add constraint_to option to SSL middleware

Author: gregmolnar

actionpack/lib/action_dispatch/middleware/ssl.rb

@@ -34,6 +34,10 @@ module ActionDispatch
   # original HSTS directive until it expires. Instead, use the header to tell browsers to
   # expire HSTS immediately. Setting `hsts: false` is a shortcut for
   # `hsts: { expires: 0 }`.
+  #
+  # Redirection can be constrained to only whitelisted requests with `constrain_to`:
+  #
+  #    config.ssl_options = { redirect: { constrain_to: -> request { request.path !~ /healthcheck/ } } }
   class SSL
     # Default to 180 days, the low end for https://www.ssllabs.com/ssltest/
     # and greater than the 18-week requirement for browser preload lists.
@@ -55,7 +59,7 @@ def initialize(app, redirect: {}, hsts: {}, secure_cookies: true, **options)
       else
         @redirect = redirect
       end
-
+      @constrain_to = @redirect && @redirect[:constrain_to] || proc { @redirect }
       @secure_cookies = secure_cookies
 
       if hsts != true && hsts != false && hsts[:subdomains].nil?
@@ -80,7 +84,7 @@ def call(env)
           flag_cookies_as_secure! headers if @secure_cookies
         end
       else
-        return redirect_to_https request if @redirect
+        return redirect_to_https request if @constrain_to.call(request)
         @app.call(env)
       end
     end

actionpack/test/dispatch/ssl_test.rb

@@ -39,6 +39,13 @@ def assert_redirected(redirect: {}, deprecated_host: nil, deprecated_port: nil,
     assert_equal redirect[:body].join, @response.body
   end
 
+  test 'constrain to can avoid redirect' do
+    constraining = { constrain_to: -> request { request.path !~ /healthcheck/ } }
+
+    assert_not_redirected 'http://example.org/healthcheck', redirect: constraining
+    assert_redirected from: 'http://example.org/', redirect: constraining
+  end
+
   test 'https is not redirected' do
     assert_not_redirected 'https://example.org'
   end