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docs: move pack format docs to man section 5

Browse Source 
Continue the move of existing Documentation/technical/* protocol and
file-format documentation into our main documentation space by moving
the various documentation pertaining to the *.pack format and related
files, and updating things that refer to it to link to the new
location.

By moving these we can properly link from the newly created
gitformat-commit-graph to a gitformat-chunk-format page.

Integrating "Documentation/technical/bitmap-format.txt" and
"Documentation/technical/cruft-packs.txt" might logically be part of
this change, but as those cover parts of the wider "pack
format" (including associated files) that's documented outside of
"Documentation/technical/pack-format.txt" let's leave those for now,
subsequent commit(s) will address those.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Ævar Arnfjörð Bjarmason
2022-08-04 18:28:39 +02:00
committed by Junio C Hamano
parent 20516890dc
commit 977c47b46d
15 changed files with 68 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files

116
Documentation/technical/chunk-format.txt
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Chunk-based file formats
========================
Some file formats in Git use a common concept of "chunks" to describe
sections of the file. This allows structured access to a large file by
scanning a small "table of contents" for the remaining data. This common
format is used by the `commit-graph` and `multi-pack-index` files. See
link:technical/pack-format.html[the `multi-pack-index` format] and
the `commit-graph` format in linkgit:gitformat-commit-graph[5] for
how they use the chunks to describe structured data.
A chunk-based file format begins with some header information custom to
that format. That header should include enough information to identify
the file type, format version, and number of chunks in the file. From this
information, that file can determine the start of the chunk-based region.
The chunk-based region starts with a table of contents describing where
each chunk starts and ends. This consists of (C+1) rows of 12 bytes each,
where C is the number of chunks. Consider the following table:
| Chunk ID (4 bytes) | Chunk Offset (8 bytes) |
|--------------------|------------------------|
| ID[0] | OFFSET[0] |
| ... | ... |
| ID[C] | OFFSET[C] |
| 0x0000 | OFFSET[C+1] |
Each row consists of a 4-byte chunk identifier (ID) and an 8-byte offset.
Each integer is stored in network-byte order.
The chunk identifier `ID[i]` is a label for the data stored within this
fill from `OFFSET[i]` (inclusive) to `OFFSET[i+1]` (exclusive). Thus, the
size of the `i`th chunk is equal to the difference between `OFFSET[i+1]`
and `OFFSET[i]`. This requires that the chunk data appears contiguously
in the same order as the table of contents.
The final entry in the table of contents must be four zero bytes. This
confirms that the table of contents is ending and provides the offset for
the end of the chunk-based data.
Note: The chunk-based format expects that the file contains _at least_ a
trailing hash after `OFFSET[C+1]`.
Functions for working with chunk-based file formats are declared in
`chunk-format.h`. Using these methods provide extra checks that assist
developers when creating new file formats.
Writing chunk-based file formats
--------------------------------
To write a chunk-based file format, create a `struct chunkfile` by
calling `init_chunkfile()` and pass a `struct hashfile` pointer. The
caller is responsible for opening the `hashfile` and writing header
information so the file format is identifiable before the chunk-based
format begins.
Then, call `add_chunk()` for each chunk that is intended for write. This
populates the `chunkfile` with information about the order and size of
each chunk to write. Provide a `chunk_write_fn` function pointer to
perform the write of the chunk data upon request.
Call `write_chunkfile()` to write the table of contents to the `hashfile`
followed by each of the chunks. This will verify that each chunk wrote
the expected amount of data so the table of contents is correct.
Finally, call `free_chunkfile()` to clear the `struct chunkfile` data. The
caller is responsible for finalizing the `hashfile` by writing the trailing
hash and closing the file.
Reading chunk-based file formats
--------------------------------
To read a chunk-based file format, the file must be opened as a
memory-mapped region. The chunk-format API expects that the entire file
is mapped as a contiguous memory region.
Initialize a `struct chunkfile` pointer with `init_chunkfile(NULL)`.
After reading the header information from the beginning of the file,
including the chunk count, call `read_table_of_contents()` to populate
the `struct chunkfile` with the list of chunks, their offsets, and their
sizes.
