Implement opt-in PERFLAB_PLACE_OBJECT_CACHE_DROPIN for Server-Timing object-cache.php placement

plugins/performance-lab/load.php

@@ -145,11 +145,13 @@
  * the frontend.
  *
  * This function will short-circuit if at least one of the constants
- * 'PERFLAB_DISABLE_SERVER_TIMING' or 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' is
- * set as true.
+ * 'PERFLAB_DISABLE_SERVER_TIMING' or
+ * 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' is set as true or if the
+ * 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN' constant is not set to a truthy value.
  *
  * @since 1.8.0
  * @since 2.1.0 No longer attempts to use two of the drop-ins together.
+ * @since n.e.x.t No longer places the drop-in on new sites by default, unless the `PERFLAB_PLACE_OBJECT_CACHE_DROPIN` constant is set to true.
  *
  * @global WP_Filesystem_Base $wp_filesystem WordPress filesystem subclass.
  */
@@ -161,7 +163,14 @@
 		return;
 	}
 
+	// Bail if the drop-in is not enabled.
+	if ( ! defined( 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN' ) || ! PERFLAB_PLACE_OBJECT_CACHE_DROPIN ) {
+		return;
+	}
+
 	// Bail if disabled via constant.
+	// This constant is maintained only for backward compatibility and should not be relied upon in new implementations.
+	// Use the 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN' constant instead to control drop-in placement.
 	if ( defined( 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN' ) && PERFLAB_DISABLE_OBJECT_CACHE_DROPIN ) {
 		return;
 	}

plugins/performance-lab/tests/test-load.php

@@ -7,6 +7,21 @@
 
 class Test_Load extends WP_UnitTestCase {
 
+	/**
+	 * Runs the routine before each test is executed.
+	 */
+	public function set_up(): void {
+		parent::set_up();
+
+		/*
+		 * This constant is not set by default in production.
+		 * However, it is needed for all tests that cover placement of the object cache drop-in.
+		 */
+		if ( ! defined( 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN' ) ) {
+			define( 'PERFLAB_PLACE_OBJECT_CACHE_DROPIN', true );
+		}
+	}
+
 	public function test_perflab_get_generator_content(): void {
 		$expected = 'performance-lab ' . PERFLAB_VERSION . '; plugins: ';
 		$content  = perflab_get_generator_content();