Add some basic pack handling code.

It has classes for the index and data parts. It supports lookup of an object
name in the index, and then access to the object in the data part using the
offset returned from the index lookup.

There are many problems with it so far.

  * The mmap in python has no offset, so the whole files are mapped.
  * There is no support for delta objects.
  * There is no consistency checking.
  * The code is not hooked up to provide a simple API.
  * The code is not hooked in to the repo, so that objects are still not
    retrieved from packs.

git/objects.py

@@ -132,6 +132,19 @@ class ShaFile(object):
     finally:
       f.close()
 
+  @classmethod
+  def from_raw_string(cls, type, string):
+    """Creates an object of the indicated type from the raw string given.
+
+    Type is the numeric type of an object. String is the raw uncompressed
+    contents.
+    """
+    real_class = num_type_map[type]
+    obj = real_class()
+    obj._text = string
+    obj._update_contents()
+    return obj
+
   def _header(self):
     return "%s %lu\0" % (self._type, len(self._contents))
 

git/pack.py

@@ -0,0 +1,219 @@
+# pack.py -- For dealing wih packed git objects.
+# Copyright (C) 2007 James Westby <jw+debian@jameswestby.net>
+# The code is loosely based on that in the sha1_file.c file from git itself,
+# which is Copyright (C) Linus Torvalds, 2005 and distributed under the
+# GPL version 2.
+# 
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; version 2
+# of the License.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+# MA  02110-1301, USA.
+
+"""Classes for dealing with packed git objects.
+
+A pack is a compact representation of a bunch of objects, stored
+using deltas where possible.
+
+They have two parts, the pack file, which stores the data, and an index
+that tells you where the data is.
+
+To find an object you look in all of the index files 'til you find a
+match for the object name. You then use the pointer got from this as
+a pointer in to the corresponding packfile.
+"""
+
+import mmap
+import os
+
+from objects import (ShaFile,
+                     _decompress,
+                     )
+
+def hex_to_sha(hex):
+  """Converts a hex value to the number it represents"""
+  mapping = { '0' : 0, '1' : 1, '2' : 2, '3' : 3, '4' : 4, '5' : 5, '6' : 6,
+              '7' : 7, '8' : 8, '9' : 9, 'a' : 10, 'b' : 11, 'c' : 12,
+              'd' : 13, 'e' : 14, 'f' : 15}
+  value = 0
+  for c in hex:
+    value = (16 * value) + mapping[c]
+  return value
+
+def multi_ord(map, start, count):
+  value = 0
+  for i in range(count):
+    value = value * 256 + ord(map[start+i])
+  return value
+
+max_size = 256 * 1024 * 1024
+
+class PackIndex(object):
+  """An index in to a packfile.
+
+  Given a sha id of an object a pack index can tell you the location in the
+  packfile of that object if it has it.
+
+  To do the looup it opens the file, and indexes first 256 4 byte groups
+  with the first byte of the sha id. The value in the four byte group indexed
+  is the end of the group that shares the same starting byte. Subtract one
+  from the starting byte and index again to find the start of the group.
+  The values are sorted by sha id within the group, so do the math to find
+  the start and end offset and then bisect in to find if the value is present.
+  """
+
+  header_record_size = 4
+  header_size = 256 * header_record_size
+  index_size = 4
+  sha_bytes = 20
+  record_size = sha_bytes + index_size
+
+  def __init__(self, filename):
+    """Create a pack index object.
+
+    Provide it with the name of the index file to consider, and it will map
+    it whenever required.
+    """
+    self._filename = filename
+    assert os.path.exists(filename), "%s is not a pack index" % filename
+    # Take the size now, so it can be checked each time we map the file to
+    # ensure that it hasn't changed.
+    self._size = os.path.getsize(filename)
+    assert self._size > self.header_size, "%s is too small to be a packfile" % \
+        filename
+    assert self._size < max_size, "%s is larger than 256 meg, and it " \
+        "might not be a good idea to mmap it. If you want to go ahead " \
+        "delete this check, or get python to support mmap offsets so that " \
+        "I can map the files sensibly"
+
+  def object_index(self, sha):
+    """Return the index in to the corresponding packfile for the object.
+
+    Given the name of an object it will return the offset that object lives
+    at within the corresponding pack file. If the pack file doesn't have the
+    object then None will be returned.
+    """
+    size = os.path.getsize(self._filename)
+    assert size == self._size, "Pack index %s has changed size, I don't " \
