Allow dynamic (immmutable) extension of the mount manager

src/MountManager.php

@@ -33,6 +33,18 @@ public function __construct(array $filesystems = [], array $config = [])
         $this->config = new Config($config);
     }
 
+    /**
+     * @param array<string,FilesystemOperator> $filesystems
+     */
+    public function extend(array $filesystems, array $config = []): MountManager
+    {
+        $clone = clone $this;
+        $clone->config = $this->config->extend($config);
+        $clone->mountFilesystems($filesystems);
+
+        return $clone;
+    }
+
     public function fileExists(string $location): bool
     {
         /** @var FilesystemOperator $filesystem */
@@ -329,7 +341,7 @@ private function mountFilesystem(string $key, FilesystemOperator $filesystem): v
     /**
      * @param string $path
      *
-     * @return array{0:FilesystemOperator, 1:string}
+     * @return array{0:FilesystemOperator, 1:string, 2:string}
      */
     private function determineFilesystemAndPath(string $path): array
     {

src/MountManagerTest.php

@@ -76,6 +76,58 @@ public function copying_without_retaining_visibility(): void
         self::assertEquals('private', $visibility);
     }
 
+    /**
+     * @test
+     */
+    public function extending_without_new_mounts_is_equal_but_not_the_same(): void
+    {
+        $mountManager = $this->mountManager->extend([]);
+
+        $this->assertNotSame($this->mountManager, $mountManager);
+        $this->assertEquals($this->mountManager, $mountManager);
+    }
+
+    /**
+     * @test
+     */
+    public function extending_with_new_mounts_is_not_equal(): void
+    {
+        $mountManager = $this->mountManager->extend([
+            'third' => new Filesystem(new InMemoryFilesystemAdapter()),
+        ]);
+
+        $this->assertNotEquals($this->mountManager, $mountManager);
+    }
+
+    /**
+     * @test
+     */
+    public function extending_exposes_a_usable_mount_on_the_extension(): void
+    {
+        $mountManager = $this->mountManager->extend([
+            'third' => new Filesystem(new InMemoryFilesystemAdapter()),
+        ]);
+
+        $mountManager->write('third://path.txt', 'this');
+        $contents = $mountManager->read('third://path.txt');
+
+        $this->assertEquals('this', $contents);
+    }
+
+    /**
+     * @test
+     */
+    public function extending_does_not_mount_on_the_original_mount_manager(): void
+    {
+        $this->mountManager->extend([
+            'third' => new Filesystem(new InMemoryFilesystemAdapter()),
+        ]);
+
+        $this->expectException(UnableToResolveFilesystemMount::class);
+
+        $this->mountManager->write('third://path.txt', 'this');
+    }
+
     /**
      * @test
      */