2 files changed, 58 insertions, 2 deletions

diff --git a/core/modules/package_manager/src/ExecutableFinder.php b/core/modules/package_manager/src/ExecutableFinder.php
index d8a2f1c21d5..a26de302187 100644
--- a/core/modules/package_manager/src/ExecutableFinder.php
+++ b/core/modules/package_manager/src/ExecutableFinder.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Drupal\package_manager;
 
+use Composer\InstalledVersions;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use PhpTuf\ComposerStager\API\Finder\Service\ExecutableFinderInterface;
 
@@ -17,10 +18,19 @@ use PhpTuf\ComposerStager\API\Finder\Service\ExecutableFinderInterface;
  */
 final class ExecutableFinder implements ExecutableFinderInterface {
 
+  /**
+   * The path where Composer is installed in the project, or FALSE if it's not.
+   */
+  private string|false|null $composerPath = NULL;
+
   public function __construct(
     private readonly ExecutableFinderInterface $decorated,
     private readonly ConfigFactoryInterface $configFactory,
-  ) {}
+  ) {
+    $this->composerPath = InstalledVersions::isInstalled('composer/composer')
+      ? InstalledVersions::getInstallPath('composer/composer') . '/bin/composer'
+      : FALSE;
+  }
 
   /**
    * {@inheritdoc}
@@ -29,7 +39,15 @@ final class ExecutableFinder implements ExecutableFinderInterface {
     $executables = $this->configFactory->get('package_manager.settings')
       ->get('executables');
 
-    return $executables[$name] ?? $this->decorated->find($name);
+    if (isset($executables[$name])) {
+      return $executables[$name];
+    }
+
+    // If we're looking for Composer, use the project's local copy if available.
+    if ($name === 'composer' && $this->composerPath && file_exists($this->composerPath)) {
+      return $this->composerPath;
+    }
+    return $this->decorated->find($name);
   }
 
 }
diff --git a/core/modules/package_manager/tests/src/Unit/ExecutableFinderTest.php b/core/modules/package_manager/tests/src/Unit/ExecutableFinderTest.php
index 811d7547e04..69becb1fd8d 100644
--- a/core/modules/package_manager/tests/src/Unit/ExecutableFinderTest.php
+++ b/core/modules/package_manager/tests/src/Unit/ExecutableFinderTest.php
@@ -6,6 +6,7 @@ namespace Drupal\Tests\package_manager\Unit;
 
 use Drupal\package_manager\ExecutableFinder;
 use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
 use PhpTuf\ComposerStager\API\Finder\Service\ExecutableFinderInterface;
 
 /**
@@ -36,4 +37,41 @@ class ExecutableFinderTest extends UnitTestCase {
     $finder->find('rsync');
   }
 
+  /**
+   * Tests that the executable finder tries to use a local copy of Composer.
+   */
+  public function testComposerInstalledInProject(): void {
+    vfsStream::setup('root', NULL, [
+      'composer-path' => [
+        'bin' => [],
+      ],
+    ]);
+    $composer_path = 'vfs://root/composer-path/bin/composer';
+    touch($composer_path);
+    $this->assertFileExists($composer_path);
+
+    $decorated = $this->prophesize(ExecutableFinderInterface::class);
+    $decorated->find('composer')->willReturn('the real Composer');
+
+    $finder = new ExecutableFinder(
+      $decorated->reveal(),
+      $this->getConfigFactoryStub([
+        'package_manager.settings' => [
+          'executables' => [],
+        ],
+      ]),
+    );
+    $reflector = new \ReflectionProperty($finder, 'composerPath');
+    $reflector->setValue($finder, $composer_path);
+    $this->assertSame($composer_path, $finder->find('composer'));
+
+    // If the executable disappears, or Composer isn't locally installed, the
+    // decorated executable finder should be called.
+    unlink($composer_path);
+    $this->assertSame('the real Composer', $finder->find('composer'));
+
+    $reflector->setValue($finder, FALSE);
+    $this->assertSame('the real Composer', $finder->find('composer'));
+  }
+
 }