config: add a switch ton control truncate before update

To avoid extra truncate on non WORM file systems, add a new config
option `localstorage.unlink_on_truncate`, which defaults to false.

The OC\Files\Storage\Local is update to respect that option.

config/config.sample.php

@@ -1769,6 +1769,14 @@
  */
 'localstorage.allowsymlinks' => false,
 
+/**
+ * This options allows storage systems that don't allow to modify existing files
+ * to overcome this limitation by removing the files before overwriting.
+ *
+ * Defaults to ``false``
+ */
+'localstorage.unlink_on_truncate' => false,
+
 /**
  * EXPERIMENTAL: option whether to include external storage in quota
  * calculation, defaults to false.

lib/private/Files/Storage/Local.php

@@ -60,6 +60,8 @@ class Local extends \OC\Files\Storage\Common {
 
 	protected $realDataDir;
 
+	protected $unlinkOnTruncate;
+
 	public function __construct($arguments) {
 		if (!isset($arguments['datadir']) || !is_string($arguments['datadir'])) {
 			throw new \InvalidArgumentException('No data directory set for local storage');
@@ -76,6 +78,9 @@ public function __construct($arguments) {
 			$this->datadir .= '/';
 		}
 		$this->dataDirLength = strlen($this->realDataDir);
+
+		// support Write-Once-Read-Many file systems
+		$unlinkOnTruncate = \OC::$server->getConfig()->getSystemValue('localstorage.unlink_on_truncate', false);
 	}
 
 	public function __destruct() {
@@ -279,8 +284,9 @@ public function file_get_contents($path) {
 
 	public function file_put_contents($path, $data) {
 		$oldMask = umask(022);
-		// support Write-Once-Read-Many filesystems
-		$this->unlink($path);
+		if ($unlinkOnTruncate) {
+			$this->unlink($path);
+		}
 		$result = file_put_contents($this->getSourcePath($path), $data);
 		umask($oldMask);
 		return $result;
@@ -354,8 +360,9 @@ public function copy($path1, $path2) {
 			return parent::copy($path1, $path2);
 		} else {
 			$oldMask = umask(022);
-			// support Write-Once-Read-Many filesystems
-			$this->unlink($path2);
+			if ($unlinkOnTruncate) {
+				$this->unlink($path2);
+			}
 			$result = copy($this->getSourcePath($path1), $this->getSourcePath($path2));
 			umask($oldMask);
 			return $result;
@@ -364,8 +371,7 @@ public function copy($path1, $path2) {
 
 	public function fopen($path, $mode) {
 		$oldMask = umask(022);
-		if ($mode === 'w') {
-			// support Write-Once-Read-Many filesystems
+		if ($mode === 'w' && $unlinkOnTruncate) {
 			$this->unlink($path);
 		}
 		$result = fopen($this->getSourcePath($path), $mode);