressing modes with only one of public addressing
or follower addressing are mangled — and most importantly:
any belatedly received DM or follower-only post
also got public adressing added!
Shockingly this last point was actually asserted as "correct" in tests;
it appears to be a mistake from mindless match adjustments
while fixing crashes on nil adressing in
10c792110e.

Clean up this sloppy logic up, making sure no more duplicates are
added by us, all instances of relevant adresses are purged and only
readded when they actually existed to begin with.

CHANGELOG.md

@@ -23,6 +23,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - AP objects with additional JSON-LD profiles beyond ActivityStreams can now be fetched
 - Single-selection polls no longer expose the voter_count; MastoAPI demands it be null
   and this confused some clients leading to vote distributions >100%
+- ObjectAge policy no longer lets unlisted posts slip through
+- ObjectAge policy no longer leaks belated DMs and follower-only posts
 
 ## Changed
 - Refactored Rich Media to cache the content in the database. Fetching operations that could block status rendering have been eliminated.

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

@@ -34,16 +34,34 @@ defp check_reject(message, actions) do
     end
   end
 
+  @spec delete_and_count(list(), term()) :: {integer(), list()}
+  defp delete_and_count(list, element), do: delete_and_count(list, element, {0, [], list})
+
+  defp delete_and_count([], _element, {0, _nlist, olist}), do: {0, olist}
+  defp delete_and_count([], _element, {count, nlist, _olist}), do: {count, Enum.reverse(nlist)}
+
+  defp delete_and_count([h | r], h, {count, nlist, olist}),
+    do: delete_and_count(r, h, {count + 1, nlist, olist})
+
+  defp delete_and_count([h | r], element, {count, nlist, olist}),
+    do: delete_and_count(r, element, {count, [h | nlist], olist})
+
+  defp insert_if_needed(list, oldcount, element) do
+    if oldcount <= 0 || Enum.member?(list, element) do
+      list
+    else
+      [element | list]
+    end
+  end
+
   defp check_delist(message, actions) do
     if :delist in actions do
       with %User{} = user <- User.get_cached_by_ap_id(message["actor"]) do
-        to =
-          List.delete(message["to"] || [], Pleroma.Constants.as_public()) ++
-            [user.follower_address]
+        {pubcnt, to} = delete_and_count(message["to"] || [], Pleroma.Constants.as_public())
+        {flwcnt, cc} = delete_and_count(message["cc"] || [], user.follower_address)
 
-        cc =
-          List.delete(message["cc"] || [], user.follower_address) ++
-            [Pleroma.Constants.as_public()]
+        cc = insert_if_needed(cc, pubcnt, Pleroma.Constants.as_public())
+        to = insert_if_needed(to, flwcnt, user.follower_address)
 
         message =
           message
@@ -65,8 +83,8 @@ defp check_delist(message, actions) do
   defp check_strip_followers(message, actions) do
     if :strip_followers in actions do
       with %User{} = user <- User.get_cached_by_ap_id(message["actor"]) do
-        to = List.delete(message["to"] || [], user.follower_address)
-        cc = List.delete(message["cc"] || [], user.follower_address)
+        {_, to} = delete_and_count(message["to"] || [], user.follower_address)
+        {_, cc} = delete_and_count(message["cc"] || [], user.follower_address)
 
         message =
           message

test/pleroma/web/activity_pub/mrf/object_age_policy_test.exs

@@ -79,7 +79,7 @@ test "works with objects with empty to or cc fields" do
 
       {:ok, data} = ObjectAgePolicy.filter(data)
 
-      assert Visibility.get_visibility(%{data: data}) == "unlisted"
+      assert Visibility.get_visibility(%{data: data}) == "direct"
     end
 
     test "it delists an old post" do