From b63eaa34be79ecc3b469c20cc82fc5a365d9e00c Mon Sep 17 00:00:00 2001
From: miraikumiko <miraikumiko@disroot.org>
Date: Thu, 12 Jun 2025 16:05:18 +0200
Subject: [PATCH] Add Webfinger

---
 lib/nulla/activitypub.ex                      | 13 +++++++
 lib/nulla/models/user.ex                      |  2 ++
 .../controllers/webfinger_controller.ex       | 34 +++++++++++++++++++
 lib/nulla_web/router.ex                       |  2 ++
 4 files changed, 51 insertions(+)
 create mode 100644 lib/nulla_web/controllers/webfinger_controller.ex

diff --git a/lib/nulla/activitypub.ex b/lib/nulla/activitypub.ex
index e3c93f8..2e58798 100644
--- a/lib/nulla/activitypub.ex
+++ b/lib/nulla/activitypub.ex
@@ -243,4 +243,17 @@ defmodule Nulla.ActivityPub do
 
     Jason.OrderedObject.new(data)
   end
+
+  def webfinger(domain, username, resource) do
+    Jason.OrderedObject.new(
+      subject: resource,
+      links: [
+        Jason.OrderedObject.new(
+          rel: "self",
+          type: "application/activity+json",
+          href: "https://#{domain}/@#{username}"
+        )
+      ]
+    )
+  end
 end
diff --git a/lib/nulla/models/user.ex b/lib/nulla/models/user.ex
index cd20f48..caf90c5 100644
--- a/lib/nulla/models/user.ex
+++ b/lib/nulla/models/user.ex
@@ -77,5 +77,7 @@ defmodule Nulla.Models.User do
     ])
   end
 
+  def get_user_by_username(username), do: Repo.get_by(User, username: username)
+
   def get_user_by_username!(username), do: Repo.get_by!(User, username: username)
 end
diff --git a/lib/nulla_web/controllers/webfinger_controller.ex b/lib/nulla_web/controllers/webfinger_controller.ex
new file mode 100644
index 0000000..ab6e2bf
--- /dev/null
+++ b/lib/nulla_web/controllers/webfinger_controller.ex
@@ -0,0 +1,34 @@
+defmodule NullaWeb.WebfingerController do
+  use NullaWeb, :controller
+  alias Nulla.Repo
+  alias Nulla.ActivityPub
+  alias Nulla.Models.User
+  alias Nulla.Models.InstanceSettings
+
+  def show(conn, %{"resource" => resource}) do
+    case Regex.run(~r/^acct:([^@]+)@(.+)$/, resource) do
+      [_, username, domain] ->
+        case User.get_user_by_username(username) do
+          nil ->
+            conn
+            |> put_status(:not_found)
+            |> json(%{error: "Not Found"})
+
+          user ->
+            instance_settings = InstanceSettings.get_instance_settings!()
+
+            if domain == instance_settings.domain do
+              json(conn, ActivityPub.webfinger(domain, username, resource))
+            else
+              conn
+              |> put_status(:not_found)
+              |> json(%{error: "Not Found"})
+            end
+        end
+      _ ->
+        conn
+        |> put_status(:bad_request)
+        |> json(%{error: "Bad Request"})
+    end
+  end
+end
diff --git a/lib/nulla_web/router.ex b/lib/nulla_web/router.ex
index 04268ed..0ad10df 100644
--- a/lib/nulla_web/router.ex
+++ b/lib/nulla_web/router.ex
@@ -21,6 +21,8 @@ defmodule NullaWeb.Router do
   scope "/", NullaWeb do
     pipe_through :browser
 
+    get "/.well-known/webfinger", WebfingerController, :show
+
     get "/@:username", UserController, :show
     get "/@:username/following", FollowController, :following
     get "/@:username/followers", FollowController, :followers