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While trying to implement a pack reader in Java I was mislead by some facts listed in this documentation as well as found a few details to be missing about the pack header. Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>
116 lines
3.8 KiB
Text
116 lines
3.8 KiB
Text
GIT pack format
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===============
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= pack-*.pack file has the following format:
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- The 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 (4-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 (4-bit type, (n-1)*7+4-bit length)
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20-byte base object name
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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 SHA1 checksum of all of the above.
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= pack-*.idx file has 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 are smaller than N. This is called the
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'first-level fan-out' table.
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Observation: we would need to extend this to an array of
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8-byte integers to go beyond 4G objects per pack, but it is
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not strictly necessary.
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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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Observation: we would definitely need to extend this to
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8-byte integer plus 20-byte object name to handle a packfile
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that is larger than 4GB.
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- The file is concluded with a trailer:
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A copy of the 20-byte SHA1 checksum at the end of
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corresponding packfile.
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20-byte SHA1-checksum of all of the above.
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Pack Idx file:
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idx
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+--------------------------------+
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| fanout[0] = 2 |-.
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+--------------------------------+ |
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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] | |
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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 type (upper 4-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 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 DELTA, then
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20-byte base object name SHA1 (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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