Working commit

.gitignore

@@ -0,0 +1,4 @@
+coverage
+doc
+.yardoc
+*.gem

README.md

@@ -1,2 +1,39 @@
 OmniAuth::Mastodon
 ==================
+
+[![Gem Version](http://img.shields.io/gem/v/omniauth-mastodon.svg)][gem]
+
+[gem]: https://rubygems.org/gems/omniauth-mastodon
+
+Authentication strategy for federated Mastodon instances. This is just slightly more complicated than a traditional OAuth2 flow: We do not know the URL of the OAuth end-points in advance, nor can we be sure that
+we already have client credentials for that Mastodon instance.
+
+## Installation
+
+    gem 'mastodon-api', require: 'mastodon'
+    gem 'omniauth-mastodon'
+    gem 'omniauth'
+
+## Configuration
+
+Example:
+
+```ruby
+Rails.application.config.middleware.use OmniAuth::Builder do
+  provider :mastodon, credentials: lambda { |domain, callback_url|
+    Rails.logger.info "Requested credentials for #{domain} with callback URL #{callback_url}"
+
+    existing = MastodonClient.find_by(domain: domain)
+    return [existing.client_id, existing.client_secret] unless existing.nil?
+
+    client = Mastodon::REST::Client.new(base_url: "https://#{domain}")
+    app = client.create_app('OmniAuth Test Harness', callback_url)
+
+    MastodonClient.create!(domain: domain, client_id: app.client_id, client_secret: app.client_secret)
+
+    [app.client_id, app.client_secret]
+  }
+end
+```
+
+The only configuration key you need to set is a lambda for `:credentials`. That lambda will be called whenever we need to get client credentials for OAuth2 requests. The example above uses an ActiveRecord model to store client credentials for different Mastodon domains, and uses the `mastodon-api` gem to fetch them dynamically if they're not stored yet.

lib/omniauth/mastodon/version.rb

@@ -8,11 +8,11 @@ module OmniAuth
       end
 
       def minor
-        0
+        9
       end
 
       def patch
-        1
+        0
       end
 
       def pre

lib/omniauth/strategies/mastodon.rb

@@ -5,9 +5,15 @@ module OmniAuth
     class Mastodon < OmniAuth::Strategies::OAuth2
       option :name, 'mastodon'
 
-      option :domain
+      option :credentials
+      option :identifier
 
-      uid { [raw_info['username'], '@', options.domain].join }
+      option :client_options, {
+        authorize_url: '/oauth/authorize',
+        token_url: '/oauth/token'
+      }
+
+      uid { identifier }
 
       info do
         {
@@ -22,9 +28,55 @@ module OmniAuth
         { raw_info: raw_info }
       end
 
+      # Before we can redirect the user to authorize access, we must know where the user is from
+      # If the identifier param is not already present, a form will be shown for entering it
+      def request_phase
+        identifier ? start_oauth : get_identifier
+      end
+
+      def callback_phase
+        set_options_from_identifier
+        super
+      end
+
       def raw_info
         @raw_info ||= access_token.get('api/v1/accounts/verify_credentials').parsed
       end
+
+      def callback_url
+        full_host + script_name + callback_path
+      end
+
+      private
+
+      def get_identifier
+        form = OmniAuth::Form.new(title: 'Mastodon Login')
+        form.text_field 'Your full Mastodon identifier', 'identifier'
+        form.button 'Login'
+        form.to_response
+      end
+
+      def start_oauth
+        set_options_from_identifier
+        redirect client.auth_code.authorize_url({:redirect_uri => callback_url}.merge(authorize_params))
+      end
+
+      def identifier
+        i = options.identifier || request.params['identifier'] || (env['omniauth.params'].is_a?(Hash) ? env['omniauth.params']['identifier'] : nil)
+        i = i.downcase.strip unless i.nil?
+        i = nil if i == ''
+        i
+      end
+
+      def set_options_from_identifier
+        username, domain         = identifier.split('@')
+        client_id, client_secret = options.credentials.call(domain, callback_url)
+
+        options.identifier            = identifier
+        options.client_options[:site] = "https://#{domain}"
+        options.client_id             = client_id
+        options.client_secret         = client_secret
+      end
     end
   end
 end