ChatMessage: Tolerate attachment field set to an empty array

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/3224

changelog.d/chat-attachment-empty-array.fix

@@ -0,0 +1 @@
+ChatMessage: Tolerate attachment field set to an empty array

lib/pleroma/web/activity_pub/object_validators/chat_message_validator.ex

@@ -57,6 +57,11 @@ def fix_attachment(%{"attachment" => [attachment | _]} = data) do
     |> Map.put("attachment", attachment)
   end
 
+  def fix_attachment(%{"attachment" => attachment} = data) when attachment == [] do
+    data
+    |> Map.drop(["attachment"])
+  end
+
   def fix_attachment(data), do: data
 
   def changeset(struct, data) do

test/pleroma/web/activity_pub/object_validators/chat_validation_test.exs

@@ -147,6 +147,21 @@ test "validates for a basic object with an attachment but without content", %{
       assert object["attachment"]
     end
 
+    test "validates for a basic object with content but attachment set to empty array", %{
+      user: user,
+      recipient: recipient
+    } do
+      {:ok, valid_chat_message, _} = Builder.chat_message(user, recipient.ap_id, "Hello!")
+
+      valid_chat_message =
+        valid_chat_message
+        |> Map.put("attachment", [])
+
+      assert {:ok, object, _meta} = ObjectValidator.validate(valid_chat_message, [])
+
+      assert object == Map.drop(valid_chat_message, ["attachment"])
+    end
+
     test "does not validate if the message has no content", %{
       valid_chat_message: valid_chat_message
     } do