feat: Allow to enforce Windows compatible file and folder names

This will:
* Deny characters forbidden on Windows
* Deny files with names which are reserved on Windows
* Deny trailing dot or space
* Deny files or folders which are not case-insensitive unique in a folder

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

config/config.sample.php

@@ -1960,6 +1960,21 @@
  */
 'updatedirectory' => '',
 
+/**
+ * Allow to enforce Windows compatible file and folder names.
+ * Nextcloud by default supports all files valid on Linux,
+ * but as Windows has some stricter filename rules this can lead to sync errors when using Windows clients.
+ *
+ * To enforce only Windows compatible filenames, also on the webui, set this value to ``true``.
+ *
+ * This will deny filenames with characters not valid on Windows, as well as some reserved filenames and nameing rules (no trailing dot or space).
+ * Additionally this will enforce files to be case in-sensitivly unique in a folder.
+ * 
+ * Defaults to ``false``
+ * 
+ */
+'enforce_windows_compatibility' => false,
+
 /**
  * Deny a specific file or files and disallow the upload of files
  * with this name. ``.htaccess`` is blocked by default.

lib/private/Files/Storage/Common.php

@@ -63,6 +63,7 @@
 use OCP\Files\Storage\IStorage;
 use OCP\Files\Storage\IWriteStreamStorage;
 use OCP\Files\StorageNotAvailableException;
+use OCP\IConfig;
 use OCP\Lock\ILockingProvider;
 use OCP\Lock\LockedException;
 use OCP\Server;
@@ -375,7 +376,7 @@ public function getWatcher($path = '', $storage = null) {
 		}
 		if (!isset($this->watcher)) {
 			$this->watcher = new Watcher($storage);
-			$globalPolicy = \OC::$server->getConfig()->getSystemValue('filesystem_check_changes', Watcher::CHECK_NEVER);
+			$globalPolicy = \OCP\Server::get(IConfig::class)->getSystemValue('filesystem_check_changes', Watcher::CHECK_NEVER);
 			$this->watcher->setPolicy((int)$this->getMountOption('filesystem_check_changes', $globalPolicy));
 		}
 		return $this->watcher;
@@ -565,6 +566,32 @@ public function verifyPath($path, $fileName) {
 		if (\OC\Files\Filesystem::hasFilenameInvalidCharacters($fileName)) {
 			throw new InvalidCharacterInPathException();
 		}
+
+		$config = \OCP\Server::get(IConfig::class);
+		if ($config->getSystemValueBool('enforce_windows_compatibility', false)) {
+			// Windows does not allow filenames to end with a trailing dot or space
+			if (str_ends_with($fileName, '.') || str_ends_with($fileName, ' ')) {
+				throw new InvalidCharacterInPathException('Filenames must not end with a dot or space');
+			}
+
+			// Windows has path namespaces so e.g. `NUL` is a reserved word,
+			// but `NUL.txt` or `NUL.tar.gz` is considered the same and thus also reserved.
+			$basename = substr($fileName, 0, strpos($fileName, '.') ?: null);
+			if (\OC\Files\Filesystem::isFileBlacklisted($basename)) {
+				throw new ReservedWordException();
+			}
+
+			// Some windows systems are case insensitive,
+			// so to guarantee files can be synced we need to enfore case insensitivity
+			$content = $this->getDirectoryContent(dirname($path));
+			$fileName = strtolower($fileName);
+			foreach ($content as $subPath) {
+				if (strtolower($subPath['name']) === $fileName) {
+					throw new InvalidPathException('Filename is not case insensitivly unique');
+				}
+			}
+		}
+
 		// NOTE: $path will remain unverified for now
 	}