api/masto/instance: use WebFinger domain for URI

Despite its name this property is not supposed to be a full URI,
but just a bare domain witout protocol. Furthermore, it’s supposed
to be the WebFinger domain used in userhandles and NOT the domain used
for API and ActivityPub objects (which every caller will already know
anyway).

Not following this caused issues for Pachli and Tusky.

Reported-by: nikclayton

lib/pleroma/web/mastodon_api/views/instance_view.ex

@@ -14,7 +14,7 @@ def render("show.json", _) do
     instance = Config.get(:instance)
 
     %{
-      uri: Pleroma.Web.Endpoint.url(),
+      uri: Pleroma.Web.WebFinger.domain(),
       title: Keyword.get(instance, :name),
       description: Keyword.get(instance, :description),
       short_description:

test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs

@@ -18,9 +18,12 @@ test "get instance information", %{conn: conn} do
     thumbnail = Pleroma.Web.Endpoint.url() <> Pleroma.Config.get([:instance, :instance_thumbnail])
     background = Pleroma.Web.Endpoint.url() <> Pleroma.Config.get([:instance, :background_image])
 
+    # if no WebFinger domain is configured (without protocol)
+    uri = Pleroma.Web.Endpoint.host()
+
     # Note: not checking for "max_toot_chars" since it's optional
     assert %{
-             "uri" => _,
+             "uri" => ^uri,
              "title" => _,
              "description" => _,
              "short_description" => _,
@@ -55,6 +58,15 @@ test "get instance information", %{conn: conn} do
     assert background == from_config_background
   end
 
+  test "get instance information prefers WebFinger domain for uri", %{conn: conn} do
+    webfinger_domain = "webfinger.example"
+    clear_config([Pleroma.Web.WebFinger, :domain], webfinger_domain)
+    conn = get(conn, "/api/v1/instance")
+
+    assert result = json_response_and_validate_schema(conn, 200)
+    assert match?(%{"uri" => ^webfinger_domain}, result)
+  end
+
   test "get instance stats", %{conn: conn} do
     user = insert(:user, %{local: true})