1 # pack.py -- For dealing wih packed git objects.
2 # Copyright (C) 2007 James Westby <jw+debian@jameswestby.net>
3 # The code is loosely based on that in the sha1_file.c file from git itself,
4 # which is Copyright (C) Linus Torvalds, 2005 and distributed under the
7 # This program is free software; you can redistribute it and/or
8 # modify it under the terms of the GNU General Public License
9 # as published by the Free Software Foundation; version 2
12 # This program is distributed in the hope that it will be useful,
13 # but WITHOUT ANY WARRANTY; without even the implied warranty of
14 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 # GNU General Public License for more details.
17 # You should have received a copy of the GNU General Public License
18 # along with this program; if not, write to the Free Software
19 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
22 """Classes for dealing with packed git objects.
24 A pack is a compact representation of a bunch of objects, stored
25 using deltas where possible.
27 They have two parts, the pack file, which stores the data, and an index
28 that tells you where the data is.
30 To find an object you look in all of the index files 'til you find a
31 match for the object name. You then use the pointer got from this as
32 a pointer in to the corresponding packfile.
38 from objects import (ShaFile,
43 """Converts a hex value to the number it represents"""
44 mapping = { '0' : 0, '1' : 1, '2' : 2, '3' : 3, '4' : 4, '5' : 5, '6' : 6,
45 '7' : 7, '8' : 8, '9' : 9, 'a' : 10, 'b' : 11, 'c' : 12,
46 'd' : 13, 'e' : 14, 'f' : 15}
49 value = (16 * value) + mapping[c]
52 def multi_ord(map, start, count):
54 for i in range(count):
55 value = value * 256 + ord(map[start+i])
58 max_size = 256 * 1024 * 1024
60 class PackIndex(object):
61 """An index in to a packfile.
63 Given a sha id of an object a pack index can tell you the location in the
64 packfile of that object if it has it.
66 To do the looup it opens the file, and indexes first 256 4 byte groups
67 with the first byte of the sha id. The value in the four byte group indexed
68 is the end of the group that shares the same starting byte. Subtract one
69 from the starting byte and index again to find the start of the group.
70 The values are sorted by sha id within the group, so do the math to find
71 the start and end offset and then bisect in to find if the value is present.
74 header_record_size = 4
75 header_size = 256 * header_record_size
78 record_size = sha_bytes + index_size
80 def __init__(self, filename):
81 """Create a pack index object.
83 Provide it with the name of the index file to consider, and it will map
86 self._filename = filename
87 assert os.path.exists(filename), "%s is not a pack index" % filename
88 # Take the size now, so it can be checked each time we map the file to
89 # ensure that it hasn't changed.
90 self._size = os.path.getsize(filename)
91 assert self._size > self.header_size, "%s is too small to be a packfile" % \
93 assert self._size < max_size, "%s is larger than 256 meg, and it " \
94 "might not be a good idea to mmap it. If you want to go ahead " \
95 "delete this check, or get python to support mmap offsets so that " \
96 "I can map the files sensibly"
98 def object_index(self, sha):
99 """Return the index in to the corresponding packfile for the object.
101 Given the name of an object it will return the offset that object lives
102 at within the corresponding pack file. If the pack file doesn't have the
103 object then None will be returned.
105 size = os.path.getsize(self._filename)
106 assert size == self._size, "Pack index %s has changed size, I don't " \
107 "like that" % self._filename
108 f = open(self._filename, 'rb')
110 map = mmap.mmap(f.fileno(), size, access=mmap.ACCESS_READ)
111 return self._object_index(map, sha)
115 def _object_index(self, map, hexsha):
116 """See object_index"""
117 first_byte = hex_to_sha(hexsha[:2])
118 header_offset = self.header_record_size * first_byte
119 start = multi_ord(map, header_offset-self.header_record_size, self.header_record_size)
120 end = multi_ord(map, header_offset, self.header_record_size)
121 sha = hex_to_sha(hexsha)
124 offset = self.header_size + (i * self.record_size)
125 file_sha = multi_ord(map, offset + self.index_size, self.sha_bytes)
127 return multi_ord(map, offset, self.index_size)
135 class PackData(object):
136 """The data contained in a packfile.
138 Pack files can be accessed both sequentially for exploding a pack, and
139 directly with the help of an index to retrieve a specific object.
141 The objects within are either complete or a delta aginst another.
143 The header is variable length. If the MSB of each byte is set then it
144 indicates that the subsequent byte is still part of the header.
145 For the first byte the next MS bits are the type, which tells you the type
146 of object, and whether it is a delta. The LS byte is the lowest bits of the
147 size. For each subsequent byte the LS 7 bits are the next MS bits of the
148 size, i.e. the last byte of the header contains the MS bits of the size.
150 For the complete objects the data is stored as zlib deflated data.
151 The size in the header is the uncompressed object size, so to uncompress
152 you need to just keep feeding data to zlib until you get an object back,
153 or it errors on bad data. This is done here by just giving the complete
154 buffer from the start of the deflated object on. This is bad, but until I
155 get mmap sorted out it will have to do.
157 Currently there are no integrity checks done. Also no attempt is made to try
158 and detect the delta case, or a request for an object at the wrong position.
159 It will all just throw a zlib or KeyError.
162 def __init__(self, filename):
163 """Create a PackData object that represents the pack in the given filename.
165 The file must exist and stay readable until the object is disposed of. It
166 must also stay the same size. It will be mapped whenever needed.
168 Currently there is a restriction on the size of the pack as the python
169 mmap implementation is flawed.
171 self._filename = filename
172 assert os.path.exists(filename), "%s is not a packfile" % filename
173 self._size = os.path.getsize(filename)
174 assert self._size < max_size, "%s is larger than 256 meg, and it " \
175 "might not be a good idea to mmap it. If you want to go ahead " \
176 "delete this check, or get python to support mmap offsets so that " \
177 "I can map the files sensibly"
179 def get_object_at(self, offset):
180 """Given an offset in to the packfile return the object that is there.
182 Using the associated index the location of an object can be looked up, and
183 then the packfile can be asked directly for that object using this
186 Currently only non-delta objects are supported.
188 size = os.path.getsize(self._filename)
189 assert size == self._size, "Pack data %s has changed size, I don't " \
190 "like that" % self._filename
191 f = open(self._filename, 'rb')
193 map = mmap.mmap(f.fileno(), size, access=mmap.ACCESS_READ)
194 return self._get_object_at(map, offset)
198 def _get_object_at(self, map, offset):
199 first_byte = ord(map[offset])
200 sign_extend = first_byte & 0x80
201 type = (first_byte >> 4) & 0x07
202 size = first_byte & 0x0f
204 while sign_extend > 0:
205 byte = ord(map[offset+cur_offset+1])
206 sign_extend = byte & 0x80
207 size_part = byte & 0x7f
208 size += size_part << ((cur_offset * 7) + 4)
210 raw_base = offset+cur_offset+1
211 # The size is the inflated size, so we have no idea what the deflated size
212 # is, so for now give it as much as we have. It should really iterate
213 # feeding it more data if it doesn't decompress, but as we have the whole
214 # thing then just use it.
216 uncomp = _decompress(raw)
217 obj = ShaFile.from_raw_string(type, uncomp)