]> arthur.barton.de Git - bup.git/blobdiff - lib/bup/git.py
get: adjust for python 3 and test there
[bup.git] / lib / bup / git.py
index 0f770a45261ecef7255b7ed329d37ca638695ec8..f4e27ad327658168004a69bcd746630c524a99dd 100644 (file)
@@ -3,28 +3,41 @@ bup repositories are in Git format. This library allows us to
 interact with the Git data structures.
 """
 
+from __future__ import absolute_import, print_function
 import errno, os, sys, zlib, time, subprocess, struct, stat, re, tempfile, glob
+from array import array
+from binascii import hexlify, unhexlify
 from collections import namedtuple
 from itertools import islice
-
-from bup import _helpers, hashsplit, path, midx, bloom, xstat
+from numbers import Integral
+
+from bup import _helpers, compat, hashsplit, path, midx, bloom, xstat
+from bup.compat import (buffer,
+                        byte_int, bytes_from_byte, bytes_from_uint,
+                        environ,
+                        items,
+                        range,
+                        reraise)
+from bup.io import path_msg
 from bup.helpers import (Sha1, add_error, chunkyreader, debug1, debug2,
                          fdatasync,
-                         hostname, localtime, log, merge_iter,
+                         hostname, localtime, log,
+                         merge_dict,
+                         merge_iter,
                          mmap_read, mmap_readwrite,
-                         progress, qprogress, unlink, username, userfullname,
+                         parse_num,
+                         progress, qprogress, stat_if_exists,
+                         unlink,
                          utc_offset_str)
+from bup.pwdgrp import username, userfullname
 
 
-max_pack_size = 1000*1000*1000  # larger packs will slow down pruning
-max_pack_objects = 200*1000  # cache memory usage is about 83 bytes per object
-
 verbose = 0
-ignore_midx = 0
-repodir = None
+repodir = None  # The default repository, once initialized
+
+_typemap =  {b'blob': 3, b'tree': 2, b'commit': 1, b'tag': 4}
+_typermap = {v: k for k, v in items(_typemap)}
 
-_typemap =  { 'blob':3, 'tree':2, 'commit':1, 'tag':4 }
-_typermap = { 3:'blob', 2:'tree', 1:'commit', 4:'tag' }
 
 _total_searches = 0
 _total_steps = 0
@@ -34,34 +47,61 @@ class GitError(Exception):
     pass
 
 
+def _gitenv(repo_dir=None):
+    if not repo_dir:
+        repo_dir = repo()
+    return merge_dict(environ, {b'GIT_DIR': os.path.abspath(repo_dir)})
+
+def _git_wait(cmd, p):
+    rv = p.wait()
+    if rv != 0:
+        raise GitError('%r returned %d' % (cmd, rv))
+
+def git_config_get(option, repo_dir=None):
+    cmd = (b'git', b'config', b'--get', option)
+    p = subprocess.Popen(cmd, stdout=subprocess.PIPE,
+                         env=_gitenv(repo_dir=repo_dir))
+    r = p.stdout.read()
+    rc = p.wait()
+    if rc == 0:
+        return r
+    if rc != 1:
+        raise GitError('%r returned %d' % (cmd, rc))
+    return None
+
+
 def parse_tz_offset(s):
     """UTC offset in seconds."""
     tz_off = (int(s[1:3]) * 60 * 60) + (int(s[3:5]) * 60)
-    if s[0] == '-':
+    if bytes_from_byte(s[0]) == b'-':
         return - tz_off
     return tz_off
 
 
 # FIXME: derived from http://git.rsbx.net/Documents/Git_Data_Formats.txt
 # Make sure that's authoritative.
-_start_end_char = r'[^ .,:;<>"\'\0\n]'
-_content_char = r'[^\0\n<>]'
-_safe_str_rx = '(?:%s{1,2}|(?:%s%s*%s))' \
+_start_end_char = br'[^ .,:;<>"\'\0\n]'
+_content_char = br'[^\0\n<>]'
+_safe_str_rx = br'(?:%s{1,2}|(?:%s%s*%s))' \
     % (_start_end_char,
        _start_end_char, _content_char, _start_end_char)
-_tz_rx = r'[-+]\d\d[0-5]\d'
-_parent_rx = r'(?:parent [abcdefABCDEF0123456789]{40}\n)'
-_commit_rx = re.compile(r'''tree (?P<tree>[abcdefABCDEF0123456789]{40})
+_tz_rx = br'[-+]\d\d[0-5]\d'
+_parent_rx = br'(?:parent [abcdefABCDEF0123456789]{40}\n)'
+# Assumes every following line starting with a space is part of the
+# mergetag.  Is there a formal commit blob spec?
+_mergetag_rx = br'(?:\nmergetag object [abcdefABCDEF0123456789]{40}(?:\n [^\0\n]*)*)'
+_commit_rx = re.compile(br'''tree (?P<tree>[abcdefABCDEF0123456789]{40})
 (?P<parents>%s*)author (?P<author_name>%s) <(?P<author_mail>%s)> (?P<asec>\d+) (?P<atz>%s)
-committer (?P<committer_name>%s) <(?P<committer_mail>%s)> (?P<csec>\d+) (?P<ctz>%s)
+committer (?P<committer_name>%s) <(?P<committer_mail>%s)> (?P<csec>\d+) (?P<ctz>%s)(?P<mergetag>%s?)
 
 (?P<message>(?:.|\n)*)''' % (_parent_rx,
                              _safe_str_rx, _safe_str_rx, _tz_rx,
-                             _safe_str_rx, _safe_str_rx, _tz_rx))
-_parent_hash_rx = re.compile(r'\s*parent ([abcdefABCDEF0123456789]{40})\s*')
-
+                             _safe_str_rx, _safe_str_rx, _tz_rx,
+                             _mergetag_rx))
+_parent_hash_rx = re.compile(br'\s*parent ([abcdefABCDEF0123456789]{40})\s*')
 
-# Note that the author_sec and committer_sec values are (UTC) epoch seconds.
+# Note that the author_sec and committer_sec values are (UTC) epoch
+# seconds, and for now the mergetag is not included.
 CommitInfo = namedtuple('CommitInfo', ['tree', 'parents',
                                        'author_name', 'author_mail',
                                        'author_sec', 'author_offset',
@@ -87,68 +127,71 @@ def parse_commit(content):
                       message=matches['message'])
 
 
-def get_commit_items(id, cp):
-    commit_it = cp.get(id)
-    assert(commit_it.next() == 'commit')
-    commit_content = ''.join(commit_it)
-    return parse_commit(commit_content)
+def get_cat_data(cat_iterator, expected_type):
+    _, kind, _ = next(cat_iterator)
+    if kind != expected_type:
+        raise Exception('expected %r, saw %r' % (expected_type, kind))
+    return b''.join(cat_iterator)
 
+def get_commit_items(id, cp):
+    return parse_commit(get_cat_data(cp.get(id), b'commit'))
 
 def _local_git_date_str(epoch_sec):
-    return '%d %s' % (epoch_sec, utc_offset_str(epoch_sec))
+    return b'%d %s' % (epoch_sec, utc_offset_str(epoch_sec))
 
 
 def _git_date_str(epoch_sec, tz_offset_sec):
     offs =  tz_offset_sec // 60
-    return '%d %s%02d%02d' \
+    return b'%d %s%02d%02d' \
         % (epoch_sec,
-           '+' if offs >= 0 else '-',
+           b'+' if offs >= 0 else b'-',
            abs(offs) // 60,
            abs(offs) % 60)
 