Extract the data information for each chunk using `pair_chunk()` or
`read_chunk()`:
* `pair_chunk()` assigns a given pointer with the location inside the
memory-mapped file corresponding to that chunk's offset. If the chunk
does not exist, then the pointer is not modified.
* `read_chunk()` takes a `chunk_read_fn` function pointer and calls it
with the appropriate initial pointer and size information. The function
is not called if the chunk does not exist. Use this method to read chunks
if you need to perform immediate parsing or if you need to execute logic
based on the size of the chunk.
After calling these methods, call `free_chunkfile()` to clear the
`struct chunkfile` data. This will not close the memory-mapped region.
Callers are expected to own that data for the timeframe the pointers into
the region are needed.
Examples
--------
These file formats use the chunk-format API, and can be used as examples
for future formats:
* *commit-graph:* see `write_commit_graph_file()` and `parse_commit_graph()`
in `commit-graph.c` for how the chunk-format API is used to write and
parse the commit-graph file format documented in
the commit-graph file format in linkgit:gitformat-commit-graph[5].
* *multi-pack-index:* see `write_midx_internal()` and `load_multi_pack_index()`
in `midx.c` for how the chunk-format API is used to write and
parse the multi-pack-index file format documented in
link:technical/pack-format.html[the multi-pack-index file format].

2
Documentation/technical/hash-function-transition.txt
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@ -205,7 +205,7 @@ SHA-1 content.
Object storage
~~~~~~~~~~~~~~
Loose objects use zlib compression and packed objects use the packed
format described in Documentation/technical/pack-format.txt, just like
format described in linkgit:gitformat-pack[5], just like
today. The content that is compressed and stored uses SHA-256 content
instead of SHA-1 content.

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Documentation/technical/pack-format.txt
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Git pack format
===============
== Checksums and object IDs
In a repository using the traditional SHA-1, pack checksums, index checksums,
and object IDs (object names) mentioned below are all computed using SHA-1.
Similarly, in SHA-256 repositories, these values are computed using SHA-256.
== pack-*.pack files have the following format:
- A header appears at the beginning and consists of the following:
4-byte signature:
The signature is: {'P', 'A', 'C', 'K'}
4-byte version number (network byte order):
Git currently accepts version number 2 or 3 but
generates version 2 only.
4-byte number of objects contained in the pack (network byte order)
Observation: we cannot have more than 4G versions ;-) and
more than 4G objects in a pack.
- The header is followed by number of object entries, each of
which looks like this:
(undeltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
compressed data
(deltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
base object name if OBJ_REF_DELTA or a negative relative
offset from the delta object's position in the pack if this
is an OBJ_OFS_DELTA object
compressed delta data
Observation: length of each object is encoded in a variable
length format and is not constrained to 32-bit or anything.
- The trailer records a pack checksum of all of the above.
=== Object types
Valid object types are:
- OBJ_COMMIT (1)
- OBJ_TREE (2)
- OBJ_BLOB (3)
- OBJ_TAG (4)
- OBJ_OFS_DELTA (6)
- OBJ_REF_DELTA (7)
Type 5 is reserved for future expansion. Type 0 is invalid.
=== Size encoding
This document uses the following "size encoding" of non-negative
integers: From each byte, the seven least significant bits are
used to form the resulting integer. As long as the most significant
bit is 1, this process continues; the byte with MSB 0 provides the
last seven bits. The seven-bit chunks are concatenated. Later
values are more significant.
This size encoding should not be confused with the "offset encoding",
which is also used in this document.
=== Deltified representation
Conceptually there are only four object types: commit, tree, tag and
blob. However to save space, an object could be stored as a "delta" of
another "base" object. These representations are assigned new types
ofs-delta and ref-delta, which is only valid in a pack file.
Both ofs-delta and ref-delta store the "delta" to be applied to
another object (called 'base object') to reconstruct the object. The
difference between them is, ref-delta directly encodes base object
name. If the base object is in the same pack, ofs-delta encodes
the offset of the base object in the pack instead.
The base object could also be deltified if it's in the same pack.
Ref-delta can also refer to an object outside the pack (i.e. the
so-called "thin pack"). When stored on disk however, the pack should
be self contained to avoid cyclic dependency.
