Add OAuth scopes

https://github.com/foodcoops/foodsoft/issues/582#issuecomment-442513237

app/controllers/concerns/auth_api.rb

@@ -0,0 +1,66 @@
+# Controller concern for authentication methods
+#
+# Split off from main +ApplicationController+ to allow e.g.
+# Doorkeeper to use it too.
+module Concerns::AuthApi
+  extend ActiveSupport::Concern
+
+  protected
+
+  def authenticate
+    # authenticate does not look at scopes, we just want to know if there's a valid token
+    # @see https://github.com/doorkeeper-gem/doorkeeper/blob/v5.0.1/lib/doorkeeper/models/access_token_mixin.rb#L218
+    doorkeeper_render_error unless doorkeeper_token && doorkeeper_token.accessible?
+    super if current_user
+  end
+
+  # @return [User] Current user, or +nil+ if no valid token.
+  def current_user
+    @current_user ||= User.undeleted.find(doorkeeper_token.resource_owner_id) if doorkeeper_token
+  end
+
+  def doorkeeper_authorize!(*scopes)
+    super(*scopes)
+    # In addition to Doorkeeper's authorization and scope check, we also verify
+    # that the user has permissions for the scope (through its workgroups).
+    # Unless no scopes were supplied, which means we only want to make sure there
+    # is a valid user.
+    #
+    # If Doorkeeper's +handle_auth_errors+ is set to +:raise+, we don't get here,
+    # but otherwise we need to check whether +super+ rendered an error.
+    doorkeeper_authorize_roles!(*scopes) if scopes.present? && !performed?
+  end
+
+  private
+
+  # Make sure that at least one the given OAuth scopes is valid for the current user's permissions.
+  # @raise Api::Errors::PermissionsRequired
+  def doorkeeper_authorize_roles!(*scopes)
+    unless scopes.any? {|scope| doorkeeper_scope_permitted?(scope) }
+      raise Api::Errors::PermissionRequired.new('Forbidden, no permission')
+    end
+  end
+
+  # Check whether a given OAuth scope is permitted for the current user.
+  # @note This does not validate whether the given scope is a valid scope.
+  # @return [Boolean] Whether the given scope is valid for the current user.
+  # @see https://github.com/foodcoops/foodsoft/issues/582#issuecomment-442513237
+  def doorkeeper_scope_permitted?(scope)
+    scope_parts = scope.split(':')
+    # user sub-scopes like +config:user+ are always permitted
+    if scope_parts.last == 'user'
+      return true
+    end
+
+    case scope_parts.first
+    when 'user'           then true # access to the current user's own profile
+    when 'config'         then current_user.role_admin?
+    when 'users'          then current_user.role_admin?
+    when 'workgroups'     then current_user.role_admin?
+    when 'suppliers'      then current_user.role_suppliers?
+    when 'group_orders'   then current_user.role_orders?
+    when 'finance'        then current_user.role_finance?
+    # please note that offline_access does not belong here, since it is not used for permission checking
+    end
+  end
+end