331 lines
12 KiB
Text
331 lines
12 KiB
Text
Git pack format
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===============
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== pack-*.pack files have the following format:
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- A header appears at the beginning and consists of the following:
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4-byte signature:
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The signature is: {'P', 'A', 'C', 'K'}
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4-byte version number (network byte order):
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Git currently accepts version number 2 or 3 but
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generates version 2 only.
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4-byte number of objects contained in the pack (network byte order)
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Observation: we cannot have more than 4G versions ;-) and
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more than 4G objects in a pack.
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- The header is followed by number of object entries, each of
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which looks like this:
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(undeltified representation)
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n-byte type and length (3-bit type, (n-1)*7+4-bit length)
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compressed data
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(deltified representation)
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n-byte type and length (3-bit type, (n-1)*7+4-bit length)
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20-byte base object name if OBJ_REF_DELTA or a negative relative
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offset from the delta object's position in the pack if this
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is an OBJ_OFS_DELTA object
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compressed delta data
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Observation: length of each object is encoded in a variable
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length format and is not constrained to 32-bit or anything.
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- The trailer records 20-byte SHA-1 checksum of all of the above.
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=== Object types
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Valid object types are:
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- OBJ_COMMIT (1)
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- OBJ_TREE (2)
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- OBJ_BLOB (3)
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- OBJ_TAG (4)
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- OBJ_OFS_DELTA (6)
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- OBJ_REF_DELTA (7)
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Type 5 is reserved for future expansion. Type 0 is invalid.
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=== Deltified representation
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Conceptually there are only four object types: commit, tree, tag and
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blob. However to save space, an object could be stored as a "delta" of
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another "base" object. These representations are assigned new types
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ofs-delta and ref-delta, which is only valid in a pack file.
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Both ofs-delta and ref-delta store the "delta" to be applied to
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another object (called 'base object') to reconstruct the object. The
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difference between them is, ref-delta directly encodes 20-byte base
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object name. If the base object is in the same pack, ofs-delta encodes
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the offset of the base object in the pack instead.
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The base object could also be deltified if it's in the same pack.
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Ref-delta can also refer to an object outside the pack (i.e. the
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so-called "thin pack"). When stored on disk however, the pack should
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be self contained to avoid cyclic dependency.
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The delta data is a sequence of instructions to reconstruct an object
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from the base object. If the base object is deltified, it must be
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converted to canonical form first. Each instruction appends more and
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more data to the target object until it's complete. There are two
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supported instructions so far: one for copy a byte range from the
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source object and one for inserting new data embedded in the
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instruction itself.
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Each instruction has variable length. Instruction type is determined
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by the seventh bit of the first octet. The following diagrams follow
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the convention in RFC 1951 (Deflate compressed data format).
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==== Instruction to copy from base object
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+----------+---------+---------+---------+---------+-------+-------+-------+
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| 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
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+----------+---------+---------+---------+---------+-------+-------+-------+
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This is the instruction format to copy a byte range from the source
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object. It encodes the offset to copy from and the number of bytes to
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copy. Offset and size are in little-endian order.
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All offset and size bytes are optional. This is to reduce the
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instruction size when encoding small offsets or sizes. The first seven
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bits in the first octet determines which of the next seven octets is
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present. If bit zero is set, offset1 is present. If bit one is set
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offset2 is present and so on.
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Note that a more compact instruction does not change offset and size
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encoding. For example, if only offset2 is omitted like below, offset3
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still contains bits 16-23. It does not become offset2 and contains
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bits 8-15 even if it's right next to offset1.
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+----------+---------+---------+
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| 10000101 | offset1 | offset3 |
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+----------+---------+---------+
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In its most compact form, this instruction only takes up one byte
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(0x80) with both offset and size omitted, which will have default
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values zero. There is another exception: size zero is automatically
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converted to 0x10000.
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==== Instruction to add new data
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+----------+============+
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| 0xxxxxxx | data |
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+----------+============+
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This is the instruction to construct target object without the base
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object. The following data is appended to the target object. The first
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seven bits of the first octet determines the size of data in
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bytes. The size must be non-zero.
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==== Reserved instruction
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+----------+============
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| 00000000 |
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+----------+============
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This is the instruction reserved for future expansion.
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== Original (version 1) pack-*.idx files have the following format:
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- The header consists of 256 4-byte network byte order
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integers. N-th entry of this table records the number of
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objects in the corresponding pack, the first byte of whose
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object name is less than or equal to N. This is called the
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'first-level fan-out' table.
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- The header is followed by sorted 24-byte entries, one entry
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per object in the pack. Each entry is:
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4-byte network byte order integer, recording where the
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object is stored in the packfile as the offset from the
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beginning.
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20-byte object name.
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- The file is concluded with a trailer:
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A copy of the 20-byte SHA-1 checksum at the end of
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corresponding packfile.
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20-byte SHA-1-checksum of all of the above.
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Pack Idx file:
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-- +--------------------------------+
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fanout | fanout[0] = 2 (for example) |-.
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table +--------------------------------+ |
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| fanout[1] | |
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+--------------------------------+ |
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| fanout[2] | |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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| fanout[255] = total objects |---.