The delta data starts with the size of the base object and the
size of the object to be reconstructed. These sizes are
encoded using the size encoding from above. The remainder of
the delta data is a sequence of instructions to reconstruct the object
from the base object. If the base object is deltified, it must be
converted to canonical form first. Each instruction appends more and
more data to the target object until it's complete. There are two
supported instructions so far: one for copy a byte range from the
source object and one for inserting new data embedded in the
instruction itself.
Each instruction has variable length. Instruction type is determined
by the seventh bit of the first octet. The following diagrams follow
the convention in RFC 1951 (Deflate compressed data format).
==== Instruction to copy from base object
+----------+---------+---------+---------+---------+-------+-------+-------+
| 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
+----------+---------+---------+---------+---------+-------+-------+-------+
This is the instruction format to copy a byte range from the source
object. It encodes the offset to copy from and the number of bytes to
copy. Offset and size are in little-endian order.
All offset and size bytes are optional. This is to reduce the
instruction size when encoding small offsets or sizes. The first seven
bits in the first octet determines which of the next seven octets is
present. If bit zero is set, offset1 is present. If bit one is set
offset2 is present and so on.
Note that a more compact instruction does not change offset and size
encoding. For example, if only offset2 is omitted like below, offset3
still contains bits 16-23. It does not become offset2 and contains
bits 8-15 even if it's right next to offset1.
+----------+---------+---------+
| 10000101 | offset1 | offset3 |
+----------+---------+---------+
In its most compact form, this instruction only takes up one byte
(0x80) with both offset and size omitted, which will have default
values zero. There is another exception: size zero is automatically
converted to 0x10000.
==== Instruction to add new data
+----------+============+
| 0xxxxxxx | data |
+----------+============+
This is the instruction to construct target object without the base
object. The following data is appended to the target object. The first
seven bits of the first octet determines the size of data in
bytes. The size must be non-zero.
==== Reserved instruction
+----------+============
| 00000000 |
+----------+============
This is the instruction reserved for future expansion.
== Original (version 1) pack-*.idx files have the following format:
- The header consists of 256 4-byte network byte order
integers. N-th entry of this table records the number of
objects in the corresponding pack, the first byte of whose
object name is less than or equal to N. This is called the
'first-level fan-out' table.
- The header is followed by sorted 24-byte entries, one entry
per object in the pack. Each entry is:
4-byte network byte order integer, recording where the
object is stored in the packfile as the offset from the
beginning.
one object name of the appropriate size.
- The file is concluded with a trailer:
A copy of the pack checksum at the end of the corresponding
packfile.
Index checksum of all of the above.
Pack Idx file:
-- +--------------------------------+
fanout | fanout[0] = 2 (for example) |-.
table +--------------------------------+ |
| fanout[1] | |
+--------------------------------+ |
| fanout[2] | |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| fanout[255] = total objects |---.
-- +--------------------------------+ | |
main | offset | | |
index | object name 00XXXXXXXXXXXXXXXX | | |
table +--------------------------------+ | |
| offset | | |
| object name 00XXXXXXXXXXXXXXXX | | |
+--------------------------------+<+ |
.-| offset | |
| | object name 01XXXXXXXXXXXXXXXX | |
| +--------------------------------+ |
| | offset | |
| | object name 01XXXXXXXXXXXXXXXX | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| | offset | |
| | object name FFXXXXXXXXXXXXXXXX | |
--| +--------------------------------+<--+
trailer | | packfile checksum |
| +--------------------------------+
| | idxfile checksum |
| +--------------------------------+
.-------.
|
Pack file entry: <+
packed object header:
1-byte size extension bit (MSB)
type (next 3 bit)
size0 (lower 4-bit)
n-byte sizeN (as long as MSB is set, each 7-bit)
size0..sizeN form 4+7+7+..+7 bit integer, size0
is the least significant part, and sizeN is the
most significant part.
packed object data:
If it is not DELTA, then deflated bytes (the size above
is the size before compression).
If it is REF_DELTA, then
base object name (the size above is the
size of the delta data that follows).
delta data, deflated.