+         "like that" % self._filename
+    f = open(self._filename, 'rb')
+    try:
+      map = mmap.mmap(f.fileno(), size, access=mmap.ACCESS_READ)
+      return self._object_index(map, sha)
+    finally:
+      f.close()
+
+  def _object_index(self, map, hexsha):
+    """See object_index"""
+    first_byte = hex_to_sha(hexsha[:2])
+    header_offset = self.header_record_size * first_byte
+    start = multi_ord(map, header_offset-self.header_record_size, self.header_record_size)
+    end = multi_ord(map, header_offset, self.header_record_size)
+    sha = hex_to_sha(hexsha)
+    while start < end:
+      i = (start + end)/2
+      offset = self.header_size + (i * self.record_size)
+      file_sha = multi_ord(map, offset + self.index_size, self.sha_bytes)
+      if file_sha == sha:
+        return multi_ord(map, offset, self.index_size)
+      elif file_sha < sha:
+        start = offset + 1
+      else:
+        end = offset - 1
+    return None
+
+
+class PackData(object):
+  """The data contained in a packfile.
+
+  Pack files can be accessed both sequentially for exploding a pack, and
+  directly with the help of an index to retrieve a specific object.
+
+  The objects within are either complete or a delta aginst another.
+
+  The header is variable length. If the MSB of each byte is set then it
+  indicates that the subsequent byte is still part of the header.
+  For the first byte the next MS bits are the type, which tells you the type
+  of object, and whether it is a delta. The LS byte is the lowest bits of the
+  size. For each subsequent byte the LS 7 bits are the next MS bits of the
+  size, i.e. the last byte of the header contains the MS bits of the size.
+
+  For the complete objects the data is stored as zlib deflated data.
+  The size in the header is the uncompressed object size, so to uncompress
+  you need to just keep feeding data to zlib until you get an object back,
+  or it errors on bad data. This is done here by just giving the complete
+  buffer from the start of the deflated object on. This is bad, but until I
+  get mmap sorted out it will have to do.
+
+  Currently there are no integrity checks done. Also no attempt is made to try
+  and detect the delta case, or a request for an object at the wrong position.
+  It will all just throw a zlib or KeyError.
+  """
+
+  def __init__(self, filename):
+    """Create a PackData object that represents the pack in the given filename.
+
+    The file must exist and stay readable until the object is disposed of. It
+    must also stay the same size. It will be mapped whenever needed.
+
+    Currently there is a restriction on the size of the pack as the python
+    mmap implementation is flawed.
+    """
+    self._filename = filename
+    assert os.path.exists(filename), "%s is not a packfile" % filename
+    self._size = os.path.getsize(filename)
+    assert self._size < max_size, "%s is larger than 256 meg, and it " \
+        "might not be a good idea to mmap it. If you want to go ahead " \
+        "delete this check, or get python to support mmap offsets so that " \
+        "I can map the files sensibly"
+
+  def get_object_at(self, offset):
+    """Given an offset in to the packfile return the object that is there.
+
+    Using the associated index the location of an object can be looked up, and
+    then the packfile can be asked directly for that object using this
+    function.
+
+    Currently only non-delta objects are supported.
+    """
+    size = os.path.getsize(self._filename)
+    assert size == self._size, "Pack data %s has changed size, I don't " \
+         "like that" % self._filename
+    f = open(self._filename, 'rb')
+    try:
+      map = mmap.mmap(f.fileno(), size, access=mmap.ACCESS_READ)
+      return self._get_object_at(map, offset)
+    finally:
+      f.close()
+
+  def _get_object_at(self, map, offset):
+    first_byte = ord(map[offset])
+    sign_extend = first_byte & 0x80
+    type = (first_byte >> 4) & 0x07
+    size = first_byte & 0x0f
+    cur_offset = 0
+    while sign_extend > 0:
+      byte = ord(map[offset+cur_offset+1])
+      sign_extend = byte & 0x80
+      size_part = byte & 0x7f
+      size += size_part << ((cur_offset * 7) + 4)
+      cur_offset += 1
+    raw_base = offset+cur_offset+1
+    # The size is the inflated size, so we have no idea what the deflated size
+    # is, so for now give it as much as we have. It should really iterate
+    # feeding it more data if it doesn't decompress, but as we have the whole
+    # thing then just use it.
+    raw = map[raw_base:]
+    uncomp = _decompress(raw)
+    obj = ShaFile.from_raw_string(type, uncomp)
+    return obj
+

