Store non-global compiled DI areas as their diff from global

lib/internal/Magento/Framework/App/ObjectManager/Environment/Compiled.php

@@ -8,6 +8,7 @@
 
 use Magento\Framework\App\EnvironmentInterface;
 use Magento\Framework\App\Interception\Cache\CompiledConfig;
+use Magento\Framework\ObjectManager\ConfigLoaderInterface;
 use Magento\Framework\ObjectManager\FactoryInterface;
 use Magento\Framework\App\Area;
 use Magento\Framework\Interception\ObjectManager\ConfigInterface;
@@ -64,7 +65,8 @@ public function getDiConfig()
     {
         if (!$this->config) {
             $this->config = new \Magento\Framework\Interception\ObjectManager\Config\Compiled(
-                $this->getConfigData()
+                $this->getConfigData(),
+                $this->getObjectManagerConfigLoader()
             );
         }
 
@@ -78,7 +80,8 @@ public function getDiConfig()
      */
     protected function getConfigData()
     {
-        return $this->getObjectManagerConfigLoader()->load(Area::AREA_GLOBAL);
+        return $this->getObjectManagerConfigLoader()->load(Area::AREA_GLOBAL)
+            + [ConfigLoaderInterface::AREA_KEY => Area::AREA_GLOBAL];
     }
 
     /**
@@ -103,11 +106,7 @@ public function configureObjectManager(ConfigInterface $diConfig, &$sharedInstan
     {
         $objectManager = ObjectManager::getInstance();
 
-        $objectManager->configure(
-            $objectManager
-                ->get(\Magento\Framework\ObjectManager\ConfigLoaderInterface::class)
-                ->load(Area::AREA_GLOBAL)
-        );
+        $objectManager->configure($this->getConfigData());
         $objectManager->get(\Magento\Framework\Config\ScopeInterface::class)
             ->setCurrentScope('global');
         $diConfig->setInterceptionConfig(

lib/internal/Magento/Framework/ObjectManager/Config/Compiled.php

@@ -7,6 +7,7 @@
 
 use Magento\Framework\ObjectManager\ConfigInterface;
 use Magento\Framework\ObjectManager\ConfigCacheInterface;
+use Magento\Framework\ObjectManager\ConfigLoaderInterface;
 use Magento\Framework\ObjectManager\LazyTypeAwareInterface;
 use Magento\Framework\ObjectManager\RelationsInterface;
 
@@ -37,11 +38,26 @@ class Compiled implements ConfigInterface, LazyTypeAwareInterface
      */
     private array $lazyTypes = [];
 
+    /**
+     * Area whose configuration is currently applied, null when the state matches no single area
+     *
+     * @var string|null
+     */
+    private $appliedArea;
+
+    /**
+     * @var ConfigLoaderInterface|null
+     */
+    private $configLoader;
+
     /**
      * @param array $data
+     * @param ConfigLoaderInterface|null $configLoader
      */
-    public function __construct($data)
+    public function __construct($data, ?ConfigLoaderInterface $configLoader = null)
     {
+        $this->configLoader = $configLoader;
+        $this->appliedArea = $data[ConfigLoaderInterface::AREA_KEY] ?? null;
         $this->arguments = isset($data['arguments']) && is_array($data['arguments'])
             ? $data['arguments'] : [];
         $this->virtualTypes = isset($data['instanceTypes']) && is_array($data['instanceTypes'])
@@ -164,6 +180,8 @@ public function getPreference($type)
      */
     public function extend(array $configuration)
     {
+        $configuration = $this->resolveAgainstAppliedArea($configuration);
+
         $this->arguments = isset($configuration['arguments']) && is_array($configuration['arguments'])
             ? array_replace($this->arguments, $configuration['arguments'])
             : $this->arguments;
@@ -178,6 +196,42 @@ public function extend(array $configuration)
             : $this->lazyTypes;
     }
 