If it is OFS_DELTA, then
n-byte offset (see below) interpreted as a negative
offset from the type-byte of the header of the
ofs-delta entry (the size above is the size of
the delta data that follows).
delta data, deflated.
offset encoding:
n bytes with MSB set in all but the last one.
The offset is then the number constructed by
concatenating the lower 7 bit of each byte, and
for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
to the result.
== Version 2 pack-*.idx files support packs larger than 4 GiB, and
have some other reorganizations. They have the format:
- A 4-byte magic number '\377tOc' which is an unreasonable
fanout[0] value.
- A 4-byte version number (= 2)
- A 256-entry fan-out table just like v1.
- A table of sorted object names. These are packed together
without offset values to reduce the cache footprint of the
binary search for a specific object name.
- A table of 4-byte CRC32 values of the packed object data.
This is new in v2 so compressed data can be copied directly
from pack to pack during repacking without undetected
data corruption.
- A table of 4-byte offset values (in network byte order).
These are usually 31-bit pack file offsets, but large
offsets are encoded as an index into the next table with
the msbit set.
- A table of 8-byte offset entries (empty for pack files less
than 2 GiB). Pack files are organized with heavily used
objects toward the front, so most object references should
not need to refer to this table.
- The same trailer as a v1 pack file:
A copy of the pack checksum at the end of
corresponding packfile.
Index checksum of all of the above.
== pack-*.rev files have the format:
- A 4-byte magic number '0x52494458' ('RIDX').
- A 4-byte version identifier (= 1).
- A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
- A table of index positions (one per packed object, num_objects in
total, each a 4-byte unsigned integer in network order), sorted by
their corresponding offsets in the packfile.
- A trailer, containing a:
checksum of the corresponding packfile, and
a checksum of all of the above.
All 4-byte numbers are in network order.
== pack-*.mtimes files have the format:
All 4-byte numbers are in network byte order.
- A 4-byte magic number '0x4d544d45' ('MTME').
- A 4-byte version identifier (= 1).
- A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
- A table of 4-byte unsigned integers. The ith value is the
modification time (mtime) of the ith object in the corresponding
pack by lexicographic (index) order. The mtimes count standard
epoch seconds.
- A trailer, containing a checksum of the corresponding packfile,
and a checksum of all of the above (each having length according
to the specified hash function).
== multi-pack-index (MIDX) files have the following format:
The multi-pack-index files refer to multiple pack-files and loose objects.
In order to allow extensions that add extra data to the MIDX, we organize
the body into "chunks" and provide a lookup table at the beginning of the
body. The header includes certain length values, such as the number of packs,
the number of base MIDX files, hash lengths and types.
All 4-byte numbers are in network order.
HEADER:
4-byte signature:
The signature is: {'M', 'I', 'D', 'X'}
1-byte version number:
Git only writes or recognizes version 1.
1-byte Object Id Version
We infer the length of object IDs (OIDs) from this value:
1 => SHA-1
2 => SHA-256
If the hash type does not match the repository's hash algorithm,
the multi-pack-index file should be ignored with a warning
presented to the user.
1-byte number of "chunks"
1-byte number of base multi-pack-index files:
This value is currently always zero.
4-byte number of pack files
CHUNK LOOKUP:
(C + 1) * 12 bytes providing the chunk offsets:
First 4 bytes describe chunk id. Value 0 is a terminating label.
Other 8 bytes provide offset in current file for chunk to start.
(Chunks are provided in file-order, so you can infer the length
using the next chunk position if necessary.)
The CHUNK LOOKUP matches the table of contents from
link:technical/chunk-format.html[the chunk-based file format].
The remaining data in the body is described one chunk at a time, and
these chunks may be given in any order. Chunks are required unless
otherwise specified.
CHUNK DATA:
Packfile Names (ID: {'P', 'N', 'A', 'M'})
Stores the packfile names as concatenated, null-terminated strings.
Packfiles must be listed in lexicographic order for fast lookups by
name. This is the only chunk not guaranteed to be a multiple of four
bytes in length, so should be the last chunk for alignment reasons.
OID Fanout (ID: {'O', 'I', 'D', 'F'})
The ith entry, F[i], stores the number of OIDs with first
byte at most i. Thus F[255] stores the total
number of objects.
