add sync implementation to various fs servers

Fixes #1007.

libports/src/server/ffat_fs/main.cc

@@ -852,6 +852,9 @@ namespace File_system {
 			{
 				PWRN("File_system::Session::sigh not supported");
 			}
+
+			/* ffat only supports fsync(2) */
+			void sync() { }
 	};
 
 

os/src/server/lx_fs/main.cc

@@ -308,6 +308,13 @@ class File_system::Session_component : public Session_rpc_object
 		{
 			_handle_registry.sigh(node_handle, sigh);
 		}
+
+		/**
+		 * We could call sync(2) here but for now we forward just the
+		 * reminder because besides testing, there is currently no
+		 * use-case.
+		 */
+		void sync() { PWRN("sync() not implemented!"); }
 };
 
 

os/src/server/ram_fs/main.cc

@@ -411,6 +411,8 @@ namespace File_system {
 			{
 				_handle_registry.sigh(node_handle, sigh);
 			}
+
+			void sync() { }
 	};
 
 

os/src/server/tar_fs/main.cc

@@ -424,6 +424,9 @@ namespace File_system {
 			{
 				PWRN("File_system::Session::sigh not supported");
 			}
+
+			/* merely readonly, no need to sync */
+			void sync() { }
 	};