Add event listener to check schema on config save

1. +++ b/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php
   @@ -0,0 +1,94 @@
   +   * Removes stale static cache entries when configuration is saved.
   

   That's not the main thing this method does. Also, could use an @throws.

2. +++ b/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php
   @@ -0,0 +1,94 @@
   +    // Check that we have a valid for all config saved during tests. Do not
   

   A valid what?

3. +++ b/core/modules/config/src/Tests/SchemaConfigListenerTest.php
   @@ -0,0 +1,70 @@
   +  public function testTrait() {
   

   Does this test have anything to do with traits? Same for the web test.

4. +++ b/core/modules/config/src/Tests/SchemaConfigListenerTest.php
   @@ -0,0 +1,70 @@
   +    // Test a non existing schema.
   

   Nice comment. Could use one for the two other code blocks. Same for the web test.

Thanks!

Why not do this on every config save, rather than only during tests? That'd greatly improve DX. Is this really so slow? Do we have so many config writes?

Only found silly nitpicks in review.

1. +++ b/core/lib/Drupal/Core/Config/Testing/ConfigSchemaChecker.php
   @@ -0,0 +1,93 @@
   +   * @param ConfigCrudEvent $event
   

   Namespace missing.

2. +++ b/core/modules/config/src/Tests/SchemaConfigListenerTest.php
   @@ -0,0 +1,72 @@
   +    // Test a non existing schema.
   
   +++ b/core/modules/config/src/Tests/SchemaConfigListenerWebTest.php
   @@ -0,0 +1,77 @@
   +    // Test a non existing schema.
   

   s/non existing/non-existing/

3. +++ b/core/modules/config/src/Tests/SchemaConfigListenerWebTest.php
   @@ -0,0 +1,77 @@
   +    $msg = 'Expected SchemaIncompleteException thrown';
   

   I don't know of Drupal using such shorthand variable names anywhere else.

4. +++ b/core/modules/config/src/Tests/SchemaConfigListenerWebTest.php
   @@ -0,0 +1,77 @@
   +      $this->pass('Schema errors for config_test.types with the following errors: config_test.types:foo: Missing schema., config_test.types:array: Variable type is integer but applied schema class is Drupal\Core\Config\Schema\Sequence.');
   

   <message>., <next> looks weird (period followed by comma).

diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 730f84f..6792b6f 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -816,6 +816,16 @@ protected function setUp() {
       // testing specific overrides
       file_put_contents($directory . '/settings.php', "\n\$test_class = '" . get_class($this) ."';\n" . 'include DRUPAL_ROOT . \'/\' . $site_path . \'/settings.testing.php\';' ."\n", FILE_APPEND);
     }
+    if ($this->strictConfigSchema) {
+      $yaml = new \Symfony\Component\Yaml\Yaml();
+      $services = $yaml->parse($directory . '/services.yml');
+      $services['services']['simpletest.config_schema_checker'] = [
+        'class' => 'Drupal\Core\Config\Testing\ConfigSchemaChecker',
+        'arguments' => ['@config.typed'],
+        'tags' => [['name' => 'event_subscriber']]
+      ];
+      file_put_contents($directory . '/services.yml', $yaml->dump($services));
+    }
     $settings_services_file = DRUPAL_ROOT . '/' . $this->originalSite . '/testing.services.yml';
     if (file_exists($settings_services_file)) {
       // Copy the testing-specific service overrides in place.

Sorry for coming so late to this, but this breaks the testing.services.yml support introduced in #2229011: Tests are no longer modifiable. Specifically, the line that comes directly after the cutoff in the hunk above, copies the testing services file (if it exists) to $directory . '/services.yml', thus, it conflicts with the addition here. Both should be possible to have at the same time.

Yes, that's very neat. I hadn't thought of that.

What about #5?

Why not do this on every config save, rather than only during tests? That'd greatly improve DX. Is this really so slow? Do we have so many config writes?

Follow-up, because we don't want to block this issue on that discussion?

D'oh, right, I think I've been told this in IRC already. Very much looking forward to getting instant feedback when a config entity (or config schema!) is wrong! :) That'll be a great DX improvement.

Committed/pushed to 8.0.x, thanks!