implemented remote users unfollow me. untested.

lib/vonbraun/activity_pub/handler/undo.ex

@@ -1,11 +1,35 @@
 defmodule Vonbraun.ActivityPub.Handler.Undo do
   @behaviour Vonbraun.ActivityPub.HandlerBehaviour
 
+  require Logger
+  alias Vonbraun.Ecto.Schema.Actor
+  alias Vonbraun.ActivityPub.Object
+  import Vonbraun.ActivityPub.Handler.Accept, only: [extract_follow_object_actor: 1]
+
   @verb "Undo"
 
   def type, do: @verb
 
   # Lots of different kinds of things can be undone.
+
+  def handle(%{"type" => @verb, "actor" => actor_id, "object" => object = %{"type" => "Follow"}}) do
+    with {:actor, {:ok, follow_actor_id}} <- {:actor, extract_follow_object_actor(object)},
+         {:match, true} <- {:match, follow_actor_id == Object.my_id()},
+         {:asked, {:ok, %Actor{:following_state => nil}}} <-
+           {:asked, Actor.remove_follower(actor_id)} do
+      {:ok, :removed_follower}
+    else
+      {:asked, {:error, error}} ->
+        {:error, error}
+
+      {:match, false} ->
+        {:ok, :unauthorized}
+
+      {:actor, {:error, _error}} ->
+        {:ok, :unauthorized}
+    end
+  end
+
   def handle(%{"type" => @verb}, %{}) do
     {:ok, :unknown}
   end

lib/vonbraun/activity_pub/handler_behaviour.ex

@@ -1,5 +1,5 @@
 defmodule Vonbraun.ActivityPub.HandlerBehaviour do
   @callback type() :: String.t()
   @callback handle(activity :: map(), actor_object :: map()) ::
-              :ok | {:ok, atom()} | {:error, any()}
+              {:ok, atom()} | {:error, any()}
 end

lib/vonbraun/ecto/schema/actor.ex

@@ -49,6 +49,11 @@ defmodule Vonbraun.Ecto.Schema.Actor do
     end
   end
 
+  @doc """
+  Mark a remote actor as following me, or pending follow, depending on if
+  approval of followers is enabled. Tries to intelligently ignore repeat and
+  spam follows.
+  """
   @spec maybe_add_follower(String.t()) :: {:ok, __MODULE__.t()} | {:error, Changeset.t()}
   def maybe_add_follower(id) when is_binary(id) do
     with {:actor, {:ok, actor = %{:follows_me_state => follows_me_state, :blocked => blocked?}}} <-
@@ -79,6 +84,9 @@ defmodule Vonbraun.Ecto.Schema.Actor do
     end
   end
 
+  @doc """
+  Mark a remote actor as no longer following me.
+  """
   @spec remove_follower(String.t()) :: {:ok, __MODULE__.t() | nil} | {:error, Changeset.t()}
   def remove_follower(id) when is_binary(id) do
     case Repo.get(__MODULE__, id) do
@@ -86,10 +94,11 @@ defmodule Vonbraun.Ecto.Schema.Actor do
         {:ok, nil}
 
       actor = %{:follows_me_state => follows_me_state} ->
-        if follows_me_state == "accepted" do
+        if follows_me_state in ["accepted", "pending"] do
           changeset(actor, %{follows_me_state: nil, follows_me_ts: DateTime.now!("Etc/UTC")})
           |> Repo.update()
         else
+          # nil or rejected already.
           {:ok, actor}
         end
     end