 
-def repo(sub = '', repo_dir=None):
+def repo(sub = b'', repo_dir=None):
     """Get the path to the git repository or one of its subdirectories."""
-    global repodir
     repo_dir = repo_dir or repodir
     if not repo_dir:
         raise GitError('You should call check_repo_or_die()')
 
     # If there's a .git subdirectory, then the actual repo is in there.
-    gd = os.path.join(repo_dir, '.git')
+    gd = os.path.join(repo_dir, b'.git')
     if os.path.exists(gd):
-        repodir = gd
+        repo_dir = gd
 
     return os.path.join(repo_dir, sub)
 
 
+_shorten_hash_rx = \
+    re.compile(br'([^0-9a-z]|\b)([0-9a-z]{7})[0-9a-z]{33}([^0-9a-z]|\b)')
+
 def shorten_hash(s):
-    return re.sub(r'([^0-9a-z]|\b)([0-9a-z]{7})[0-9a-z]{33}([^0-9a-z]|\b)',
-                  r'\1\2*\3', s)
+    return _shorten_hash_rx.sub(br'\1\2*\3', s)
 
 
 def repo_rel(path):
     full = os.path.abspath(path)
-    fullrepo = os.path.abspath(repo(''))
-    if not fullrepo.endswith('/'):
-        fullrepo += '/'
+    fullrepo = os.path.abspath(repo(b''))
+    if not fullrepo.endswith(b'/'):
+        fullrepo += b'/'
     if full.startswith(fullrepo):
         path = full[len(fullrepo):]
-    if path.startswith('index-cache/'):
-        path = path[len('index-cache/'):]
+    if path.startswith(b'index-cache/'):
+        path = path[len(b'index-cache/'):]
     return shorten_hash(path)
 
 
 def all_packdirs():
-    paths = [repo('objects/pack')]
-    paths += glob.glob(repo('index-cache/*/.'))
+    paths = [repo(b'objects/pack')]
+    paths += glob.glob(repo(b'index-cache/*/.'))
     return paths
 
 
 def auto_midx(objdir):
-    args = [path.exe(), 'midx', '--auto', '--dir', objdir]
+    args = [path.exe(), b'midx', b'--auto', b'--dir', objdir]
     try:
-        rv = subprocess.call(args, stdout=open('/dev/null', 'w'))
+        rv = subprocess.call(args, stdout=open(os.devnull, 'w'))
     except OSError as e:
         # make sure 'args' gets printed to help with debugging
         add_error('%r: exception: %s' % (args, e))
@@ -156,9 +199,9 @@ def auto_midx(objdir):
     if rv:
         add_error('%r: returned %d' % (args, rv))
 
-    args = [path.exe(), 'bloom', '--dir', objdir]
+    args = [path.exe(), b'bloom', b'--dir', objdir]
     try:
-        rv = subprocess.call(args, stdout=open('/dev/null', 'w'))
+        rv = subprocess.call(args, stdout=open(os.devnull, 'w'))
     except OSError as e:
         # make sure 'args' gets printed to help with debugging
         add_error('%r: exception: %s' % (args, e))
@@ -175,9 +218,9 @@ def mangle_name(name, mode, gitmode):
     """
     if stat.S_ISREG(mode) and not stat.S_ISREG(gitmode):
         assert(stat.S_ISDIR(gitmode))
-        return name + '.bup'
-    elif name.endswith('.bup') or name[:-1].endswith('.bup'):
-        return name + '.bupl'
+        return name + b'.bup'
+    elif name.endswith(b'.bup') or name[:-1].endswith(b'.bup'):
+        return name + b'.bupl'
     else:
         return name
 
@@ -194,11 +237,11 @@ def demangle_name(name, mode):
 
     For more information on the name mangling algorithm, see mangle_name()
     """
-    if name.endswith('.bupl'):
+    if name.endswith(b'.bupl'):
         return (name[:-5], BUP_NORMAL)
-    elif name.endswith('.bup'):
+    elif name.endswith(b'.bup'):
         return (name[:-4], BUP_CHUNKED)
-    elif name.endswith('.bupm'):
+    elif name.endswith(b'.bupm'):
         return (name[:-5],
                 BUP_CHUNKED if stat.S_ISDIR(mode) else BUP_NORMAL)
     else:
@@ -207,7 +250,7 @@ def demangle_name(name, mode):
 
 def calc_hash(type, content):
     """Calculate some content's hash in the Git fashion."""
-    header = '%s %d\0' % (type, len(content))
+    header = b'%s %d\0' % (type, len(content))
     sum = Sha1(header)
     sum.update(content)
     return sum.digest()
@@ -217,7 +260,7 @@ def shalist_item_sort_key(ent):
     (mode, name, id) = ent
     assert(mode+0 == mode)
     if stat.S_ISDIR(mode):
-        return name + '/'
+        return name + b'/'
     else:
         return name
 
@@ -231,19 +274,19 @@ def tree_encode(shalist):
         assert(mode+0 == mode)
         assert(name)
         assert(len(bin) == 20)
-        s = '%o %s\0%s' % (mode,name,bin)
-        assert(s[0] != '0')  # 0-padded octal is not acceptable in a git tree
+        s = b'%o %s\0%s' % (mode,name,bin)
+        assert s[0] != b'0'  # 0-padded octal is not acceptable in a git tree
         l.append(s)
-    return ''.join(l)
+    return b''.join(l)
 
 
 def tree_decode(buf):
     """Generate a list of (mode,name,hash) from the git tree object in buf."""
     ofs = 0
     while ofs < len(buf):
-        z = buf.find('\0', ofs)
+        z = buf.find(b'\0', ofs)
         assert(z > ofs)
-        spl = buf[ofs:z].split(' ', 1)
+        spl = buf[ofs:z].split(b' ', 1)
         assert(len(spl) == 2)
         mode,name = spl
         sha = buf[z+1:z+1+20]
@@ -252,21 +295,19 @@ def tree_decode(buf):
 
 
 def _encode_packobj(type, content, compression_level=1):
-    szout = ''
+    if compression_level not in (0, 1, 2, 3, 4, 5, 6, 7, 8, 9):
+        raise ValueError('invalid compression level %s' % compression_level)
+    szout = b''
     sz = len(content)
     szbits = (sz & 0x0f) | (_typemap[type]<<4)
     sz >>= 4
     while 1:
         if sz: szbits |= 0x80
-        szout += chr(szbits)
+        szout += bytes_from_uint(szbits)
         if not sz:
             break
         szbits = sz & 0x7f
         sz >>= 7
-    if compression_level > 9:
-        compression_level = 9
-    elif compression_level < 0:
-        compression_level = 0
     z = zlib.compressobj(compression_level)
     yield szout
     yield z.compress(content)
@@ -275,7 +316,7 @@ def _encode_packobj(type, content, compression_level=1):
 
 def _encode_looseobj(type, content, compression_level=1):
     z = zlib.compressobj(compression_level)
-    yield z.compress('%s %d\0' % (type, len(content)))
+    yield z.compress(b'%s %d\0' % (type, len(content)))
     yield z.compress(content)
     yield z.flush()
 
