add a disk_free_space check before writing config

Signed-off-by: Simon L <szaimen@e.mail.de>

lib/private/Config.php

@@ -277,6 +277,12 @@ class Config {
 				'This can usually be fixed by giving the webserver write access to the config directory.');
 		}
 
+		// Never write file back if disk space should be low (less than 100 KiB)
+		$df = disk_free_space($this->configDir);
+		if ($df !== false && (int)$df < 102400) {
+			throw new \Exception($this->configDir . " does not have enough space for writing the config file! Not writing it back!");
+		}
+
 		// Try to acquire a file lock
 		if (!flock($filePointer, LOCK_EX)) {
 			throw new \Exception(sprintf('Could not acquire an exclusive lock on the config file %s', $this->configFilePath));