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-- +--------------------------------+ | |
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main | offset | | |
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index | object name 00XXXXXXXXXXXXXXXX | | |
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table +--------------------------------+ | |
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| offset | | |
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| object name 00XXXXXXXXXXXXXXXX | | |
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+--------------------------------+<+ |
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.-| offset | |
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| | object name 01XXXXXXXXXXXXXXXX | |
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| +--------------------------------+ |
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| | offset | |
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| | object name 01XXXXXXXXXXXXXXXX | |
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| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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| | offset | |
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| | object name FFXXXXXXXXXXXXXXXX | |
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--| +--------------------------------+<--+
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trailer | | packfile checksum |
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| +--------------------------------+
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| | idxfile checksum |
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| +--------------------------------+
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.-------.
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Pack file entry: <+
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packed object header:
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1-byte size extension bit (MSB)
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type (next 3 bit)
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size0 (lower 4-bit)
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n-byte sizeN (as long as MSB is set, each 7-bit)
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size0..sizeN form 4+7+7+..+7 bit integer, size0
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is the least significant part, and sizeN is the
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most significant part.
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packed object data:
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If it is not DELTA, then deflated bytes (the size above
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is the size before compression).
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If it is REF_DELTA, then
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20-byte base object name SHA-1 (the size above is the
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size of the delta data that follows).
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delta data, deflated.
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If it is OFS_DELTA, then
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n-byte offset (see below) interpreted as a negative
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offset from the type-byte of the header of the
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ofs-delta entry (the size above is the size of
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the delta data that follows).
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delta data, deflated.
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offset encoding:
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n bytes with MSB set in all but the last one.
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The offset is then the number constructed by
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concatenating the lower 7 bit of each byte, and
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for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
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to the result.
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== Version 2 pack-*.idx files support packs larger than 4 GiB, and
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have some other reorganizations. They have the format:
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- A 4-byte magic number '\377tOc' which is an unreasonable
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fanout[0] value.
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- A 4-byte version number (= 2)
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- A 256-entry fan-out table just like v1.
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- A table of sorted 20-byte SHA-1 object names. These are
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packed together without offset values to reduce the cache
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footprint of the binary search for a specific object name.
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- A table of 4-byte CRC32 values of the packed object data.
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This is new in v2 so compressed data can be copied directly
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from pack to pack during repacking without undetected
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data corruption.
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- A table of 4-byte offset values (in network byte order).
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These are usually 31-bit pack file offsets, but large
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offsets are encoded as an index into the next table with
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the msbit set.
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- A table of 8-byte offset entries (empty for pack files less
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than 2 GiB). Pack files are organized with heavily used
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objects toward the front, so most object references should
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not need to refer to this table.
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- The same trailer as a v1 pack file:
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A copy of the 20-byte SHA-1 checksum at the end of
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corresponding packfile.
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20-byte SHA-1-checksum of all of the above.
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== multi-pack-index (MIDX) files have the following format:
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The multi-pack-index files refer to multiple pack-files and loose objects.
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In order to allow extensions that add extra data to the MIDX, we organize
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the body into "chunks" and provide a lookup table at the beginning of the
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body. The header includes certain length values, such as the number of packs,
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the number of base MIDX files, hash lengths and types.
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All 4-byte numbers are in network order.
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HEADER:
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4-byte signature:
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The signature is: {'M', 'I', 'D', 'X'}
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1-byte version number:
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Git only writes or recognizes version 1.
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1-byte Object Id Version
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Git only writes or recognizes version 1 (SHA1).
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1-byte number of "chunks"
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1-byte number of base multi-pack-index files:
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This value is currently always zero.
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4-byte number of pack files
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CHUNK LOOKUP:
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(C + 1) * 12 bytes providing the chunk offsets:
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First 4 bytes describe chunk id. Value 0 is a terminating label.
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Other 8 bytes provide offset in current file for chunk to start.
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(Chunks are provided in file-order, so you can infer the length
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using the next chunk position if necessary.)
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The remaining data in the body is described one chunk at a time, and
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these chunks may be given in any order. Chunks are required unless
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otherwise specified.
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CHUNK DATA:
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Packfile Names (ID: {'P', 'N', 'A', 'M'})
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Stores the packfile names as concatenated, null-terminated strings.
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Packfiles must be listed in lexicographic order for fast lookups by
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name. This is the only chunk not guaranteed to be a multiple of four
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bytes in length, so should be the last chunk for alignment reasons.
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OID Fanout (ID: {'O', 'I', 'D', 'F'})
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The ith entry, F[i], stores the number of OIDs with first
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byte at most i. Thus F[255] stores the total
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number of objects.
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OID Lookup (ID: {'O', 'I', 'D', 'L'})
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The OIDs for all objects in the MIDX are stored in lexicographic
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order in this chunk.
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Object Offsets (ID: {'O', 'O', 'F', 'F'})
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Stores two 4-byte values for every object.
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1: The pack-int-id for the pack storing this object.
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2: The offset within the pack.
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If all offsets are less than 2^31, then the large offset chunk
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will not exist and offsets are stored as in IDX v1.
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If there is at least one offset value larger than 2^32-1, then
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the large offset chunk must exist. If the large offset chunk
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exists and the 31st bit is on, then removing that bit reveals
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the row in the large offsets containing the 8-byte offset of
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this object.
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[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
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8-byte offsets into large packfiles.
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TRAILER:
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20-byte SHA1-checksum of the above contents.
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