@@ -283,9 +324,9 @@ def _encode_looseobj(type, content, compression_level=1):
 def _decode_looseobj(buf):
     assert(buf);
     s = zlib.decompress(buf)
-    i = s.find('\0')
+    i = s.find(b'\0')
     assert(i > 0)
-    l = s[:i].split(' ')
+    l = s[:i].split(b' ')
     type = l[0]
     sz = int(l[1])
     content = s[i+1:]
@@ -296,14 +337,14 @@ def _decode_looseobj(buf):
 
 def _decode_packobj(buf):
     assert(buf)
-    c = ord(buf[0])
+    c = byte_int(buf[0])
     type = _typermap[(c & 0x70) >> 4]
     sz = c & 0x0f
     shift = 4
     i = 0
     while c & 0x80:
         i += 1
-        c = ord(buf[i])
+        c = byte_int(buf[i])
         sz |= (c & 0x7f) << shift
         shift += 7
         if not (c & 0x80):
@@ -328,21 +369,18 @@ class PackIdx:
             return want_source and os.path.basename(self.name) or True
         return None
 
-    def __len__(self):
-        return int(self.fanout[255])
-
     def _idx_from_hash(self, hash):
         global _total_searches, _total_steps
         _total_searches += 1
         assert(len(hash) == 20)
-        b1 = ord(hash[0])
+        b1 = byte_int(hash[0])
         start = self.fanout[b1-1] # range -1..254
         end = self.fanout[b1] # range 0..255
-        want = str(hash)
+        want = hash
         _total_steps += 1  # lookup table is a step
         while start < end:
             _total_steps += 1
-            mid = start + (end-start)/2
+            mid = start + (end - start) // 2
             v = self._idx_to_hash(mid)
             if v < want:
                 start = mid+1
@@ -359,22 +397,33 @@ class PackIdxV1(PackIdx):
         self.name = filename
         self.idxnames = [self.name]
         self.map = mmap_read(f)
-        self.fanout = list(struct.unpack('!256I',
-                                         str(buffer(self.map, 0, 256*4))))
+        # Min size for 'L' is 4, which is sufficient for struct's '!I'
+        self.fanout = array('L', struct.unpack('!256I', self.map))
         self.fanout.append(0)  # entry "-1"
-        nsha = self.fanout[255]
-        self.sha_ofs = 256*4
-        self.shatable = buffer(self.map, self.sha_ofs, nsha*24)
+        self.nsha = self.fanout[255]
+        self.sha_ofs = 256 * 4
+        # Avoid slicing shatable for individual hashes (very high overhead)
+        self.shatable = buffer(self.map, self.sha_ofs, self.nsha * 24)
+
+    def __len__(self):
+        return int(self.nsha)  # int() from long for python 2
 
     def _ofs_from_idx(self, idx):
-        return struct.unpack('!I', str(self.shatable[idx*24 : idx*24+4]))[0]
+        if idx >= self.nsha or idx < 0:
+            raise IndexError('invalid pack index index %d' % idx)
+        ofs = self.sha_ofs + idx * 24
+        return struct.unpack_from('!I', self.map, offset=ofs)[0]
 
     def _idx_to_hash(self, idx):
-        return str(self.shatable[idx*24+4 : idx*24+24])
+        if idx >= self.nsha or idx < 0:
+            raise IndexError('invalid pack index index %d' % idx)
+        ofs = self.sha_ofs + idx * 24 + 4
+        return self.map[ofs : ofs + 20]
 
     def __iter__(self):
-        for i in xrange(self.fanout[255]):
-            yield buffer(self.map, 256*4 + 24*i + 4, 20)
+        start = self.sha_ofs + 4
+        for ofs in range(start, start + 24 * self.nsha, 24):
+            yield self.map[ofs : ofs + 20]
 
 
 class PackIdxV2(PackIdx):
@@ -383,38 +432,46 @@ class PackIdxV2(PackIdx):
         self.name = filename
         self.idxnames = [self.name]
         self.map = mmap_read(f)
-        assert(str(self.map[0:8]) == '\377tOc\0\0\0\2')
-        self.fanout = list(struct.unpack('!256I',
-                                         str(buffer(self.map, 8, 256*4))))
-        self.fanout.append(0)  # entry "-1"
-        nsha = self.fanout[255]
+        assert self.map[0:8] == b'\377tOc\0\0\0\2'
+        # Min size for 'L' is 4, which is sufficient for struct's '!I'
+        self.fanout = array('L', struct.unpack_from('!256I', self.map, offset=8))
+        self.fanout.append(0)
+        self.nsha = self.fanout[255]
         self.sha_ofs = 8 + 256*4
-        self.shatable = buffer(self.map, self.sha_ofs, nsha*20)
-        self.ofstable = buffer(self.map,
-                               self.sha_ofs + nsha*20 + nsha*4,
-                               nsha*4)
-        self.ofs64table = buffer(self.map,
-                                 8 + 256*4 + nsha*20 + nsha*4 + nsha*4)
+        self.ofstable_ofs = self.sha_ofs + self.nsha * 20 + self.nsha * 4
+        self.ofs64table_ofs = self.ofstable_ofs + self.nsha * 4
+        # Avoid slicing this for individual hashes (very high overhead)
+        self.shatable = buffer(self.map, self.sha_ofs, self.nsha*20)
+
+    def __len__(self):
+        return int(self.nsha)  # int() from long for python 2
 
     def _ofs_from_idx(self, idx):
-        ofs = struct.unpack('!I', str(buffer(self.ofstable, idx*4, 4)))[0]
+        if idx >= self.nsha or idx < 0:
+            raise IndexError('invalid pack index index %d' % idx)
+        ofs_ofs = self.ofstable_ofs + idx * 4
+        ofs = struct.unpack_from('!I', self.map, offset=ofs_ofs)[0]
         if ofs & 0x80000000:
             idx64 = ofs & 0x7fffffff
-            ofs = struct.unpack('!Q',
-                                str(buffer(self.ofs64table, idx64*8, 8)))[0]
+            ofs64_ofs = self.ofs64table_ofs + idx64 * 8
+            ofs = struct.unpack_from('!Q', self.map, offset=ofs64_ofs)[0]
         return ofs
 
     def _idx_to_hash(self, idx):
-        return str(self.shatable[idx*20:(idx+1)*20])
+        if idx >= self.nsha or idx < 0:
+            raise IndexError('invalid pack index index %d' % idx)
+        ofs = self.sha_ofs + idx * 20
+        return self.map[ofs : ofs + 20]
 
     def __iter__(self):
-        for i in xrange(self.fanout[255]):
-            yield buffer(self.map, 8 + 256*4 + 20*i, 20)
+        start = self.sha_ofs
+        for ofs in range(start, start + 20 * self.nsha, 20):
+            yield self.map[ofs : ofs + 20]
 
 
 _mpi_count = 0
 class PackIdxList:
-    def __init__(self, dir):
+    def __init__(self, dir, ignore_midx=False):
         global _mpi_count
         assert(_mpi_count == 0) # these things suck tons of VM; don't waste it
         _mpi_count += 1
@@ -423,6 +480,7 @@ class PackIdxList:
         self.packs = []
         self.do_bloom = False
         self.bloom = None
+        self.ignore_midx = ignore_midx
         self.refresh()
 
     def __del__(self):
@@ -448,7 +506,7 @@ class PackIdxList:
             else:
                 _total_searches -= 1  # was counted by bloom
                 return None
-        for i in xrange(len(self.packs)):
+        for i in range(len(self.packs)):
             p = self.packs[i]
             _total_searches -= 1  # will be incremented by sub-pack
             ix = p.exists(hash, want_source=want_source)
@@ -468,12 +526,12 @@ class PackIdxList:
         If skip_midx is True, all work on .midx files will be skipped and .midx
         files will be removed from the list.
 
