borg/borg/archive.py

from binascii import hexlify
from datetime import datetime, timezone
from getpass import getuser
from itertools import groupby
import errno

from .logger import create_logger
logger = create_logger()
from .key import key_factory
from .remote import cache_if_remote

import os
import socket
import stat
import sys
import time
from io import BytesIO
from . import xattr
from .helpers import Error, uid2user, user2uid, gid2group, group2gid, \
    parse_timestamp, to_localtime, format_time, format_timedelta, \
    Manifest, Statistics, decode_dict, make_path_safe, StableDict, int_to_bigint, bigint_to_int, \
    ProgressIndicatorPercent
from .platform import acl_get, acl_set
from .chunker import Chunker
from .hashindex import ChunkIndex
import msgpack

ITEMS_BUFFER = 1024 * 1024

CHUNK_MIN_EXP = 19  # 2**19 == 512kiB
CHUNK_MAX_EXP = 23  # 2**23 == 8MiB
HASH_WINDOW_SIZE = 0xfff  # 4095B
HASH_MASK_BITS = 21  # results in ~2MiB chunks statistically

# defaults, use --chunker-params to override
CHUNKER_PARAMS = (CHUNK_MIN_EXP, CHUNK_MAX_EXP, HASH_MASK_BITS, HASH_WINDOW_SIZE)

# chunker params for the items metadata stream, finer granularity
ITEMS_CHUNKER_PARAMS = (12, 16, 14, HASH_WINDOW_SIZE)

has_lchmod = hasattr(os, 'lchmod')
has_lchflags = hasattr(os, 'lchflags')

flags_normal = os.O_RDONLY | getattr(os, 'O_BINARY', 0)
flags_noatime = flags_normal | getattr(os, 'O_NOATIME', 0)


class DownloadPipeline:

    def __init__(self, repository, key):
        self.repository = repository
        self.key = key

    def unpack_many(self, ids, filter=None, preload=False):
        unpacker = msgpack.Unpacker(use_list=False)
        for data in self.fetch_many(ids):
            unpacker.feed(data)
            items = [decode_dict(item, (b'path', b'source', b'user', b'group')) for item in unpacker]
            if filter:
                items = [item for item in items if filter(item)]
            if preload:
                for item in items:
                    if b'chunks' in item:
                        self.repository.preload([c[0] for c in item[b'chunks']])
            for item in items:
                yield item

    def fetch_many(self, ids, is_preloaded=False):
        for id_, data in zip(ids, self.repository.get_many(ids, is_preloaded=is_preloaded)):
            yield self.key.decrypt(id_, data)


class ChunkBuffer:
    BUFFER_SIZE = 1 * 1024 * 1024

    def __init__(self, key, chunker_params=ITEMS_CHUNKER_PARAMS):
        self.buffer = BytesIO()
        self.packer = msgpack.Packer(unicode_errors='surrogateescape')
        self.chunks = []
        self.key = key
        self.chunker = Chunker(self.key.chunk_seed, *chunker_params)

    def add(self, item):
        self.buffer.write(self.packer.pack(StableDict(item)))
        if self.is_full():
            self.flush()

    def write_chunk(self, chunk):
        raise NotImplementedError

    def flush(self, flush=False):
        if self.buffer.tell() == 0:
            return
        self.buffer.seek(0)
        chunks = list(bytes(s) for s in self.chunker.chunkify(self.buffer))
        self.buffer.seek(0)
        self.buffer.truncate(0)
        # Leave the last partial chunk in the buffer unless flush is True
        end = None if flush or len(chunks) == 1 else -1
        for chunk in chunks[:end]:
            self.chunks.append(self.write_chunk(chunk))
        if end == -1:
            self.buffer.write(chunks[-1])

    def is_full(self):
        return self.buffer.tell() > self.BUFFER_SIZE


class CacheChunkBuffer(ChunkBuffer):

    def __init__(self, cache, key, stats, chunker_params=ITEMS_CHUNKER_PARAMS):
        super().__init__(key, chunker_params)
        self.cache = cache
        self.stats = stats

    def write_chunk(self, chunk):
        id_, _, _ = self.cache.add_chunk(self.key.id_hash(chunk), chunk, self.stats)
        return id_


class Archive:

    class DoesNotExist(Error):
        """Archive {} does not exist"""

    class AlreadyExists(Error):
        """Archive {} already exists"""

    class IncompatibleFilesystemEncodingError(Error):
        """Failed to encode filename "{}" into file system encoding "{}". Consider configuring the LANG environment variable."""

    def __init__(self, repository, key, manifest, name, cache=None, create=False,
                 checkpoint_interval=300, numeric_owner=False, progress=False,
                 chunker_params=CHUNKER_PARAMS, start=None, end=None):
        self.cwd = os.getcwd()
        self.key = key
        self.repository = repository
        self.cache = cache
        self.manifest = manifest
        self.hard_links = {}
        self.stats = Statistics()
        self.show_progress = progress
        self.name = name
        self.checkpoint_interval = checkpoint_interval
        self.numeric_owner = numeric_owner
        if start is None:
            start = datetime.utcnow()
        self.chunker_params = chunker_params
        self.start = start
        if end is None:
            end = datetime.utcnow()
        self.end = end
        self.pipeline = DownloadPipeline(self.repository, self.key)
        if create:
            self.items_buffer = CacheChunkBuffer(self.cache, self.key, self.stats)
            self.chunker = Chunker(self.key.chunk_seed, *chunker_params)
            if name in manifest.archives:
                raise self.AlreadyExists(name)
            self.last_checkpoint = time.time()
            i = 0
            while True:
                self.checkpoint_name = '%s.checkpoint%s' % (name, i and ('.%d' % i) or '')
                if self.checkpoint_name not in manifest.archives:
                    break
                i += 1
        else:
            if name not in self.manifest.archives:
                raise self.DoesNotExist(name)
            info = self.manifest.archives[name]
            self.load(info[b'id'])
            self.zeros = b'\0' * (1 << chunker_params[1])

