SimpleTestTest is broken

The Simpletest test executes the test runner within a child site, which sets up a grandchild site.

However, the grandchild site is accessed through the internal browser, so the test runs in a different process.

Consequently, I do not understand this patch. The global variable should not have any impact.

I still don't understand.

The grandchild site is set up in a separate request. Which means that the test prefix of the child site should no longer be known in the grandchild site. The setUp() method of the child site and the setUp() of the grandchild site are executed in different environments.

Is the actual problem that SimpletestTest::setUp() does not set up a new database/test prefix for the grandchild site before checking drupal_valid_test_ua()?
http://drupalcode.org/project/drupal.git/blob/HEAD:/core/modules/simplet...

Also:

+++ b/core/includes/bootstrap.inc
@@ -504,7 +504,7 @@ function find_conf_path($http_host, $script_name, $require_settings = TRUE) {
 function config_get_config_directory($type = CONFIG_ACTIVE_DIRECTORY) {
...
-  if ($test_prefix = drupal_valid_test_ua()) {
+  if (empty($GLOBALS['config_directories_force']) && $test_prefix = drupal_valid_test_ua()) {
     // @see Drupal\simpletest\WebTestBase::setUp()
     $path = conf_path() . '/files/simpletest/' . substr($test_prefix, 10) . '/config_' . $type;
   }

The new condition means that a test might NOT use the config directories prepared by Simpletest for the test run.

That's not what we want, because it means that a test may write into the wrong config directory -- namely, the config directory of the parent site. That must never ever happen.

Yeah, that makes a little more sense. The static in drupal_valid_test_ua() is likely the culprit. We can replace it with a drupal_static() and reset only this one from TestBase::prepareEnvironment(), or alternatively, add a $reset argument. (That said, there's also #1436684: Remove static cache in drupal_valid_test_ua(), but I still think it's in the critical path and called relatively often.)

TestBase::prepareEnvironment() already sets up the new $GLOBALS['drupal_test_info'], but we probably need to move that above the creation of config directories and the container rebuild (right in the same method), so the call to config_get_config_directory() originating from drupal_container() will pick up the new database prefix already.

For sqlite we have more troubles - it creates new database and does not uses table prefixes #1799310: WebTestBase->setUp() broken for sqlite

Current solution extracted from #1608842: Replace list of bootstrap modules, enabled modules, enabled themes, and default theme with config

#1436684: Remove static cache in drupal_valid_test_ua() might be a better solution instead of this, if the I/O and performance impact is acceptable.

#14 went in with #1608842: Replace list of bootstrap modules, enabled modules, enabled themes, and default theme with config, but the fix is actually wrong.

To be continued in #1436684: Remove static cache in drupal_valid_test_ua()

Updated issue summary.