-        The module-global variable 'ignore_midx' can force this function to
+        The instance variable 'ignore_midx' can force this function to
         always act as if skip_midx was True.
         """
         self.bloom = None # Always reopen the bloom as it may have been relaced
         self.do_bloom = False
-        skip_midx = skip_midx or ignore_midx
+        skip_midx = skip_midx or self.ignore_midx
         d = dict((p.name, p) for p in self.packs
                  if not skip_midx or not isinstance(p, midx.PackMidx))
         if os.path.exists(self.dir):
@@ -483,15 +541,16 @@ class PackIdxList:
                     if isinstance(ix, midx.PackMidx):
                         for name in ix.idxnames:
                             d[os.path.join(self.dir, name)] = ix
-                for full in glob.glob(os.path.join(self.dir,'*.midx')):
+                for full in glob.glob(os.path.join(self.dir,b'*.midx')):
                     if not d.get(full):
                         mx = midx.PackMidx(full)
                         (mxd, mxf) = os.path.split(mx.name)
                         broken = False
                         for n in mx.idxnames:
                             if not os.path.exists(os.path.join(mxd, n)):
-                                log(('warning: index %s missing\n' +
-                                    '  used by %s\n') % (n, mxf))
+                                log(('warning: index %s missing\n'
+                                     '  used by %s\n')
+                                    % (path_msg(n), path_msg(mxf)))
                                 broken = True
                         if broken:
                             mx.close()
@@ -515,10 +574,10 @@ class PackIdxList:
                             d[os.path.join(self.dir, name)] = ix
                     elif not ix.force_keep:
                         debug1('midx: removing redundant: %s\n'
-                               % os.path.basename(ix.name))
+                               % path_msg(os.path.basename(ix.name)))
                         ix.close()
                         unlink(ix.name)
-            for full in glob.glob(os.path.join(self.dir,'*.idx')):
+            for full in glob.glob(os.path.join(self.dir, b'*.idx')):
                 if not d.get(full):
                     try:
                         ix = open_idx(full)
@@ -526,11 +585,11 @@ class PackIdxList:
                         add_error(e)
                         continue
                     d[full] = ix
-            bfull = os.path.join(self.dir, 'bup.bloom')
+            bfull = os.path.join(self.dir, b'bup.bloom')
             if self.bloom is None and os.path.exists(bfull):
                 self.bloom = bloom.ShaBloom(bfull)
             self.packs = list(set(d.values()))
-            self.packs.sort(lambda x,y: -cmp(len(x),len(y)))
+            self.packs.sort(reverse=True, key=lambda x: len(x))
             if self.bloom and self.bloom.valid() and len(self.bloom) >= len(self):
                 self.do_bloom = True
             else:
@@ -544,21 +603,22 @@ class PackIdxList:
 
 
 def open_idx(filename):
-    if filename.endswith('.idx'):
+    if filename.endswith(b'.idx'):
         f = open(filename, 'rb')
         header = f.read(8)
-        if header[0:4] == '\377tOc':
+        if header[0:4] == b'\377tOc':
             version = struct.unpack('!I', header[4:8])[0]
             if version == 2:
                 return PackIdxV2(filename, f)
             else:
                 raise GitError('%s: expected idx file version 2, got %d'
-                               % (filename, version))
-        elif len(header) == 8 and header[0:4] < '\377tOc':
+                               % (path_msg(filename), version))
+        elif len(header) == 8 and header[0:4] < b'\377tOc':
             return PackIdxV1(filename, f)
         else:
-            raise GitError('%s: unrecognized idx file header' % filename)
-    elif filename.endswith('.midx'):
+            raise GitError('%s: unrecognized idx file header'
+                           % path_msg(filename))
+    elif filename.endswith(b'.midx'):
         return midx.PackMidx(filename)
     else:
         raise GitError('idx filenames must end with .idx or .midx')
@@ -577,7 +637,7 @@ def idxmerge(idxlist, final_progress=True):
 
 
 def _make_objcache():
-    return PackIdxList(repo('objects/pack'))
+    return PackIdxList(repo(b'objects/pack'))
 
 # bup-gc assumes that it can disable all PackWriter activities
 # (bloom/midx/cache) via the constructor and close() arguments.
@@ -585,7 +645,9 @@ def _make_objcache():
 class PackWriter:
     """Writes Git objects inside a pack file."""
     def __init__(self, objcache_maker=_make_objcache, compression_level=1,
-                 run_midx=True, on_pack_finish=None):
+                 run_midx=True, on_pack_finish=None,
+                 max_pack_size=None, max_pack_objects=None, repo_dir=None):
+        self.repo_dir = repo_dir or repo()
         self.file = None
         self.parentfd = None
         self.count = 0
@@ -597,14 +659,32 @@ class PackWriter:
         self.compression_level = compression_level
         self.run_midx=run_midx
         self.on_pack_finish = on_pack_finish
+        if not max_pack_size:
+            max_pack_size = git_config_get(b'pack.packSizeLimit',
+                                           repo_dir=self.repo_dir)
+            if max_pack_size is not None:
+                max_pack_size = parse_num(max_pack_size)
+            if not max_pack_size:
+                # larger packs slow down pruning
+                max_pack_size = 1000 * 1000 * 1000
+        self.max_pack_size = max_pack_size
+        # cache memory usage is about 83 bytes per object
+        self.max_pack_objects = max_pack_objects if max_pack_objects \
+                                else max(1, self.max_pack_size // 5000)
 
     def __del__(self):
         self.close()
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type, value, traceback):
+        self.close()
+
     def _open(self):
         if not self.file:
-            objdir = dir=repo('objects')
-            fd, name = tempfile.mkstemp(suffix='.pack', dir=objdir)
+            objdir = dir = os.path.join(self.repo_dir, b'objects')
+            fd, name = tempfile.mkstemp(suffix=b'.pack', dir=objdir)
             try:
                 self.file = os.fdopen(fd, 'w+b')
             except:
@@ -617,10 +697,10 @@ class PackWriter:
                 self.file = None
                 f.close()
                 raise
-            assert(name.endswith('.pack'))
+            assert name.endswith(b'.pack')
             self.filename = name[:-5]
-            self.file.write('PACK\0\0\0\2\0\0\0\0')
-            self.idx = list(list() for i in xrange(256))
+            self.file.write(b'PACK\0\0\0\2\0\0\0\0')
+            self.idx = list(list() for i in range(256))
 
     def _raw_write(self, datalist, sha):
         self._open()
@@ -630,11 +710,11 @@ class PackWriter:
         # all-or-nothing.  (The blob shouldn't be very big anyway, thanks
         # to our hashsplit algorithm.)  f.write() does its own buffering,
         # but that's okay because we'll flush it in _end().
-        oneblob = ''.join(datalist)
+        oneblob = b''.join(datalist)
         try:
             f.write(oneblob)
         except IOError as e:
-            raise GitError, e, sys.exc_info()[2]
+            reraise(GitError(e))
         nw = len(oneblob)
         crc = zlib.crc32(oneblob) & 0xffffffff
         self._update_idx(sha, crc, nw)
@@ -645,7 +725,8 @@ class PackWriter:
     def _update_idx(self, sha, crc, size):
         assert(sha)
         if self.idx:
-            self.idx[ord(sha[0])].append((sha, crc, self.file.tell() - size))
+            self.idx[byte_int(sha[0])].append((sha, crc,
+                                               self.file.tell() - size))
 
     def _write(self, sha, type, content):
         if verbose:
@@ -655,7 +736,8 @@ class PackWriter:
         size, crc = self._raw_write(_encode_packobj(type, content,
                                                     self.compression_level),
                                     sha=sha)
-        if self.outbytes >= max_pack_size or self.count >= max_pack_objects:
+        if self.outbytes >= self.max_pack_size \
+           or self.count >= self.max_pack_objects:
             self.breakpoint()
         return sha
 