OID Lookup (ID: {'O', 'I', 'D', 'L'})
The OIDs for all objects in the MIDX are stored in lexicographic
order in this chunk.
Object Offsets (ID: {'O', 'O', 'F', 'F'})
Stores two 4-byte values for every object.
1: The pack-int-id for the pack storing this object.
2: The offset within the pack.
If all offsets are less than 2^32, then the large offset chunk
will not exist and offsets are stored as in IDX v1.
If there is at least one offset value larger than 2^32-1, then
the large offset chunk must exist, and offsets larger than
2^31-1 must be stored in it instead. If the large offset chunk
exists and the 31st bit is on, then removing that bit reveals
the row in the large offsets containing the 8-byte offset of
this object.
[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
8-byte offsets into large packfiles.
[Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'})
A list of MIDX positions (one per object in the MIDX, num_objects in
total, each a 4-byte unsigned integer in network byte order), sorted
according to their relative bitmap/pseudo-pack positions.
TRAILER:
Index checksum of the above contents.
== multi-pack-index reverse indexes
Similar to the pack-based reverse index, the multi-pack index can also
be used to generate a reverse index.
Instead of mapping between offset, pack-, and index position, this
reverse index maps between an object's position within the MIDX, and
that object's position within a pseudo-pack that the MIDX describes
(i.e., the ith entry of the multi-pack reverse index holds the MIDX
position of ith object in pseudo-pack order).
To clarify the difference between these orderings, consider a multi-pack
reachability bitmap (which does not yet exist, but is what we are
building towards here). Each bit needs to correspond to an object in the
MIDX, and so we need an efficient mapping from bit position to MIDX
position.
One solution is to let bits occupy the same position in the oid-sorted
index stored by the MIDX. But because oids are effectively random, their
resulting reachability bitmaps would have no locality, and thus compress
poorly. (This is the reason that single-pack bitmaps use the pack
ordering, and not the .idx ordering, for the same purpose.)
So we'd like to define an ordering for the whole MIDX based around
pack ordering, which has far better locality (and thus compresses more
efficiently). We can think of a pseudo-pack created by the concatenation
of all of the packs in the MIDX. E.g., if we had a MIDX with three packs
(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an
ordering of the objects like:
|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
where the ordering of the packs is defined by the MIDX's pack list,
and then the ordering of objects within each pack is the same as the
order in the actual packfile.
Given the list of packs and their counts of objects, you can
naïvely reconstruct that pseudo-pack ordering (e.g., the object at
position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
slots). But there's a catch. Objects may be duplicated between packs, in
which case the MIDX only stores one pointer to the object (and thus we'd
want only one slot in the bitmap).
Callers could handle duplicates themselves by reading objects in order
of their bit-position, but that's linear in the number of objects, and
much too expensive for ordinary bitmap lookups. Building a reverse index
solves this, since it is the logical inverse of the index, and that
index has already removed duplicates. But, building a reverse index on
the fly can be expensive. Since we already have an on-disk format for
pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack,
too.
Objects from the MIDX are ordered as follows to string together the
pseudo-pack. Let `pack(o)` return the pack from which `o` was selected
by the MIDX, and define an ordering of packs based on their numeric ID
(as stored by the MIDX). Let `offset(o)` return the object offset of `o`
within `pack(o)`. Then, compare `o1` and `o2` as follows:
- If one of `pack(o1)` and `pack(o2)` is preferred and the other
is not, then the preferred one sorts first.
+
(This is a detail that allows the MIDX bitmap to determine which
pack should be used by the pack-reuse mechanism, since it can ask
the MIDX for the pack containing the object at bit position 0).
- If `pack(o1) ≠ pack(o2)`, then sort the two objects in descending
order based on the pack ID.
- Otherwise, `pack(o1) = pack(o2)`, and the objects are sorted in
pack-order (i.e., `o1` sorts ahead of `o2` exactly when `offset(o1)
< offset(o2)`).
In short, a MIDX's pseudo-pack is the de-duplicated concatenation of
objects in packs stored by the MIDX, laid out in pack order, and the
packs arranged in MIDX order (with the preferred pack coming first).
The MIDX's reverse index is stored in the optional 'RIDX' chunk within
the MIDX itself.