git/tests/__init__.py

@@ -18,12 +18,19 @@
 
 import unittest
 import test_objects
+import test_repository
+import test_pack
 
 def test_suite():
-  test_modules = [test_objects]
+  test_modules = [test_objects, test_repository, test_pack]
   loader = unittest.TestLoader()
   suite = unittest.TestSuite()
   for mod in test_modules:
     suite.addTest(loader.loadTestsFromModule(mod))
   return suite
 
+if __name__ == '__main__':
+  suite = test_suite()
+  from unittest import TextTestRunner
+  TextTestRunner().run(suite)
+

git/tests/test_pack.py

@@ -0,0 +1,77 @@
+# test_pack.py -- Tests for the handling of git packs.
+# Copyright (C) 2007 James Westby <jw+debian@jameswestby.net>
+# 
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; version 2
+# of the License.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+# MA  02110-1301, USA.
+
+import os
+import unittest
+
+from git.pack import (PackIndex,
+                      PackData,
+                      )
+
+pack1_sha = 'bc63ddad95e7321ee734ea11a7a62d314e0d7481'
+
+a_sha = '6f670c0fb53f9463760b7295fbb814e965fb20c8'
+tree_sha = 'b2a2766a2879c209ab1176e7e778b81ae422eeaa'
+commit_sha = 'f18faa16531ac570a3fdc8c7ca16682548dafd12'
+
+class PackTests(unittest.TestCase):
+  """Base class for testing packs"""
+
+  datadir = os.path.join(os.path.dirname(__file__), 'data/packs')
+
+  def get_pack_index(self, sha):
+    """Returns a PackIndex from the datadir with the given sha"""
+    return PackIndex(os.path.join(self.datadir, 'pack-%s.idx' % sha))
+
+  def get_pack_data(self, sha):
+    """Returns a PackData object from the datadir with the given sha"""
+    return PackData(os.path.join(self.datadir, 'pack-%s.pack' % sha))
+
+
+class PackIndexTests(PackTests):
+  """Class that tests the index of packfiles"""
+
+  def test_object_index(self):
+    """Tests that the correct object offset is returned from the index."""
+    p = self.get_pack_index(pack1_sha)
+    self.assertEqual(p.object_index(pack1_sha), None)
+    self.assertEqual(p.object_index(a_sha), 178)
+    self.assertEqual(p.object_index(tree_sha), 138)
+    self.assertEqual(p.object_index(commit_sha), 12)
+
+
+class TestPackData(PackTests):
+  """Tests getting the data from the packfile."""
+
+  def test_create_pack(self):
+    p = self.get_pack_data(pack1_sha)
+
+  def test_get_object_at(self):
+    """Tests random access for non-delta objects"""
+    p = self.get_pack_data(pack1_sha)
+    idx = self.get_pack_index(pack1_sha)
+    obj = p.get_object_at(idx.object_index(a_sha))
+    self.assertEqual(obj._type, 'blob')
+    self.assertEqual(obj.sha().hexdigest(), a_sha)
+    obj = p.get_object_at(idx.object_index(tree_sha))
+    self.assertEqual(obj._type, 'tree')
+    self.assertEqual(obj.sha().hexdigest(), tree_sha)
+    obj = p.get_object_at(idx.object_index(commit_sha))
+    self.assertEqual(obj._type, 'commit')
+    self.assertEqual(obj.sha().hexdigest(), commit_sha)
+