1 files changed, 168 insertions, 0 deletions
diff --git a/actionpack/lib/action_dispatch/http/feature_policy.rb b/actionpack/lib/action_dispatch/http/feature_policy.rb
new file mode 100644
index 0000000000..c09690f9fc
--- /dev/null
+++ b/actionpack/lib/action_dispatch/http/feature_policy.rb
@@ -0,0 +1,168 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/object/deep_dup"
+
+module ActionDispatch #:nodoc:
+  class FeaturePolicy
+    class Middleware
+      CONTENT_TYPE = "Content-Type"
+      POLICY       = "Feature-Policy"
+
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        request = ActionDispatch::Request.new(env)
+        _, headers, _ = response = @app.call(env)
+
+        return response unless html_response?(headers)
+        return response if policy_present?(headers)
+
+        if policy = request.feature_policy
+          headers[POLICY] = policy.build(request.controller_instance)
+        end
+
+        if policy_empty?(policy)
+          headers.delete(POLICY)
+        end
+
+        response
+      end
+
+      private
+        def html_response?(headers)
+          if content_type = headers[CONTENT_TYPE]
+            /html/.match?(content_type)
+          end
+        end
+
+        def policy_present?(headers)
+          headers[POLICY]
+        end
+
+        def policy_empty?(policy)
+          policy&.directives&.empty?
+        end
+    end
+
+    module Request
+      POLICY = "action_dispatch.feature_policy"
+
+      def feature_policy
+        get_header(POLICY)
+      end
+
+      def feature_policy=(policy)
+        set_header(POLICY, policy)
+      end
+    end
+
+    MAPPINGS = {
+      self: "'self'",
+      none: "'none'",
+    }.freeze
+
+    # List of available features can be found at
+    # https://github.com/WICG/feature-policy/blob/master/features.md#policy-controlled-features
+    DIRECTIVES = {
+      accelerometer:        "accelerometer",
+      ambient_light_sensor: "ambient-light-sensor",
+      autoplay:             "autoplay",
+      camera:               "camera",
+      encrypted_media:      "encrypted-media",
+      fullscreen:           "fullscreen",
+      geolocation:          "geolocation",
+      gyroscope:            "gyroscope",
+      magnetometer:         "magnetometer",
+      microphone:           "microphone",
+      midi:                 "midi",
+      payment:              "payment",
+      picture_in_picture:   "picture-in-picture",
+      speaker:              "speaker",
+      usb:                  "usb",
+      vibrate:              "vibrate",
+      vr:                   "vr",
+    }.freeze
+
+    private_constant :MAPPINGS, :DIRECTIVES
+
+    attr_reader :directives
+
+    def initialize
+      @directives = {}
+      yield self if block_given?
+    end
+
+    def initialize_copy(other)
+      @directives = other.directives.deep_dup
+    end
+
+    DIRECTIVES.each do |name, directive|
+      define_method(name) do |*sources|
+        if sources.first
+          @directives[directive] = apply_mappings(sources)
+        else
+          @directives.delete(directive)
+        end
+      end
+    end
+
+    def build(context = nil)
+      build_directives(context).compact.join("; ")
+    end
+
+    private
+      def apply_mappings(sources)
+        sources.map do |source|
+          case source
+          when Symbol
+            apply_mapping(source)
+          when String, Proc
+            source
+          else
+            raise ArgumentError, "Invalid HTTP feature policy source: #{source.inspect}"
+          end
+        end
+      end
+
+      def apply_mapping(source)
+        MAPPINGS.fetch(source) do
+          raise ArgumentError, "Unknown HTTP feature policy source mapping: #{source.inspect}"
+        end
+      end
+
+      def build_directives(context)
+        @directives.map do |directive, sources|
+          if sources.is_a?(Array)
+            "#{directive} #{build_directive(sources, context).join(' ')}"
+          elsif sources
+            directive
+          else
+            nil
+          end
+        end
+      end
+
+      def build_directive(sources, context)
+        sources.map { |source| resolve_source(source, context) }
+      end
+
+      def resolve_source(source, context)
+        case source
+        when String
+          source
+        when Symbol
+          source.to_s
+        when Proc
+          if context.nil?
+            raise RuntimeError, "Missing context for the dynamic feature policy source: #{source.inspect}"
+          else
+            context.instance_exec(&source)
+          end
+        else
+          raise RuntimeError, "Unexpected feature policy source: #{source.inspect}"
+        end
+      end
+  end
+end