Add initial docs page, improve adding domains

Signed-off-by: Marcin Mikołajczak <git@mkljczk.pl>

config/description.exs

@@ -1061,6 +1061,18 @@
         suggestions: [
           "en"
         ]
+      },
+      %{
+        key: :multitenancy,
+        type: :map,
+        description: "Multitenancy support",
+        children: [
+          %{
+            key: :enabled,
+            type: :boolean,
+            description: "Enables allowing multiple Webfinger domains"
+          },
+        ]
       }
     ]
   },
@@ -3466,5 +3478,5 @@
         ]
       }
     ]
-  }
+  },
 ]

docs/configuration/how_to_serve_multiple_domains_for_webfinger.md

@@ -0,0 +1,30 @@
+# How to serve multiple domains for Pleroma user identifiers
+
+It is possible to use multiple domains for WebFinger identifiers. If configured, users can select from the available domains during registration. Domains can be set by instance administrator and can be marked as either public (everyone can choose it) or private (only available when admin creates a user)
+
+## Configuring
+
+### Configuring Pleroma
+
+To enable using multiple domains, append the following to your `prod.secret.exs` or `dev.secret.exs`:
+```elixir
+config :pleroma, :instance, :multitenancy, enabled: true
+```
+
+Creating, updating and deleting domains is available from the admin API.
+
+### Configuring WebFinger domains
+
+If you recall how webfinger queries work, the first step is to query `https://example.org/.well-known/host-meta`, which will contain an URL template.
+
+Therefore, the easiest way to configure the additional domains is to redirect `/.well-known/host-meta` to the domain used by Pleroma.
+
+With nginx, it would be as simple as adding:
+
+```nginx
+location = /.well-known/host-meta {
+       return 301 https://pleroma.example.org$request_uri;
+}
+```
+
+in the additional domain's server block.

lib/pleroma/domain.ex

@@ -19,11 +19,13 @@ def changeset(%__MODULE__{} = domain, params \\ %{}) do
     domain
     |> cast(params, [:domain, :public])
     |> validate_required([:domain])
+    |> update_change(:domain, &String.downcase/1)
+    |> unique_constraint(:domain)
   end
 
   def update_changeset(%__MODULE__{} = domain, params \\ %{}) do
     domain
-    |> cast(params, [:domain])
+    |> cast(params, [:public])
   end
 
   def get(id), do: Repo.get(__MODULE__, id)

lib/pleroma/web/admin_api/controllers/domain_controller.ex

@@ -37,11 +37,13 @@ def index(conn, _) do
   end
 
   def create(%{body_params: params} = conn, _) do
-    domain =
-      params
-      |> Domain.create()
-
-    render(conn, "show.json", domain: domain)
+    with {:domain_not_used, true} <-
+           {:domain_not_used, params[:domain] !== Pleroma.Web.WebFinger.domain()},
+         {:domain, domain} <- Domain.create(params) do
+      render(conn, "show.json", domain: domain)
+    else
+      {:domain_not_used, false} -> {:error, :invalid_domain}
+    end
   end
 
   def update(%{body_params: params} = conn, %{id: id}) do

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

@@ -147,7 +147,7 @@ def fields_limits do
   end
 
   defp multitenancy do
-    enabled = Config.get([:multitenancy, :enabled])
+    enabled = Config.get([:instance, :multitenancy, :enabled])
 
     if enabled do
       domains =

lib/pleroma/web/web_finger.ex

@@ -102,7 +102,7 @@ defp get_subject(%User{nickname: nickname}) do
     end
   end
 
-  defp domain do
+  def domain do
     Pleroma.Config.get([__MODULE__, :domain]) || Pleroma.Web.Endpoint.host()
   end
 

priv/repo/migrations/20230618190919_create_domains.exs

@@ -3,7 +3,7 @@ defmodule Pleroma.Repo.Migrations.CreateDomains do
 
   def change do
     create_if_not_exists table(:domains) do
-      add(:domain, :string)
+      add(:domain, :citext)
       add(:public, :boolean)
 
       timestamps()