3 files changed, 20 insertions, 0 deletions
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index d52ccd3d5e..5123713c6b 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Always use the provided port if the protocol is relative.
+
+    Fixes #15043.
+
+    *Guilherme Cavalcanti*, *Andrew White*
+
 *   Moved `params[request_forgery_protection_token]` into its own method
     and improved tests.
 
diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index 77ea0cf658..c9860af909 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -135,6 +135,8 @@ module ActionDispatch
           return nil if options[:port].nil? || options[:port] == false
 
           case options[:protocol]
+          when "//"
+            options[:port]
           when "https://"
             options[:port].to_i == 443 ? nil : options[:port]
           else
diff --git a/actionpack/test/dispatch/url_generation_test.rb b/actionpack/test/dispatch/url_generation_test.rb
index fdea27e2d2..910ff8a80f 100644
--- a/actionpack/test/dispatch/url_generation_test.rb
+++ b/actionpack/test/dispatch/url_generation_test.rb
@@ -64,18 +64,30 @@ module TestUrlGeneration
 
     test "port is extracted from the host" do
       assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com:8080", protocol: "http://")
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com:8080", protocol: "//")
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com:80", protocol: "//")
+    end
+
+    test "port option is used" do
+      assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com", protocol: "http://", port: 8080)
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com", protocol: "//", port: 8080)
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com", protocol: "//", port: 80)
     end
 
     test "port option overrides the host" do
       assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: 8080)
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: 8080)
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com:443", protocol: "//", port: 80)
     end
 
     test "port option disables the host when set to nil" do
       assert_equal "http://www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: nil)
+      assert_equal "//www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: nil)
     end
 
     test "port option disables the host when set to false" do
       assert_equal "http://www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: false)
+      assert_equal "//www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: false)
     end
 
     test "keep subdomain when key is true" do