Layout

The Layout record contains metadata of a file or directory that should be written to the mass memory. When combined with a Content record (and possibly an Index record), it is possible to write the file or directory on disk in a reproducible way.

It is composed of a 32 bytes long header-like section and two fields of variable length.

Field	Type	Size (bytes)	Description
uid	uint	4	User ID of the owner of the file.
gid	uint	4	Group ID of the owner of the file.
mode	uint	4	File’s mode and permission bits.
tag	blob	4	Unused.
source_length	uint	2	Length of the source blob.
target_length	uint	2	Length of the target string.
file_type	uint	1	Type of file to be written.
padding	blob	11	Unused.
source	blob	Specified by source_length	”source” of the file. Its meaning varies depending on file_type.
target	str	Specified by target_length	Path where the file should be written to.

file_type

file_type is an enum that specifies the type of file that should be written on disk.

Value	Name	Description	Meaning of source
1	Regular	Regular file of data.	XXH3_128bits hash of the file. The source is a hash because our package manager, Moss, uses a CAS to store files. This hash may be also used to match an Index record, in order to identify a region of a Content record to read. In fact, the Index record also contains a XXH3_128bits-based hash.
2	Symlink	Symbolic link to another file.	String. Original file path to link somewhere else.
3	Directory	Empty directory.	Unused.
4	CharacterDevice	Character device.	Unused.
5	BlockDevice	Block device.	Unused.
6	FIFO	POSIX FIFO.	Unused.
7	Socket	POSIX communication socket.	Unused.