vfs: implement 'complete_sync()' in 'Single_file_system'

Fixes #3047

repos/os/include/vfs/single_file_system.h

@@ -48,6 +48,11 @@ class Vfs::Single_file_system : public File_system
 			virtual Write_result write(char const *src, file_size count,
 			                           file_size &out_count) = 0;
 
+			virtual Sync_result sync()
+			{
+				return SYNC_OK;
+			}
+
 			virtual bool read_ready() = 0;
 		};
 
@@ -275,6 +280,17 @@ class Vfs::Single_file_system : public File_system
 		{
 			return FTRUNCATE_ERR_NO_PERM;
 		}
+
+		Sync_result complete_sync(Vfs_handle *vfs_handle)
+		{
+			Single_vfs_handle *handle =
+				static_cast<Single_vfs_handle*>(vfs_handle);
+
+			if (handle)
+				return handle->sync();
+
+			return SYNC_ERR_INVALID;
+		}
 };
 
 #endif /* _INCLUDE__VFS__SINGLE_FILE_SYSTEM_H_ */

repos/os/src/lib/vfs/log_file_system.h

@@ -122,16 +122,17 @@ class Vfs::Log_file_system : public Single_file_system
 						count -= curr_count;
 						src   += curr_count;
 					}
-
 					return WRITE_OK;
 				}
 
 				bool read_ready() override { return false; }
 
-				void sync()
+				Sync_result sync() override
 				{
 					if (_line_pos > 0)
 						_flush();
+
+					return SYNC_OK;
 				}
 		};
 
@@ -167,12 +168,6 @@ class Vfs::Log_file_system : public Single_file_system
 			catch (Genode::Out_of_ram)  { return OPEN_ERR_OUT_OF_RAM; }
 			catch (Genode::Out_of_caps) { return OPEN_ERR_OUT_OF_CAPS; }
 		}
-
-		Sync_result complete_sync(Vfs_handle *vfs_handle)
-		{
-			static_cast<Log_vfs_handle *>(vfs_handle)->sync();
-			return SYNC_OK;
-		}
 };
 
 #endif /* _INCLUDE__VFS__LOG_FILE_SYSTEM_H_ */