+    /**
+     * Returns a configuration that is equivalent to the complete one on top of the current state
+     *
+     * @param array $configuration
+     * @return array
+     */
+    private function resolveAgainstAppliedArea(array $configuration)
+    {
+        $base = $configuration[ConfigLoaderInterface::EXTENDS_KEY] ?? null;
+        $resolved = $base !== null && $base !== $this->appliedArea && $this->configLoader !== null
+            ? self::merge($this->configLoader->load($base), $configuration)
+            : $configuration;
+
+        $this->appliedArea = $configuration[ConfigLoaderInterface::AREA_KEY] ?? null;
+
+        return $resolved;
+    }
+
+    /**
+     * Applies a configuration on top of another one, per top-level key of each section
+     *
+     * @param array $base
+     * @param array $configuration
+     * @return array
+     */
+    private static function merge(array $base, array $configuration)
+    {
+        foreach (['arguments', 'instanceTypes', 'preferences', 'lazyTypes'] as $section) {
+            if (isset($configuration[$section]) && is_array($configuration[$section])) {
+                $base[$section] = array_replace($base[$section] ?? [], $configuration[$section]);
+            }
+        }
+
+        return $base;
+    }
+
     /**
      * Retrieve all virtual types
      *

lib/internal/Magento/Framework/ObjectManager/ConfigLoaderInterface.php

@@ -14,6 +14,16 @@
  */
 interface ConfigLoaderInterface
 {
+    /**
+     * When present, names the area the loaded configuration only holds the differences against
+     */
+    public const EXTENDS_KEY = '_extends';
+
+    /**
+     * When present, names the area the loaded configuration belongs to
+     */
+    public const AREA_KEY = '_area';
+
     /**
      * Load modules DI configuration
      *

lib/internal/Magento/Framework/ObjectManager/Test/Unit/Config/CompiledTest.php

@@ -8,6 +8,7 @@
 namespace Magento\Framework\ObjectManager\Test\Unit\Config;
 
 use Magento\Framework\ObjectManager\Config\Compiled;
+use Magento\Framework\ObjectManager\ConfigLoaderInterface;
 use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
 use PHPUnit\Framework\TestCase;
 use PHPUnit\Framework\Attributes\DataProvider;
@@ -347,4 +348,90 @@ public function testExtendMergesLazyTypes(): void
         $this->assertFalse($compiled->isNonLazyType('Second\\Type'));
         $this->assertTrue($compiled->isNonLazyType('Other\\Type'));
     }
+
+    public function testExtendAppliesADeltaOnTopOfTheAreaItExtends(): void
+    {
+        $configLoader = $this->createMock(ConfigLoaderInterface::class);
+        $configLoader->expects($this->never())->method('load');
+
+        $compiled = new Compiled(
+            [
+                'preferences' => ['preference1' => 'globalValue'],
+                ConfigLoaderInterface::AREA_KEY => 'global',
+            ],
+            $configLoader
+        );
+
+        $compiled->extend([
+            ConfigLoaderInterface::EXTENDS_KEY => 'global',
+            'preferences' => ['preference1' => 'frontendValue'],
+        ]);
+
+        $this->assertSame('frontendValue', $compiled->getPreference('preference1'));
+    }
+
+    public function testExtendRebuildsFromTheBaseWhenAnotherAreaIsAlreadyApplied(): void
+    {
+        $globalConfig = ['preferences' => ['preference1' => 'globalValue']];
+        $configLoader = $this->createMock(ConfigLoaderInterface::class);
+        $configLoader->expects($this->once())
+            ->method('load')
+            ->with('global')
+            ->willReturn($globalConfig);
+
+        $compiled = new Compiled(
+            $globalConfig + [ConfigLoaderInterface::AREA_KEY => 'global'],
+            $configLoader
+        );
+
+        $compiled->extend([
+            ConfigLoaderInterface::EXTENDS_KEY => 'global',
+            'preferences' => ['preference1' => 'adminhtmlValue'],
+        ]);
+        $compiled->extend([
+            ConfigLoaderInterface::EXTENDS_KEY => 'global',
+            'preferences' => ['preference2' => 'frontendOnly'],
+        ]);
+
+        $this->assertSame('globalValue', $compiled->getPreference('preference1'));
+        $this->assertSame('frontendOnly', $compiled->getPreference('preference2'));
+    }
+
+    public function testExtendRebuildsFromTheBaseAfterAnArbitraryConfiguration(): void
+    {
+        $globalConfig = ['preferences' => ['preference1' => 'globalValue']];
+        $configLoader = $this->createMock(ConfigLoaderInterface::class);
+        $configLoader->expects($this->once())
+            ->method('load')
+            ->with('global')
+            ->willReturn($globalConfig);
+
+        $compiled = new Compiled(
+            $globalConfig + [ConfigLoaderInterface::AREA_KEY => 'global'],
+            $configLoader
+        );
+
+        $compiled->extend(['preferences' => ['preference1' => 'mockedValue']]);
+        $compiled->extend([
+            ConfigLoaderInterface::EXTENDS_KEY => 'global',
+            'preferences' => [],
+        ]);
+
+        $this->assertSame('globalValue', $compiled->getPreference('preference1'));
+    }
+
+    public function testExtendAppliesADeltaWhenNoLoaderIsAvailable(): void
+    {
+        $compiled = new Compiled([
+            'preferences' => ['preference1' => 'globalValue'],
+            ConfigLoaderInterface::AREA_KEY => 'global',
+        ]);
+
+        $compiled->extend([
+            ConfigLoaderInterface::EXTENDS_KEY => 'global',
+            'preferences' => ['preference1' => 'frontendValue'],
+        ]);
+
+        $this->assertSame('frontendValue', $compiled->getPreference('preference1'));
+    }
 }

