The repo file format / razor_set data structure

-----------------------------------------------

The repo starts with a header, containing some number of sections,

terminated by a section with type 0:

	struct razor_set_header {

		uint32_t magic;

		uint32_t version;

		struct razor_set_section sections[0];

	};

	struct razor_set_section {

		uint32_t type;

		uint32_t offset;

		uint32_t size;

	};

razor_set_open() mmaps the repo file, and creates a struct razor_set:

	struct razor_set {

		struct array string_pool;

	 	struct array packages;

	 	struct array properties;

	 	struct array files;

		struct array package_pool;

	 	struct array property_pool;

	 	struct array file_pool;

		struct razor_set_header *header;

	};

by finding the sections with those IDs and creating "struct array"s

pointing to the right places in the mmapped data. (This is the only

processing needed when reading in the file; everything else is used

exactly as-is.)

The sections

------------

RAZOR_STRING_POOL

	Stores one copy of each string that appears in the repo. (At

	the moment, this is: package names, package versions, property

	names, property versions, and (basenames of) filenames.) The

	strings are arbitrarily-sized, 0-terminated, and not in any

	particular order (although the empty string always ends up

	being at offset 0).

RAZOR_PACKAGES

	Array of struct razor_package; one for each package in the

	set, sorted by name.

RAZOR_PROPERTIES

	Array of struct razor_property; one for each unique property

	in the set, sorted by type, then name, then relation type (eg,

	"<" or ">="), then version. (Properties with no version have

	relation type RAZOR_VERSION_EQUAL, and version "".)

RAZOR_FILES

	Array of struct razor_entry; one for each file owned by any

	package in the set. The current sort order (which is subject

	to change) is breadth-first, sorted by basename. So eg: /, /bin,

	/dev, /etc, /bin/false, /bin/true, /dev/null, /etc/passwd.

RAZOR_PACKAGE_POOL

	Array of struct list, with each list item containing the index

	of a struct razor_package in the packages section. See the

	discussion of lists below.

RAZOR_PROPERTY_POOL

	Array of struct list, with each list item containing the index

	of a struct razor_property in the properties section. See the

	discussion of lists below.

RAZOR_FILE_POOL

	Array of struct list, with each list item containing the index

	of a struct razor_entry in the files section. See the

	discussion of lists below.

Data types

----------

Note that the exact layout of bits involves some historical accidents.

(Particularly the fact that the "name" field in most structs loses its

high bits to a flags field.)

struct list_head

	uint list_ptr : 24;

	uint flags    : 8;

struct list

	uint data  : 24;

	uint flags : 8;

	Used to store lists of package, property, or file IDs. "struct

	list_head" stores the head of the list, which points to one or

	more "struct list"s in the appropriate "pool" section.

	("struct list" should probably be called "struct list_item".)

	"list_first(&head, &pool)" returns a "struct list *" pointing

	to the first element of the list (or NULL for an empty list),

	and "list_next(list)" will return successive elements, until

	NULL is returned. Each "list->data" contains the index of a

	package, property, or file in the corresponding section of the

	set.

	Peeking underneath the abstraction, a list_head's "flags" is

	0xff if the list is empty, 0x80 if it contains a single

	element, or 0x00 if it contains more than one element. In the

	single-element case, that element is actually stored in the

	list_head directly rather than being stored in a pool (and so

	list_first() just casts the list_head* to a list* and returns

	it). For multi-element lists, list_ptr is the index in the

	pool of the first element of this list; the list continues

	through successive elements of the pool until one with

	non-zero flags is reached, indicating the end of the list.

struct razor_package

	uint name    : 24;

	uint flags   : 8;

	uint version : 32;

	struct list_head properties;

	struct list_head files;

	name and version are indexes into string_pool. properties is a

	list of all of the package's properties, and files is a list

	of its files. flags is currently only used during razor_set

	merging, to keep track of which set a package came from.

struct razor_property

	uint name     : 24;

	uint flags    : 6;

	uint type     : 2;

	uint relation : 32;

	uint version  : 32;

	struct list_head packages;

	name and version are indexes into string_pool. type is an enum

	razor_property_type (eg, RAZOR_PROPERTY_REQUIRES), and

	relation is an enum razor_version_relation (eg,

	RAZOR_VERSION_GREATER_OR_EQUAL). packages is a list of the

	packages that have this property. flags is currently unused.

struct razor_entry

	uint name  : 24;

	uint flags : 8;

	uint start : 32;

	struct list_head packages;

	name is an index into string_pool, giving the basename of the

	file. start is either 0, or an index pointing to another

	razor_entry that is the first child of this entry (for a

	non-empty directory). (Entry 0 is always the root of the tree,

	so no entry could have entry 0 as a child.) flags is 0x80

	(RAZOR_ENTRY_LAST) if an entry is the last entry in its

	directory. Otherwise it is 0.

	Note that given a pointer to a struct_razor_entry (eg, from a

	package's "files" list), there is no way to reconstruct its

	full name without walking the entire files array up to that

	point. Because of this and other problems (fix_file_map()), it

	seems like razor_entry should be modified to include a pointer

	to its parent. (Storing full paths instead of just basenames

	would also fix this problem, but that would use much more

	memory.)