os: handle SYNC packet in lx_fs

Call 'fsync(2)' on files and directories.

Fixes #3711.

repos/os/src/server/lx_fs/directory.h

@@ -237,6 +237,12 @@ class Lx_fs::Directory : public Node
 			return 0;
 		}
 
+		bool sync() override
+		{
+			int ret = fsync(dirfd(_fd));
+			return ret ? false : true;
+		}
+
 		Status status() override
 		{
 			struct stat st { };

repos/os/src/server/lx_fs/file.h

@@ -141,6 +141,12 @@ class Lx_fs::File : public Node
 			return ret == -1 ? 0 : ret;
 		}
 
+		bool sync() override
+		{
+			int ret = fsync(_fd);
+			return ret ? false : true;
+		}
+
 		Status status() override
 		{
 			struct stat st { };

repos/os/src/server/lx_fs/main.cc

@@ -121,13 +121,10 @@ class Lx_fs::Session_component : public Session_rpc_object
 
 			case Packet_descriptor::SYNC:
 
-				/**
-				 * We could call sync(2) here but for now we forward just the
-				 * reminder because besides testing, there is currently no
-				 * use-case.
-				 */
-				Genode::warning("SYNC not implemented!");
-				succeeded = true;
+				if (tx_sink()->packet_valid(packet)) {
+					succeeded = open_node.node().sync();
+				}
+
 				break;
 			}
 

repos/os/src/server/lx_fs/node.h

@@ -53,6 +53,8 @@ class Lx_fs::Node : public File_system::Node_base
 		virtual size_t read(char *dst, size_t len, seek_off_t) = 0;
 		virtual size_t write(char const *src, size_t len, seek_off_t) = 0;
 
+		virtual bool sync() { return true; }
+
 		virtual Status status() = 0;
 
 		/*