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authorVincent Ambo <mail@tazj.in>2021-09-21T10·03+0300
committerVincent Ambo <mail@tazj.in>2021-09-21T11·29+0300
commit43b1791ec601732ac31195df96781a848360a9ac (patch)
treedaae8d638343295d2f1f7da955e556ef4c958864 /third_party/git/Documentation/technical/pack-format.txt
parent2d8e7dc9d9c38127ec4ebd13aee8e8f586a43318 (diff)
chore(3p/git): Unvendor git and track patches instead r/2903
This was vendored a long time ago under the expectation that keeping
it in sync with cgit would be easier this way, but it has proven not
to be a big issue.

On the other hand, a vendored copy of git is an annoying maintenance
burden. It is much easier to rebase the single (dottime) patch that we
have.

This removes the vendored copy of git and instead passes the git
source code to cgit via `pkgs.srcOnly`, which includes the applied
patch so that cgit can continue rendering dottime.

Change-Id: If31f62dea7ce688fd1b9050204e9378019775f2b
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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.
-
-=== 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 is a sequence of instructions to reconstruct an 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.
-
-== 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 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.
-
-TRAILER:
-
-	Index checksum of the above contents.