Workaround for empty dir deletion for SFTP

Explicitly clear the stat cache after deleting an empty folder to make
sure it is properly detected as deleted in subsequent requests.

This works around a problem with phpseclib where the folder is properly
deleted remotely but the stat cache was not updated.

apps/files_external/lib/sftp.php

@@ -251,7 +251,11 @@ class SFTP extends \OC\Files\Storage\Common {
 	 */
 	public function rmdir($path) {
 		try {
-			return $this->getConnection()->delete($this->absPath($path), true);
+			$result = $this->getConnection()->delete($this->absPath($path), true);
+			// workaround: stray stat cache entry when deleting empty folders
+			// see https://github.com/phpseclib/phpseclib/issues/706
+			$this->getConnection()->clearStatCache();
+			return $result;
 		} catch (\Exception $e) {
 			return false;
 		}

tests/lib/files/storage/storage.php

@@ -380,6 +380,13 @@ abstract class Storage extends \Test\TestCase {
 		$this->assertFalse($this->instance->file_exists('folder'));
 	}
 
+	public function testRmdirEmptyFolder() {
+		$this->assertTrue($this->instance->mkdir('empty'));
+		$this->wait();
+		$this->assertTrue($this->instance->rmdir('empty'));
+		$this->assertFalse($this->instance->file_exists('empty'));
+	}
+
 	public function testRecursiveUnlink() {
 		$this->instance->mkdir('folder');
 		$this->instance->mkdir('folder/bar');