feat(instances): record the instance of new remote users

The instances table only got rows from fetches, reachability changes
and favicon lookups, so most hosts we know users from had none. Insert
a row when a remote user is created, keeping existing rows.

lib/pleroma/instances/instance.ex

@@ -91,6 +91,13 @@ defmodule Pleroma.Instances.Instance do
 
   def reachable?(url_or_host) when is_binary(url_or_host), do: true
 
+  @doc "Creates a record for the instance unless one exists. Existing records are kept."
+  def ensure_exists(url_or_host) when is_binary(url_or_host) do
+    %Instance{host: host(url_or_host)}
+    |> changeset(%{})
+    |> Repo.insert(on_conflict: :nothing, conflict_target: :host)
+  end
+
   def set_reachable(url_or_host) when is_binary(url_or_host) do
     host = host(url_or_host)
 

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -11,6 +11,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
   alias Pleroma.Conversation.Participation
   alias Pleroma.Filter
   alias Pleroma.Hashtag
+  alias Pleroma.Instances.Instance
   alias Pleroma.Maps
   alias Pleroma.Notification
   alias Pleroma.Object
@@ -2068,10 +2069,11 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
       else
         maybe_handle_clashing_nickname(data)
 
-        data
-        |> User.remote_user_changeset()
-        |> Repo.insert()
-        |> User.set_cache()
+        with {:ok, user} <- data |> User.remote_user_changeset() |> Repo.insert() do
+          # Keeps the instances table complete, so it can list our peers.
+          Instance.ensure_exists(user.ap_id)
+          User.set_cache({:ok, user})
+        end
       end
     end
   end