setup/src/Magento/Setup/Module/Di/App/Task/Operation/Area.php

@@ -7,6 +7,7 @@
 
 use Magento\Setup\Module\Di\App\Task\OperationInterface;
 use Magento\Framework\App;
+use Magento\Framework\ObjectManager\ConfigLoaderInterface;
 use Magento\Setup\Module\Di\Compiler\Config;
 use Magento\Setup\Module\Di\Definition\Collection as DefinitionsCollection;
 
@@ -15,6 +16,11 @@
  */
 class Area implements OperationInterface
 {
+    /**
+     * Sections of the compiled configuration that are merged per top-level key
+     */
+    private const MERGED_SECTIONS = ['arguments', 'preferences', 'instanceTypes', 'lazyTypes'];
+
     /**
      * @var App\AreaList
      */
@@ -91,6 +97,7 @@ public function doOperation()
         $this->sortDefinitions($definitionsCollection);
 
         $areaCodes = array_merge([App\Area::AREA_GLOBAL], $this->areaList->getCodes());
+        $globalConfig = [];
         foreach ($areaCodes as $areaCode) {
             $config = $this->configReader->generateCachePerScope($definitionsCollection, $areaCode);
             $config = $this->modificationChain->modify($config);
@@ -100,10 +107,46 @@ public function doOperation()
             ksort($config['preferences']);
             ksort($config['instanceTypes']);
 
+            if ($areaCode === App\Area::AREA_GLOBAL) {
+                $globalConfig = $config;
+            } else {
+                $config = $this->extractDiff($config, $globalConfig);
+            }
+
             $this->configWriter->write($areaCode, $config);
         }
     }
 
+    /**
+     * Reduces an area configuration to the entries that differ from the global one
+     *
+     * @param array $config
+     * @param array $globalConfig
+     * @return array
+     */
+    private function extractDiff(array $config, array $globalConfig)
+    {
+        $diff = [ConfigLoaderInterface::EXTENDS_KEY => App\Area::AREA_GLOBAL];
+
+        foreach ($config as $section => $values) {
+            if (!is_array($values) || !in_array($section, self::MERGED_SECTIONS, true)) {
+                $diff[$section] = $values;
+                continue;
+            }
+
+            $globalValues = $globalConfig[$section] ?? [];
+            $sectionDiff = [];
+            foreach ($values as $key => $value) {
+                if (!array_key_exists($key, $globalValues) || $globalValues[$key] !== $value) {
+                    $sectionDiff[$key] = $value;
+                }
+            }
+            $diff[$section] = $sectionDiff;
+        }
+
+        return $diff;
+    }
+
     /**
      * Returns definitions collection
      *