@@ -677,27 +759,29 @@ class PackWriter:
         self._require_objcache()
         return self.objcache.exists(id, want_source=want_source)
 
-    def write(self, sha, type, content):
-        """Write an object to the pack file.  Fails if sha exists()."""
+    def just_write(self, sha, type, content):
+        """Write an object to the pack file without checking for duplication."""
         self._write(sha, type, content)
+        # If nothing else, gc doesn't have/want an objcache
+        if self.objcache is not None:
+            self.objcache.add(sha)
 
     def maybe_write(self, type, content):
         """Write an object to the pack file if not present and return its id."""
         sha = calc_hash(type, content)
         if not self.exists(sha):
-            self.write(sha, type, content)
             self._require_objcache()
-            self.objcache.add(sha)
+            self.just_write(sha, type, content)
         return sha
 
     def new_blob(self, blob):
         """Create a blob object in the pack with the supplied content."""
-        return self.maybe_write('blob', blob)
+        return self.maybe_write(b'blob', blob)
 
     def new_tree(self, shalist):
         """Create a tree object in the pack."""
         content = tree_encode(shalist)
-        return self.maybe_write('tree', content)
+        return self.maybe_write(b'tree', content)
 
     def new_commit(self, tree, parent,
                    author, adate_sec, adate_tz,
@@ -714,13 +798,13 @@ class PackWriter:
         else:
             cdate_str = _local_git_date_str(cdate_sec)
         l = []
-        if tree: l.append('tree %s' % tree.encode('hex'))
-        if parent: l.append('parent %s' % parent.encode('hex'))
-        if author: l.append('author %s %s' % (author, adate_str))
-        if committer: l.append('committer %s %s' % (committer, cdate_str))
-        l.append('')
+        if tree: l.append(b'tree %s' % hexlify(tree))
+        if parent: l.append(b'parent %s' % hexlify(parent))
+        if author: l.append(b'author %s %s' % (author, adate_str))
+        if committer: l.append(b'committer %s %s' % (committer, cdate_str))
+        l.append(b'')
         l.append(msg)
-        return self.maybe_write('commit', '\n'.join(l))
+        return self.maybe_write(b'commit', b'\n'.join(l))
 
     def abort(self):
         """Remove the pack file from disk."""
@@ -732,7 +816,7 @@ class PackWriter:
             self.idx = None
             try:
                 try:
-                    os.unlink(self.filename + '.pack')
+                    os.unlink(self.filename + b'.pack')
                 finally:
                     f.close()
             finally:
@@ -765,20 +849,21 @@ class PackWriter:
         finally:
             f.close()
 
-        obj_list_sha = self._write_pack_idx_v2(self.filename + '.idx', idx, packbin)
-
-        nameprefix = repo('objects/pack/pack-%s' % obj_list_sha)
-        if os.path.exists(self.filename + '.map'):
-            os.unlink(self.filename + '.map')
-        os.rename(self.filename + '.pack', nameprefix + '.pack')
-        os.rename(self.filename + '.idx', nameprefix + '.idx')
+        obj_list_sha = self._write_pack_idx_v2(self.filename + b'.idx', idx,
+                                               packbin)
+        nameprefix = os.path.join(self.repo_dir,
+                                  b'objects/pack/pack-' +  obj_list_sha)
+        if os.path.exists(self.filename + b'.map'):
+            os.unlink(self.filename + b'.map')
+        os.rename(self.filename + b'.pack', nameprefix + b'.pack')
+        os.rename(self.filename + b'.idx', nameprefix + b'.idx')
         try:
             os.fsync(self.parentfd)
         finally:
             os.close(self.parentfd)
 
         if run_midx:
-            auto_midx(repo('objects/pack'))
+            auto_midx(os.path.join(self.repo_dir, b'objects/pack'))
 
         if self.on_pack_finish:
             self.on_pack_finish(nameprefix)
@@ -825,7 +910,7 @@ class PackWriter:
             for b in chunkyreader(idx_f, 20*self.count):
                 idx_sum.update(b)
                 obj_list_sum.update(b)
-            namebase = obj_list_sum.hexdigest()
+            namebase = hexlify(obj_list_sum.digest())
 
             for b in chunkyreader(idx_f):
                 idx_sum.update(b)
@@ -836,47 +921,37 @@ class PackWriter:
             idx_f.close()
 
 
-def _gitenv(repo_dir = None):
-    if not repo_dir:
-        repo_dir = repo()
-    def env():
-        os.environ['GIT_DIR'] = os.path.abspath(repo_dir)
-    return env
-
-
-def list_refs(refname=None, repo_dir=None,
+def list_refs(patterns=None, repo_dir=None,
               limit_to_heads=False, limit_to_tags=False):
-    """Yield (refname, hash) tuples for all repository refs unless a ref
-    name is specified.  Given a ref name, only include tuples for that
-    particular ref.  The limits restrict the result items to
-    refs/heads or refs/tags.  If both limits are specified, items from
-    both sources will be included.
+    """Yield (refname, hash) tuples for all repository refs unless
+    patterns are specified.  In that case, only include tuples for
+    refs matching those patterns (cf. git-show-ref(1)).  The limits
+    restrict the result items to refs/heads or refs/tags.  If both
+    limits are specified, items from both sources will be included.
 
     """
-    argv = ['git', 'show-ref']
+    argv = [b'git', b'show-ref']
     if limit_to_heads:
-        argv.append('--heads')
+        argv.append(b'--heads')
     if limit_to_tags:
-        argv.append('--tags')
-    argv.append('--')
-    if refname:
-        argv += [refname]
-    p = subprocess.Popen(argv,
-                         preexec_fn = _gitenv(repo_dir),
-                         stdout = subprocess.PIPE)
+        argv.append(b'--tags')
+    argv.append(b'--')
+    if patterns:
+        argv.extend(patterns)
+    p = subprocess.Popen(argv, env=_gitenv(repo_dir), stdout=subprocess.PIPE)
     out = p.stdout.read().strip()
     rv = p.wait()  # not fatal
     if rv:
         assert(not out)
     if out:
-        for d in out.split('\n'):
-            (sha, name) = d.split(' ', 1)
-            yield (name, sha.decode('hex'))
+        for d in out.split(b'\n'):
+            sha, name = d.split(b' ', 1)
+            yield name, unhexlify(sha)
 
 
 def read_ref(refname, repo_dir = None):
     """Get the commit id of the most recent commit made on a given ref."""
-    refs = list_refs(refname, repo_dir=repo_dir, limit_to_heads=True)
+    refs = list_refs(patterns=[refname], repo_dir=repo_dir, limit_to_heads=True)
     l = tuple(islice(refs, 2))
     if l:
         assert(len(l) == 1)
@@ -885,35 +960,57 @@ def read_ref(refname, repo_dir = None):
         return None
 
