Allow configuring the location of the appdata_[instanceid] folder

config/config.sample.php

@@ -1924,6 +1924,15 @@
  */
 'updatedirectory' => '',
 
+/**
+ * Override where Nextcloud stores the ``appdata_INSTANCEID`` directory. Useful
+ * when using remote object storage where local system disks provide faster data
+ * access. Defaults to `datadirectory` if unset.
+ * 
+ * The Web server user must have write access to this directory.
+ */
+'appdatadirectory' => '',
+
 /**
  * Blacklist a specific file or files and disallow the upload of files
  * with this name. ``.htaccess`` is blocked by default.

lib/private/Files/AppData/AppData.php

@@ -27,7 +27,11 @@
 namespace OC\Files\AppData;
 
 use OCP\Cache\CappedMemoryCache;
+use OC\Files\Filesystem;
 use OC\Files\SimpleFS\SimpleFolder;
+use OC\Files\Node\LazyRoot;
+use OC\Files\Storage\LocalRootStorage;
+use OC\Files\Mount\MountPoint;
 use OC\SystemConfig;
 use OCP\Files\Folder;
 use OCP\Files\IAppData;
@@ -55,10 +59,28 @@ class AppData implements IAppData {
 	public function __construct(IRootFolder $rootFolder,
 								SystemConfig $systemConfig,
 								string $appId) {
-		$this->rootFolder = $rootFolder;
 		$this->config = $systemConfig;
 		$this->appId = $appId;
 		$this->folders = new CappedMemoryCache();
+
+		$this->rootFolder = new LazyRoot(function () use ($rootFolder, $systemConfig) {
+			if ($appdatadirectory = $systemConfig->getValue('appdatadirectory', null)) {
+				$instanceId = $systemConfig->getValue('instanceid', null);
+				if ($instanceId === null) {
+					throw new \RuntimeException('no instance id!');
+				}
+
+				$folderName = 'appdata_' . $instanceId;
+
+				$arguments = [
+					'datadir' => $appdatadirectory,
+				];
+				$storage = new LocalRootStorage($arguments);
+				$mount = new MountPoint($storage, $folderName, $arguments);
+				Filesystem::getMountManager()->addMount($mount);
+			}
+			return $rootFolder;
+		});
 	}
 
 	private function getAppDataFolderName() {

lib/private/Files/Cache/LocalRootScanner.php

@@ -25,6 +25,8 @@
  */
 namespace OC\Files\Cache;
 
+use OC\Files\Filesystem;
+
 class LocalRootScanner extends Scanner {
 	public function scanFile($file, $reuseExisting = 0, $parentId = -1, $cacheData = null, $lock = true, $data = null) {
 		if ($this->shouldScanPath($file)) {
@@ -43,7 +45,11 @@ public function scan($path, $recursive = self::SCAN_RECURSIVE, $reuse = -1, $loc
 	}
 
 	private function shouldScanPath(string $path): bool {
+		$storageId = $this->storage->getId();
+		$mount = Filesystem::getMountManager()->findByStorageId($storageId);
+		$mountPoint = sizeof($mount) == 1 ? $mount[0]->getMountPoint() : "null";
+
 		$path = trim($path, '/');
-		return $path === '' || str_starts_with($path, 'appdata_') || str_starts_with($path, '__groupfolders');
+		return $path === '' || str_starts_with($path, 'appdata_') || str_starts_with($path, '__groupfolders') || str_starts_with($mountPoint, '/appdata_');
 	}
 }

lib/private/SystemConfig.php

@@ -38,6 +38,7 @@ class SystemConfig {
 	protected $sensitiveValues = [
 		'instanceid' => true,
 		'datadirectory' => true,
+		'appdatadirectory' => true,
 		'dbname' => true,
 		'dbhost' => true,
 		'dbpassword' => true,

tests/lib/Files/AppData/AppDataTest.php

@@ -51,8 +51,10 @@ protected function setUp(): void {
 
 		$this->systemConfig->expects($this->any())
 			->method('getValue')
-			->with('instanceid', null)
-			->willReturn('iid');
+			->willReturnMap([
+				['instanceid', null, 'iid'],
+				['appdatadirectory', null, '/path'],
+			]);
 	}
 
 	private function setupAppFolder() {