Make the kernel rewrite the base_url scheme

Seems to work nicely when I applied this on SimplyTest.me. Haven't done any further work though.

As instructed:

1. I set RequestHeader set X-Forwarded-Proto "https" in my Apache config,
2. I reloaded Apache,
3. I set $settings['reverse_proxy_proto_change'] = TRUE; and $base_url = 'http://test_d8.dev'; in settings.php, and,
4. I loaded the page over https.

Without the patch applied, all absolute links in the source (to stylesheets, etc) started with http (i.e.: used the unmodified $base_url). With the patch applied, all absolute links in the source (to stylesheets, etc) started with http (i.e.: used the modified $base_url). I'd say this is working as-intended.

The code looks good, but is missing tests.

I initially thought it might be really hard to test this because $base_url is a global, but there's already a bunch of tests in \Drupal\Tests\Core\Routing\UrlGeneratorTest::testPathBasedURLGeneration() that test generating URLs with custom $base_urls.

\Drupal\Tests\Core\Site\SettingsTest may also be useful, as it contains examples of how to set $settings within a test.

No longer applies.

ReRolled.

Test file has already been updated. Re-queuing it for testing.

The latest patch is still using ReverseProxySubscriber() for tests.

Here is updated patch.

I just did a code review. Seems to conform to Drupal coding standards.

A few things I noticed:

1. +++ b/core/tests/Drupal/Tests/Core/StackMiddleware/ReverseProxyMiddlewareTest.php
   @@ -36,11 +36,14 @@ public function setUp() {
   +    $this->assertEquals(FALSE, $settings->get('reverse_proxy'));
   
   @@ -54,9 +57,54 @@ public function testNoProxy() {
   +    $settings = new Settings(array('reverse_proxy' => 1) + $provided_settings);
   ...
   +      'reverse_proxy' => 1,
   ...
   +      'reverse_proxy' => 1,
   

   Inconsistent variable type usage (TRUE/FALSE vs 1/0).

2. +++ b/core/tests/Drupal/Tests/Core/StackMiddleware/ReverseProxyMiddlewareTest.php
   @@ -54,9 +57,54 @@ public function testNoProxy() {
   +      'reverse_proxy_addresses' => array('127.0.0.2'),
   ...
   +    $request->server->set('REMOTE_ADDR', '127.0.0.2');
   ...
   +      'reverse_proxy_addresses' => array('127.0.0.2'),
   ...
   +    $request->server->set('REMOTE_ADDR', '127.0.0.2');
   

   "127.0.0.2" seems like a "magic number (constant)". Should it be defined in a constant or variable in the test object?

   Probably not a big deal in a test.

... other than those things, it looks fine.

One more thing to note: if you want code committed to Drupal 8.0.x now that we're in beta, you'll need to fill out a Beta Evaluation to determine if it meets the criteria allowed changes during the Drupal 8 beta phase. If it doesn't apply, you'll have to mark it for Drupal 8.1.x. :S

Also reading through the core gates to see if this issue passes... a couple of notes:

Documentation gate:

1. An update to the issue summary would be nice (especially the Remaining tasks section; plus it would be good to add the User interface changes and API changes sections)
2. Is there a specification/best-practice for reverse proxies and/or the X_FORWARDED_* headers? If so, please link to it in both the issue summary and in the code so both the core maintainers looking to commit the changes and folks who stumble across the code in the future can read more about what it does / how it works and confirm you wrote code that uses those standards/best-practices.
3. You should also write a change record.

Performance gate: I don't think you need to do anything (doesn't apply).
Accessibility gate: I don't think you need to do anything (doesn't apply).
Usability gate: I don't think you need to do anything (doesn't apply).
Testing gate: I don't think you need to do anything (already done).