 
-def rev_list(ref, count=None, repo_dir=None):
-    """Generate a list of reachable commits in reverse chronological order.
+def rev_list_invocation(ref_or_refs, count=None, format=None):
+    if isinstance(ref_or_refs, bytes):
+        refs = (ref_or_refs,)
+    else:
+        refs = ref_or_refs
+    argv = [b'git', b'rev-list']
+    if isinstance(count, Integral):
+        argv.extend([b'-n', b'%d' % count])
+    elif count:
+        raise ValueError('unexpected count argument %r' % count)
+
+    if format:
+        argv.append(b'--pretty=format:' + format)
+    for ref in refs:
+        assert not ref.startswith(b'-')
+        argv.append(ref)
+    argv.append(b'--')
+    return argv
+
 
-    This generator walks through commits, from child to parent, that are
-    reachable via the specified ref and yields a series of tuples of the form
-    (date,hash).
+def rev_list(ref_or_refs, count=None, parse=None, format=None, repo_dir=None):
+    """Yield information about commits as per "git rev-list".  If a format
+    is not provided, yield one hex hash at a time.  If a format is
+    provided, pass it to rev-list and call parse(git_stdout) for each
+    commit with the stream positioned just after the rev-list "commit
+    HASH" header line.  When a format is provided yield (oidx,
+    parse(git_stdout)) for each commit.
 
-    If count is a non-zero integer, limit the number of commits to "count"
-    objects.
     """
-    assert(not ref.startswith('-'))
-    opts = []
-    if count:
-        opts += ['-n', str(atoi(count))]
-    argv = ['git', 'rev-list', '--pretty=format:%at'] + opts + [ref, '--']
-    p = subprocess.Popen(argv,
-                         preexec_fn = _gitenv(repo_dir),
+    assert bool(parse) == bool(format)
+    p = subprocess.Popen(rev_list_invocation(ref_or_refs, count=count,
+                                             format=format),
+                         env=_gitenv(repo_dir),
                          stdout = subprocess.PIPE)
-    commit = None
-    for row in p.stdout:
-        s = row.strip()
-        if s.startswith('commit '):
-            commit = s[7:].decode('hex')
-        else:
-            date = int(s)
-            yield (date, commit)
+    if not format:
+        for line in p.stdout:
+            yield line.strip()
+    else:
+        line = p.stdout.readline()
+        while line:
+            s = line.strip()
+            if not s.startswith(b'commit '):
+                raise Exception('unexpected line ' + repr(s))
+            s = s[7:]
+            assert len(s) == 40
+            yield s, parse(p.stdout)
+            line = p.stdout.readline()
+
     rv = p.wait()  # not fatal
     if rv:
-        raise GitError, 'git rev-list returned error %d' % rv
+        raise GitError('git rev-list returned error %d' % rv)
 
 
 def get_commit_dates(refs, repo_dir=None):
@@ -937,14 +1034,14 @@ def rev_parse(committish, repo_dir=None):
     """
     head = read_ref(committish, repo_dir=repo_dir)
     if head:
-        debug2("resolved from ref: commit = %s\n" % head.encode('hex'))
+        debug2("resolved from ref: commit = %s\n" % hexlify(head))
         return head
 
-    pL = PackIdxList(repo('objects/pack', repo_dir=repo_dir))
+    pL = PackIdxList(repo(b'objects/pack', repo_dir=repo_dir))
 
     if len(committish) == 40:
         try:
-            hash = committish.decode('hex')
+            hash = unhexlify(committish)
         except TypeError:
             return None
 
@@ -957,21 +1054,21 @@ def rev_parse(committish, repo_dir=None):
 def update_ref(refname, newval, oldval, repo_dir=None):
     """Update a repository reference."""
     if not oldval:
-        oldval = ''
-    assert(refname.startswith('refs/heads/') \
-           or refname.startswith('refs/tags/'))
-    p = subprocess.Popen(['git', 'update-ref', refname,
-                          newval.encode('hex'), oldval.encode('hex')],
-                         preexec_fn = _gitenv(repo_dir))
-    _git_wait('git update-ref', p)
+        oldval = b''
+    assert refname.startswith(b'refs/heads/') \
+        or refname.startswith(b'refs/tags/')
+    p = subprocess.Popen([b'git', b'update-ref', refname,
+                          hexlify(newval), hexlify(oldval)],
+                         env=_gitenv(repo_dir))
+    _git_wait(b'git update-ref', p)
 
 
 def delete_ref(refname, oldvalue=None):
     """Delete a repository reference (see git update-ref(1))."""
-    assert(refname.startswith('refs/'))
+    assert refname.startswith(b'refs/')
     oldvalue = [] if not oldvalue else [oldvalue]
-    p = subprocess.Popen(['git', 'update-ref', '-d', refname] + oldvalue,
-                         preexec_fn = _gitenv())
+    p = subprocess.Popen([b'git', b'update-ref', b'-d', refname] + oldvalue,
+                         env=_gitenv())
     _git_wait('git update-ref', p)
 
 
@@ -986,9 +1083,9 @@ def guess_repo(path=None):
     if path:
         repodir = path
     if not repodir:
-        repodir = os.environ.get('BUP_DIR')
+        repodir = environ.get(b'BUP_DIR')
         if not repodir:
-            repodir = os.path.expanduser('~/.bup')
+            repodir = os.path.expanduser(b'~/.bup')
 
 
 def init_repo(path=None):
@@ -997,39 +1094,39 @@ def init_repo(path=None):
     d = repo()  # appends a / to the path
     parent = os.path.dirname(os.path.dirname(d))
     if parent and not os.path.exists(parent):
-        raise GitError('parent directory "%s" does not exist\n' % parent)
-    if os.path.exists(d) and not os.path.isdir(os.path.join(d, '.')):
-        raise GitError('"%s" exists but is not a directory\n' % d)
-    p = subprocess.Popen(['git', '--bare', 'init'], stdout=sys.stderr,
-                         preexec_fn = _gitenv())
+        raise GitError('parent directory "%s" does not exist\n'
+                       % path_msg(parent))
+    if os.path.exists(d) and not os.path.isdir(os.path.join(d, b'.')):
+        raise GitError('"%s" exists but is not a directory\n' % path_msg(d))
+    p = subprocess.Popen([b'git', b'--bare', b'init'], stdout=sys.stderr,
+                         env=_gitenv())
     _git_wait('git init', p)
     # Force the index version configuration in order to ensure bup works
     # regardless of the version of the installed Git binary.
-    p = subprocess.Popen(['git', 'config', 'pack.indexVersion', '2'],
-                         stdout=sys.stderr, preexec_fn = _gitenv())
+    p = subprocess.Popen([b'git', b'config', b'pack.indexVersion', '2'],
+                         stdout=sys.stderr, env=_gitenv())
     _git_wait('git config', p)
     # Enable the reflog
-    p = subprocess.Popen(['git', 'config', 'core.logAllRefUpdates', 'true'],
-                         stdout=sys.stderr, preexec_fn = _gitenv())
+    p = subprocess.Popen([b'git', b'config', b'core.logAllRefUpdates', b'true'],
+                         stdout=sys.stderr, env=_gitenv())
     _git_wait('git config', p)
 
 
 def check_repo_or_die(path=None):
-    """Make sure a bup repository exists, and abort if not.
-    If the path to a particular repository was not specified, this function
-    initializes the default repository automatically.
-    """
+    """Check to see if a bup repository probably exists, and abort if not."""
     guess_repo(path)
-    try:
-        os.stat(repo('objects/pack/.'))
-    except OSError as e:
-        if e.errno == errno.ENOENT:
-            log('error: %r is not a bup repository; run "bup init"\n'
-                % repo())
+    top = repo()
+    pst = stat_if_exists(top + b'/objects/pack')
+    if pst and stat.S_ISDIR(pst.st_mode):
+        return
+    if not pst:
+        top_st = stat_if_exists(top)
+        if not top_st:
+            log('error: repository %r does not exist (see "bup help init")\n'
+                % top)
             sys.exit(15)
-        else:
-            log('error: %s\n' % e)
-            sys.exit(14)
+    log('error: %s is not a repository\n' % path_msg(top))
+    sys.exit(14)
 
