Retrieving plugins with entity context definitions from cache is expensive, which is noticeable when used with Layout Builder's FieldBlockDeriver

I'm not quite sure what the fix for this is. Perhaps the deriver could only create blocks for entity types that had layout builder enabled, or perhaps there is a way to __wakeup a context definition without calling initializeEntityContextDefinition?

I have spun off #3043330: Reduce the number of field blocks created for entities (possibly to zero) to look into reducing the size of the block plugin list.

That's a dramatic speed-up, blackfire comparison here. Looks like a 20% speed-up of the whole page when clicking the "Add block" button.

Pulling out the relevant parts from the profiler, before:

After:

Comparison:

I think a suitable fix for the test case might simply be to add a call to getConstraints. The test is using reflection, so I don't think it's testing a public API. Asserting it's there after triggering an API call seems appropriate:

--- a/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php
@@ -80,6 +80,7 @@ protected function setUp() {
    */
   public function testSerialization() {
     $definition = unserialize(serialize($this->definition));
+    $definition->getConstraints();
     $bc_layer = $this->compatibilityLayer->getValue($definition);
     $this->assertInstanceOf(EntityContextDefinition::class, $bc_layer);

Good stuff, thanks for the review and fix!

Does the fix in #14 need a test? I had a play and this seems enough to ensure the constraints are added during initialization:

diff --git a/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php b/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php
index 8053a3d669..fc861e8c62 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/Context/EntityContextDefinitionDeprecationTest.php
@@ -79,8 +79,9 @@ protected function setUp() {
    * @expectedDeprecation Constructing a ContextDefinition object for an entity type is deprecated in Drupal 8.6.0. Use Drupal\Core\Plugin\Context\EntityContextDefinition instead. See https://www.drupal.org/node/2976400 for more information.
    */
   public function testSerialization() {
+    $this->definition->addConstraint('foo');
     $definition = unserialize(serialize($this->definition));
-    $definition->getConstraints();
+    $this->assertEquals(['foo' => NULL], $definition->getConstraints());
     $bc_layer = $this->compatibilityLayer->getValue($definition);
     $this->assertInstanceOf(EntityContextDefinition::class, $bc_layer);
   }

Beyond that, looks ready to me.

Good stuff, back to RTBC.

Fail was a fluke. Updating issue summary.