add support for named appends

README.md

@@ -94,6 +94,61 @@ use SecureHeaders::Middleware
 
 All headers except for PublicKeyPins have a default value. See the [corresponding classes for their defaults](https://github.com/twitter/secureheaders/tree/master/lib/secure_headers/headers).
 
+## Named Appends
+
+Named Appends are blocks of code that can be reused and composed during requests. e.g. If a certain partial is rendered conditionally, and the csp needs to be adjusted for that partial, you can create a named append for that situation. The value returned by the block will be passed into `append_content_security_policy_directives`. The current request object is passed as an argument to the block for even more flexibility.
+
+```ruby
+def show
+  if include_widget?
+    @widget = widget.render
+    use_content_security_policy_named_append(:widget_partial)
+  end
+end
+
+
+SecureHeaders::Configuration.named_append(:widget_partial) do |request|
+  if request.controller_instance.current_user.in_test_bucket?
+    { child_src: %w(beta.thirdpartyhost.com) }
+  else
+    { child_src: %w(thirdpartyhost.com) }
+  end
+end
+```
+
+You can use as many named appends as you would like per request, but be careful because order of inclusion matters. Consider the following:
+
+```ruby
+SecureHeader::Configuration.default do |config|
+  config.csp = { default_src: %w('self')}
+end
+
+SecureHeaders::Configuration.named_append(:A) do |request|
+  { default_src: %w(myhost.com) }
+end
+
+SecureHeaders::Configuration.named_append(:B) do |request|
+  { script_src: %w('unsafe-eval') }
+end
+```
+
+The following code will produce different policies due to the way policies are normalized (e.g. providing a previously undefined directive that inherits from `default-src`, removing host source values when `*` is provided. Removing `'none'` when additional values are present, etc.):
+
+```ruby
+def index
+  use_content_security_policy_named_append(:A)
+  use_content_security_policy_named_append(:B)
+  # produces default-src 'self' myhost.com; script-src 'self' myhost.com 'unsafe-eval';
+end
+
+def show
+  use_content_security_policy_named_append(:B)
+  use_content_security_policy_named_append(:A)
+  # produces default-src 'self' myhost.com; script-src 'self' 'unsafe-eval';
+end
+```
+
+
 ## Named overrides
 
 Named overrides serve two purposes:

lib/secure_headers.rb

@@ -72,6 +72,11 @@ def append_content_security_policy_directives(request, additions)
       override_secure_headers_request_config(request, config)
     end
 
+    def use_content_security_policy_named_append(request, name)
+      additions = SecureHeaders::Configuration.named_appends(name).call(request)
+      append_content_security_policy_directives(request, additions)
+    end
+
     # Public: override X-Frame-Options settings for this request.
     #
     # value - deny, sameorigin, or allowall
@@ -267,4 +272,8 @@ def override_content_security_policy_directives(additions)
   def override_x_frame_options(value)
     SecureHeaders.override_x_frame_options(request, value)
   end
+
+  def use_content_security_policy_named_append(name)
+    SecureHeaders.use_content_security_policy_named_append(request, name)
+  end
 end

lib/secure_headers/configuration.rb

@@ -46,6 +46,17 @@ def get(name = DEFAULT_CONFIG)
         @configurations[name]
       end
 
+      def named_appends(name)
+        @appends ||= {}
+        @appends[name]
+      end
+
+      def named_append(name, target = nil, &block)
+        @appends ||= {}
+        raise "Provide a configuration block" unless block_given?
+        @appends[name] = block
+      end
+
       private
 
       # Private: add a valid configuration to the global set of named configs.

spec/lib/secure_headers_spec.rb

@@ -151,6 +151,28 @@ module SecureHeaders
           expect(hash[CSP::HEADER_NAME]).to eq("default-src 'self'; script-src mycdn.com 'unsafe-inline' anothercdn.com")
         end
 
+        it "supports named appends" do
+          Configuration.default do |config|
+            config.csp = {
+              default_src: %w('self')
+            }
+          end
+
+          Configuration.named_append(:moar_default_sources) do |request|
+            { default_src: %w(https:)}
+          end
+
+          Configuration.named_append(:how_about_a_script_src_too) do |request|
+            { script_src: %w('unsafe-inline')}
+          end
+
+          SecureHeaders.use_content_security_policy_named_append(request, :moar_default_sources)
+          SecureHeaders.use_content_security_policy_named_append(request, :how_about_a_script_src_too)
+          hash = SecureHeaders.header_hash_for(request)
+
+          expect(hash[CSP::HEADER_NAME]).to eq("default-src 'self' https:; script-src 'self' https: 'unsafe-inline'")
+        end
+
         it "dups global configuration just once when overriding n times and only calls idempotent_additions? once" do
           Configuration.default do |config|
             config.csp = {