Wrap application's fallback handler to provider debug headers

config/di-web.php

@@ -5,6 +5,7 @@
 use Cycle\Database\DatabaseProviderInterface;
 use Psr\Container\ContainerInterface;
 use Yiisoft\Db\Connection\ConnectionInterface;
+use Yiisoft\Yii\Debug\Api\Debug\Middleware\DebugFallbackHandler;
 use Yiisoft\Yii\Debug\Api\Debug\Repository\CollectorRepository;
 use Yiisoft\Yii\Debug\Api\Debug\Repository\CollectorRepositoryInterface;
 use Yiisoft\Yii\Debug\Api\Inspector\Database\Cycle\CycleSchemaProvider;
@@ -34,4 +35,9 @@
             )
         );
     },
+    DebugFallbackHandler::class => [
+        '__construct()' => [
+            'middlewareDefinitions' => $params['yiisoft/yii-debug-api']['fallbackHandler']['middlewares'],
+        ],
+    ],
 ];

config/params.php

@@ -3,6 +3,7 @@
 declare(strict_types=1);
 
 use Codeception\Extension;
+use Yiisoft\Yii\Debug\Api\Debug\Middleware\DebugHeaders;
 use Yiisoft\Yii\Debug\Api\Inspector\Command\CodeceptionCommand;
 use Yiisoft\Yii\Debug\Api\Inspector\Command\PHPUnitCommand;
 use Yiisoft\Yii\Debug\Api\Inspector\Command\PsalmCommand;
@@ -34,6 +35,11 @@
                 ],
             ],
         ],
+        'fallbackHandler' => [
+            'middlewares' => [
+                DebugHeaders::class,
+            ],
+        ]
     ],
     'yiisoft/yii-swagger' => [
         'annotation-paths' => [

src/Debug/Http/DebugHttpApplicationWrapper.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Yiisoft\Yii\Debug\Api\Debug\Http;
+
+use Closure;
+use Yiisoft\Yii\Debug\Api\Debug\Middleware\DebugFallbackHandler;
+use Yiisoft\Yii\Http\Application;
+
+final readonly class DebugHttpApplicationWrapper
+{
+    public function __construct(
+        private DebugFallbackHandler $debugFallbackHandler
+    ) {
+    }
+
+    public function wrap(Application $application): void
+    {
+        $debugFallbackHandler = $this->debugFallbackHandler;
+        $closure = Closure::bind(static function (Application $application) use ($debugFallbackHandler) {
+            $application->fallbackHandler = $debugFallbackHandler->withFallbackRequestHandler($application->fallbackHandler);
+        }, null, $application);
+
+        $closure($application);
+    }
+}

src/Debug/Middleware/DebugFallbackHandler.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Yiisoft\Yii\Debug\Api\Debug\Middleware;
+
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+use Yiisoft\Middleware\Dispatcher\MiddlewareDispatcher;
+
+final class DebugFallbackHandler implements MiddlewareInterface, RequestHandlerInterface
+{
+    private ?RequestHandlerInterface $fallbackHandler = null;
+
+    public function __construct(
+        private readonly MiddlewareDispatcher $middlewareDispatcher,
+        private readonly array $middlewareDefinitions,
+    ) {
+    }
+
+    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
+    {
+        return $this->middlewareDispatcher
+            ->withMiddlewares($this->middlewareDefinitions)
+            ->dispatch($request, $handler);
+    }
+
+    public function handle(ServerRequestInterface $request): ResponseInterface
+    {
+        if ($this->fallbackHandler === null) {
+            throw new \RuntimeException('No fallback handler defined.');
+        }
+
+        return $this->process($request, $this->fallbackHandler);
+    }
+
+    public function withFallbackRequestHandler(RequestHandlerInterface $fallbackHandler): self
+    {
+        $new = clone $this;
+        $new->fallbackHandler = $fallbackHandler;
+
+        return $new;
+    }
+}

src/Debug/Provider/DebugApiProvider.php

@@ -7,7 +7,9 @@
 use Psr\Container\ContainerInterface;
 use Yiisoft\Di\ServiceProviderInterface;
 use Yiisoft\Router\RouteCollectorInterface;
+use Yiisoft\Yii\Debug\Api\Debug\Http\DebugHttpApplicationWrapper;
 use Yiisoft\Yii\Debug\Api\Debug\Middleware\DebugHeaders;
+use Yiisoft\Yii\Http\Application;
 
 final class DebugApiProvider implements ServiceProviderInterface
 {
@@ -22,10 +24,19 @@ public function getDefinitions(): array
     public function getExtensions(): array
     {
         return [
-            RouteCollectorInterface::class => static function (ContainerInterface $container, RouteCollectorInterface $routeCollector) {
+            RouteCollectorInterface::class => static function (
+                ContainerInterface $container,
+                RouteCollectorInterface $routeCollector
+            ) {
                 $routeCollector->prependMiddleware(DebugHeaders::class);
                 return $routeCollector;
             },
+            Application::class => static function (ContainerInterface $container, Application $application) {
+                $applicationWrapper = $container->get(DebugHttpApplicationWrapper::class);
+                $applicationWrapper->wrap($application);
+
+                return $application;
+            },
         ];
     }
 }