Allow to set #cache metadata in hook_page_attachments()

Adding additional assertions to PageRenderTest seems sufficient.

In a follow-up, we should make all hook_page_attachments(), hook_page_attachments_alter(), hook_page_top() and hook_page_bottom() implementations set the right cacheability metadata.

For example, quickedit_page_attachments() only adds its asset library if the current user has the right permission (vary by the 'user.permissions' cache context) and it's not an admin route (no cache context for that yet).

For example, quickedit_page_attachments() only adds its asset library if the current user has the right permission

That is something I need for Google Analytics, too.

+++ b/core/modules/system/src/Tests/Common/PageRenderTest.php
@@ -54,6 +54,9 @@ function assertPageRenderHookExceptions($module, $hook) {
+    // Assert that hooks can set cache tags.
+    $this->assertEqual($page['#cache']['tags'], ['example']);

+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -224,6 +224,7 @@ function common_test_cron() {
+  $page['#cache']['tags'] = ['example'];

@@ -248,6 +249,7 @@ function common_test_page_attachments_alter(array &$page) {
+  $page['#cache']['tags'] = ['example'];

Can you also add a cache context, just for the sake of completeness? :) E.g. the user.permissions cache context, which is likely the one that'll be used most often in the real world.

Then this is RTBC.

(But I should file a new issue for #2.)

This should be ready then, don't think testbot will fail on this.

Can someone open a followup for #2.

This issue is a normal bug fix, and doesn't include any disruptive changes, so it is allowed per https://www.drupal.org/core/beta-changes. Committed 5b560a5 and pushed to 8.0.x. Thanks!

Is there any documentation how these cache metadata works and when and how modules need to implement it?

This is not clear to me from this case.

#11: https://www.drupal.org/developing/api/8/render/arrays/cacheability

Filed an issue for #2: #2478393: hook_page_(attachments|attachments_alter|top|bottom)() should specify the right cacheability metadata. Initial patch included.