3 files changed, 85 insertions, 1 deletions

diff --git a/lib/pleroma/web/activity_pub/activity_pub.ex b/lib/pleroma/web/activity_pub/activity_pub.ex
index d43f85ee4..491ad3705 100644
--- a/lib/pleroma/web/activity_pub/activity_pub.ex
+++ b/lib/pleroma/web/activity_pub/activity_pub.ex
@@ -144,6 +144,24 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
     end
   end
 
+  def unannounce(
+        %User{} = actor,
+        %Object{} = object,
+        activity_id \\ nil,
+        local \\ true
+      ) do
+    with %Activity{} = announce_activity <- get_existing_announce(actor.ap_id, object),
+         unannounce_data <- make_unannounce_data(actor, announce_activity, activity_id),
+         {:ok, unannounce_activity} <- insert(unannounce_data, local),
+         :ok <- maybe_federate(unannounce_activity),
+         {:ok, _activity} <- Repo.delete(announce_activity),
+         {:ok, object} <- remove_announce_from_object(announce_activity, object) do
+      {:ok, unannounce_activity, announce_activity, object}
+    else
+      _e -> {:ok, object}
+    end
+  end
+
   def follow(follower, followed, activity_id \\ nil, local \\ true) do
     with data <- make_follow_data(follower, followed, activity_id),
          {:ok, activity} <- insert(data, local),
diff --git a/lib/pleroma/web/activity_pub/transmogrifier.ex b/lib/pleroma/web/activity_pub/transmogrifier.ex
index 5f86f67cd..463d1e59d 100644
--- a/lib/pleroma/web/activity_pub/transmogrifier.ex
+++ b/lib/pleroma/web/activity_pub/transmogrifier.ex
@@ -223,9 +223,27 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     end
   end
 
+  def handle_incoming(
+        %{
+          "type" => "Undo",
+          "object" => %{"type" => "Announce", "object" => object_id},
+          "actor" => actor,
+          "id" => id
+        } = data
+      ) do
+    with %User{} = actor <- User.get_or_fetch_by_ap_id(actor),
+         {:ok, object} <-
+           get_obj_helper(object_id) || ActivityPub.fetch_object_from_id(object_id),
+         {:ok, activity, _, _} <- ActivityPub.unannounce(actor, object, id, false) do
+      {:ok, activity}
+    else
+      e -> :error
+    end
+  end
+
   # TODO
   # Accept
-  # Undo
+  # Undo for non-Announce
 
   def handle_incoming(_), do: :error
 
diff --git a/lib/pleroma/web/activity_pub/utils.ex b/lib/pleroma/web/activity_pub/utils.ex
index 7b2bf8fa7..f98545336 100644
--- a/lib/pleroma/web/activity_pub/utils.ex
+++ b/lib/pleroma/web/activity_pub/utils.ex
@@ -238,6 +238,28 @@ defmodule Pleroma.Web.ActivityPub.Utils do
   #### Announce-related helpers
 
   @doc """
+  Retruns an existing announce activity if the notice has already been announced
+  """
+  def get_existing_announce(actor, %{data: %{"id" => id}}) do
+    query =
+      from(
+        activity in Activity,
+        where: fragment("(?)->>'actor' = ?", activity.data, ^actor),
+        # this is to use the index
+        where:
+          fragment(
+            "coalesce((?)->'object'->>'id', (?)->>'object') = ?",
+            activity.data,
+            activity.data,
+            ^id
+          ),
+        where: fragment("(?)->>'type' = 'Announce'", activity.data)
+      )
+
+    Repo.one(query)
+  end
+
+  @doc """
   Make announce activity data for the given actor and object
   """
   def make_announce_data(
@@ -257,12 +279,38 @@ defmodule Pleroma.Web.ActivityPub.Utils do
     if activity_id, do: Map.put(data, "id", activity_id), else: data
   end
 
+  @doc """
+  Make unannounce activity data for the given actor and object
+  """
+  def make_unannounce_data(
+        %User{ap_id: ap_id} = user,
+        %Activity{data: %{"context" => context}} = activity,
+        activity_id
+      ) do
+    data = %{
+      "type" => "Undo",
+      "actor" => ap_id,
+      "object" => activity.data,
+      "to" => [user.follower_address, activity.data["actor"]],
+      "cc" => ["https://www.w3.org/ns/activitystreams#Public"],
+      "context" => context
+    }
+
+    if activity_id, do: Map.put(data, "id", activity_id), else: data
+  end
+
   def add_announce_to_object(%Activity{data: %{"actor" => actor}}, object) do
     with announcements <- [actor | object.data["announcements"] || []] |> Enum.uniq() do
       update_element_in_object("announcement", announcements, object)
     end
   end
 
+  def remove_announce_from_object(%Activity{data: %{"actor" => actor}}, object) do
+    with announcements <- (object.data["announcements"] || []) |> List.delete(actor) do
+      update_element_in_object("announcement", announcements, object)
+    end
+  end
+
   #### Unfollow-related helpers
 
   def make_unfollow_data(follower, followed, follow_activity) do