 
 _ver = None
@@ -1040,38 +1137,25 @@ def ver():
     representing a digit in the version tag. For example, the following tuple
     would represent version 1.6.6.9:
 
-        ('1', '6', '6', '9')
+        (1, 6, 6, 9)
     """
     global _ver
     if not _ver:
-        p = subprocess.Popen(['git', '--version'],
-                             stdout=subprocess.PIPE)
+        p = subprocess.Popen([b'git', b'--version'], stdout=subprocess.PIPE)
         gvs = p.stdout.read()
         _git_wait('git --version', p)
-        m = re.match(r'git version (\S+.\S+)', gvs)
+        m = re.match(br'git version (\S+.\S+)', gvs)
         if not m:
             raise GitError('git --version weird output: %r' % gvs)
-        _ver = tuple(m.group(1).split('.'))
-    needed = ('1','5', '3', '1')
+        _ver = tuple(int(x) for x in m.group(1).split(b'.'))
+    needed = (1, 5, 3, 1)
     if _ver < needed:
         raise GitError('git version %s or higher is required; you have %s'
-                       % ('.'.join(needed), '.'.join(_ver)))
+                       % ('.'.join(str(x) for x in needed),
+                          '.'.join(str(x) for x in _ver)))
     return _ver
 
 
-def _git_wait(cmd, p):
-    rv = p.wait()
-    if rv != 0:
-        raise GitError('%s returned %d' % (cmd, rv))
-
-
-def _git_capture(argv):
-    p = subprocess.Popen(argv, stdout=subprocess.PIPE, preexec_fn = _gitenv())
-    r = p.stdout.read()
-    _git_wait(repr(argv), p)
-    return r
-
-
 class _AbortableIter:
     def __init__(self, it, onabort = None):
         self.it = it
@@ -1081,9 +1165,9 @@ class _AbortableIter:
     def __iter__(self):
         return self
 
-    def next(self):
+    def __next__(self):
         try:
-            return self.it.next()
+            return next(self.it)
         except StopIteration as e:
             self.done = True
             raise
@@ -1091,6 +1175,8 @@ class _AbortableIter:
             self.abort()
             raise
 
+    next = __next__
+
     def abort(self):
         """Abort iteration and call the abortion callback, if needed."""
         if not self.done:
@@ -1102,22 +1188,15 @@ class _AbortableIter:
         self.abort()
 
 
-_ver_warned = 0
 class CatPipe:
     """Link to 'git cat-file' that is used to retrieve blob data."""
     def __init__(self, repo_dir = None):
-        global _ver_warned
         self.repo_dir = repo_dir
-        wanted = ('1','5','6')
+        wanted = (1, 5, 6)
         if ver() < wanted:
-            if not _ver_warned:
-                log('warning: git version < %s; bup will be slow.\n'
-                    % '.'.join(wanted))
-                _ver_warned = 1
-            self.get = self._slow_get
-        else:
-            self.p = self.inprogress = None
-            self.get = self._fast_get
+            log('error: git version must be at least 1.5.6\n')
+            sys.exit(1)
+        self.p = self.inprogress = None
 
     def _abort(self):
         if self.p:
@@ -1126,85 +1205,75 @@ class CatPipe:
         self.p = None
         self.inprogress = None
 
-    def _restart(self):
+    def restart(self):
         self._abort()
-        self.p = subprocess.Popen(['git', 'cat-file', '--batch'],
+        self.p = subprocess.Popen([b'git', b'cat-file', b'--batch'],
                                   stdin=subprocess.PIPE,
                                   stdout=subprocess.PIPE,
                                   close_fds = True,
                                   bufsize = 4096,
-                                  preexec_fn = _gitenv(self.repo_dir))
+                                  env=_gitenv(self.repo_dir))
 
-    def _fast_get(self, id):
+    def get(self, ref):
+        """Yield (oidx, type, size), followed by the data referred to by ref.
+        If ref does not exist, only yield (None, None, None).
+
+        """
         if not self.p or self.p.poll() != None:
-            self._restart()
+            self.restart()
         assert(self.p)
         poll_result = self.p.poll()
         assert(poll_result == None)
         if self.inprogress:
-            log('_fast_get: opening %r while %r is open\n'
-                % (id, self.inprogress))
+            log('get: opening %r while %r is open\n' % (ref, self.inprogress))
         assert(not self.inprogress)
-        assert(id.find('\n') < 0)
-        assert(id.find('\r') < 0)
-        assert(not id.startswith('-'))
-        self.inprogress = id
-        self.p.stdin.write('%s\n' % id)
+        assert ref.find(b'\n') < 0
+        assert ref.find(b'\r') < 0
+        assert not ref.startswith(b'-')
+        self.inprogress = ref
+        self.p.stdin.write(ref + b'\n')
         self.p.stdin.flush()
         hdr = self.p.stdout.readline()
-        if hdr.endswith(' missing\n'):
+        if hdr.endswith(b' missing\n'):
             self.inprogress = None
-            raise KeyError('blob %r is missing' % id)
-        spl = hdr.split(' ')
-        if len(spl) != 3 or len(spl[0]) != 40:
-            raise GitError('expected blob, got %r' % spl)
-        (hex, type, size) = spl
-
-        it = _AbortableIter(chunkyreader(self.p.stdout, int(spl[2])),
-                           onabort = self._abort)
+            yield None, None, None
+            return
+        info = hdr.split(b' ')
+        if len(info) != 3 or len(info[0]) != 40:
+            raise GitError('expected object (id, type, size), got %r' % info)
+        oidx, typ, size = info
+        size = int(size)
+        it = _AbortableIter(chunkyreader(self.p.stdout, size),
+                            onabort=self._abort)
         try:
-            yield type
+            yield oidx, typ, size
             for blob in it:
                 yield blob
             readline_result = self.p.stdout.readline()
-            assert(readline_result == '\n')
+            assert readline_result == b'\n'
             self.inprogress = None
         except Exception as e:
             it.abort()
             raise
 
-    def _slow_get(self, id):
-        assert(id.find('\n') < 0)
-        assert(id.find('\r') < 0)
-        assert(id[0] != '-')
-        type = _git_capture(['git', 'cat-file', '-t', id]).strip()
-        yield type
-
-        p = subprocess.Popen(['git', 'cat-file', type, id],
-                             stdout=subprocess.PIPE,
-                             preexec_fn = _gitenv(self.repo_dir))
-        for blob in chunkyreader(p.stdout):
-            yield blob
-        _git_wait('git cat-file', p)
-
     def _join(self, it):
-        type = it.next()
-        if type == 'blob':
+        _, typ, _ = next(it)
+        if typ == b'blob':
             for blob in it:
                 yield blob
-        elif type == 'tree':
-            treefile = ''.join(it)
+        elif typ == b'tree':
+            treefile = b''.join(it)
             for (mode, name, sha) in tree_decode(treefile):
-                for blob in self.join(sha.encode('hex')):
+                for blob in self.join(hexlify(sha)):
                     yield blob
-        elif type == 'commit':
-            treeline = ''.join(it).split('\n')[0]
-            assert(treeline.startswith('tree '))
+        elif typ == b'commit':
+            treeline = b''.join(it).split(b'\n')[0]
+            assert treeline.startswith(b'tree ')
             for blob in self.join(treeline[5:]):
                 yield blob
         else:
             raise GitError('invalid object type %r: expected blob/tree/commit'
-                           % type)
+                           % typ)
 
     def join(self, id):
         """Generate a list of the content of all blobs that can be reached
@@ -1223,9 +1292,9 @@ _cp = {}
 
 def cp(repo_dir=None):
     """Create a CatPipe object or reuse the already existing one."""
-    global _cp
+    global _cp, repodir
     if not repo_dir:
-        repo_dir = repo()
+        repo_dir = repodir or repo()
     repo_dir = os.path.abspath(repo_dir)
     cp = _cp.get(repo_dir)
     if not cp:
@@ -1238,7 +1307,7 @@ def tags(repo_dir = None):
     """Return a dictionary of all tags in the form {hash: [tag_names, ...]}."""
     tags = {}
     for n, c in list_refs(repo_dir = repo_dir, limit_to_tags=True):
-        assert(n.startswith('refs/tags/'))
+        assert n.startswith(b'refs/tags/')
         name = n[10:]
         if not c in tags:
             tags[c] = []
@@ -1246,7 +1315,13 @@ def tags(repo_dir = None):
     return tags
 
 
-WalkItem = namedtuple('WalkItem', ['id', 'type', 'mode',
+class MissingObject(KeyError):
+    def __init__(self, oid):
+        self.oid = oid
+        KeyError.__init__(self, 'object %r is missing' % oid.encode('hex'))
+
+
+WalkItem = namedtuple('WalkItem', ['oid', 'type', 'mode',
                                    'path', 'chunk_path', 'data'])
 # The path is the mangled path, and if an item represents a fragment
 # of a chunked file, the chunk_path will be the chunked subtree path
@@ -1260,76 +1335,70 @@ WalkItem = namedtuple('WalkItem', ['id', 'type', 'mode',
 #   ...
 
 
-def _walk_object(cat_pipe, id,
-                 parent_path, chunk_path,
-                 mode=None,
-                 stop_at=None,
-                 include_data=None):
+def walk_object(get_ref, oidx, stop_at=None, include_data=None):
+    """Yield everything reachable from oidx via get_ref (which must behave
+    like CatPipe get) as a WalkItem, stopping whenever stop_at(oidx)
+    returns true.  Throw MissingObject if a hash encountered is
+    missing from the repository, and don't read or return blob content
+    in the data field unless include_data is set.
 
-    if stop_at and stop_at(id):
-        return
-
-    item_it = cat_pipe.get(id)  # FIXME: use include_data
-    type = item_it.next()
-
-    if type not in ('blob', 'commit', 'tree'):
-        raise Exception('unexpected repository object type %r' % type)
-
-    # FIXME: set the mode based on the type when the mode is None
-
-    if type == 'blob' and not include_data:
-        # Dump data until we can ask cat_pipe not to fetch it
-        for ignored in item_it:
-            pass
-        data = None
-    else:
-        data = ''.join(item_it)
-
-    yield  WalkItem(id=id, type=type,
-                    chunk_path=chunk_path, path=parent_path,
-                    mode=mode,
-                    data=(data if include_data else None))
-
-    if type == 'commit':
-        commit_items = parse_commit(data)
-        tree_id = commit_items.tree
-        for x in _walk_object(cat_pipe, tree_id, parent_path, chunk_path,
-                              mode=hashsplit.GIT_MODE_TREE,
-                              stop_at=stop_at,
-                              include_data=include_data):
-            yield x
-        parents = commit_items.parents
-        for pid in parents:
-            for x in _walk_object(cat_pipe, pid, parent_path, chunk_path,
-                                  mode=mode, # Same mode as this child
-                                  stop_at=stop_at,
-                                  include_data=include_data):
-                yield x
-    elif type == 'tree':
-        for mode, name, ent_id in tree_decode(data):
-            demangled, bup_type = demangle_name(name, mode)
-            if chunk_path:
-                sub_path = parent_path
-                sub_chunk_path = chunk_path + [name]
-            else:
-                sub_path = parent_path + [name]
-                if bup_type == BUP_CHUNKED:
-                    sub_chunk_path = ['']
+    """
+    # Maintain the pending stack on the heap to avoid stack overflow
+    pending = [(oidx, [], [], None)]
+    while len(pending):
+        oidx, parent_path, chunk_path, mode = pending.pop()
+        oid = unhexlify(oidx)
+        if stop_at and stop_at(oidx):
+            continue
+
+        if (not include_data) and mode and stat.S_ISREG(mode):
+            # If the object is a "regular file", then it's a leaf in
+            # the graph, so we can skip reading the data if the caller
+            # hasn't requested it.
+            yield WalkItem(oid=oid, type=b'blob',
+                           chunk_path=chunk_path, path=parent_path,
+                           mode=mode,
+                           data=None)
+            continue
+
+        item_it = get_ref(oidx)
+        get_oidx, typ, _ = next(item_it)
+        if not get_oidx:
+            raise MissingObject(unhexlify(oidx))
+        if typ not in (b'blob', b'commit', b'tree'):
+            raise Exception('unexpected repository object type %r' % typ)
+
+        # FIXME: set the mode based on the type when the mode is None
+        if typ == b'blob' and not include_data:
+            # Dump data until we can ask cat_pipe not to fetch it
+            for ignored in item_it:
+                pass
+            data = None
+        else:
+            data = b''.join(item_it)
+
+        yield WalkItem(oid=oid, type=typ,
+                       chunk_path=chunk_path, path=parent_path,
+                       mode=mode,
+                       data=(data if include_data else None))
+
+        if typ == b'commit':
+            commit_items = parse_commit(data)
+            for pid in commit_items.parents:
+                pending.append((pid, parent_path, chunk_path, mode))
+            pending.append((commit_items.tree, parent_path, chunk_path,
+                            hashsplit.GIT_MODE_TREE))
+        elif typ == b'tree':
+            for mode, name, ent_id in tree_decode(data):
+                demangled, bup_type = demangle_name(name, mode)
+                if chunk_path:
+                    sub_path = parent_path
+                    sub_chunk_path = chunk_path + [name]
                 else:
-                    sub_chunk_path = chunk_path
-            for x in _walk_object(cat_pipe, ent_id.encode('hex'),
-                                  sub_path, sub_chunk_path,
-                                  mode=mode,
-                                  stop_at=stop_at,
-                                  include_data=include_data):
-                yield x
-
-
-def walk_object(cat_pipe, id,
-                stop_at=None,
-                include_data=None):
-    """Yield everything reachable from id via cat_pipe as a WalkItem,
-    stopping whenever stop_at(id) returns true."""
-    return _walk_object(cat_pipe, id, [], [],
-                        stop_at=stop_at,
-                        include_data=include_data)
+                    sub_path = parent_path + [name]
+                    if bup_type == BUP_CHUNKED:
+                        sub_chunk_path = [b'']
+                    else:
+                        sub_chunk_path = chunk_path
+                pending.append((hexlify(ent_id), sub_path, sub_chunk_path,
+                                mode))