    def _load_meta(self, id):
        data = self.key.decrypt(id, self.repository.get(id))
        metadata = msgpack.unpackb(data)
        if metadata[b'version'] != 1:
            raise Exception('Unknown archive metadata version')
        return metadata

    def load(self, id):
        self.id = id
        self.metadata = self._load_meta(self.id)
        decode_dict(self.metadata, (b'name', b'hostname', b'username', b'time', b'time_end'))
        self.metadata[b'cmdline'] = [arg.decode('utf-8', 'surrogateescape') for arg in self.metadata[b'cmdline']]
        self.name = self.metadata[b'name']

    @property
    def ts(self):
        """Timestamp of archive creation (start) in UTC"""
        ts = self.metadata[b'time']
        return parse_timestamp(ts)

    @property
    def ts_end(self):
        """Timestamp of archive creation (end) in UTC"""
        # fall back to time if there is no time_end present in metadata
        ts = self.metadata.get(b'time_end') or self.metadata[b'time']
        return parse_timestamp(ts)

    @property
    def fpr(self):
        return hexlify(self.id).decode('ascii')

    @property
    def duration(self):
        return format_timedelta(self.end - self.start)

    def __str__(self):
        return '''\
Archive name: {0.name}
Archive fingerprint: {0.fpr}
Time (start): {start}
Time (end):   {end}
Duration: {0.duration}
Number of files: {0.stats.nfiles}'''.format(
            self,
            start=format_time(to_localtime(self.start.replace(tzinfo=timezone.utc))),
            end=format_time(to_localtime(self.end.replace(tzinfo=timezone.utc))))

    def __repr__(self):
        return 'Archive(%r)' % self.name

    def iter_items(self, filter=None, preload=False):
        for item in self.pipeline.unpack_many(self.metadata[b'items'], filter=filter, preload=preload):
            yield item

    def add_item(self, item):
        unknown_keys = set(item) - ITEM_KEYS
        assert not unknown_keys, ('unknown item metadata keys detected, please update ITEM_KEYS: %s',
                                  ','.join(k.decode('ascii') for k in unknown_keys))
        if self.show_progress:
            self.stats.show_progress(item=item, dt=0.2)
        self.items_buffer.add(item)
        if time.time() - self.last_checkpoint > self.checkpoint_interval:
            self.write_checkpoint()
            self.last_checkpoint = time.time()

    def write_checkpoint(self):
        self.save(self.checkpoint_name)
        del self.manifest.archives[self.checkpoint_name]
        self.cache.chunk_decref(self.id, self.stats)

    def save(self, name=None, timestamp=None):
        name = name or self.name
        if name in self.manifest.archives:
            raise self.AlreadyExists(name)
        self.items_buffer.flush(flush=True)
        if timestamp is None:
            self.end = datetime.utcnow()
            start = self.start
            end = self.end
        else:
            self.end = timestamp
            start = timestamp
            end = timestamp  # we only have 1 value
        metadata = StableDict({
            'version': 1,
            'name': name,
            'items': self.items_buffer.chunks,
            'cmdline': sys.argv,
            'hostname': socket.gethostname(),
            'username': getuser(),
            'time': start.isoformat(),
            'time_end': end.isoformat(),
            'chunker_params': self.chunker_params,
        })
        data = msgpack.packb(metadata, unicode_errors='surrogateescape')
        self.id = self.key.id_hash(data)
        self.cache.add_chunk(self.id, data, self.stats)
        self.manifest.archives[name] = {'id': self.id, 'time': metadata['time']}
        self.manifest.write()
        self.repository.commit()
        self.cache.commit()

    def calc_stats(self, cache):
        def add(id):
            count, size, csize = cache.chunks[id]
            stats.update(size, csize, count == 1)
            cache.chunks[id] = count - 1, size, csize

        def add_file_chunks(chunks):
            for id, _, _ in chunks:
                add(id)

        # This function is a bit evil since it abuses the cache to calculate
        # the stats. The cache transaction must be rolled back afterwards
        unpacker = msgpack.Unpacker(use_list=False)
        cache.begin_txn()
        stats = Statistics()
        add(self.id)
        for id, chunk in zip(self.metadata[b'items'], self.repository.get_many(self.metadata[b'items'])):
            add(id)
            unpacker.feed(self.key.decrypt(id, chunk))
            for item in unpacker:
                if b'chunks' in item:
                    stats.nfiles += 1
                    add_file_chunks(item[b'chunks'])
        cache.rollback()
        return stats

    def extract_item(self, item, restore_attrs=True, dry_run=False, stdout=False, sparse=False):
        if dry_run or stdout:
            if b'chunks' in item:
                for data in self.pipeline.fetch_many([c[0] for c in item[b'chunks']], is_preloaded=True):
                    if stdout:
                        sys.stdout.buffer.write(data)
                if stdout:
                    sys.stdout.buffer.flush()
            return

        dest = self.cwd
        if item[b'path'].startswith('/') or item[b'path'].startswith('..'):
            raise Exception('Path should be relative and local')
        path = os.path.join(dest, item[b'path'])
        # Attempt to remove existing files, ignore errors on failure
        try:
            st = os.lstat(path)
            if stat.S_ISDIR(st.st_mode):
                os.rmdir(path)
            else:
                os.unlink(path)
        except UnicodeEncodeError:
            raise self.IncompatibleFilesystemEncodingError(path, sys.getfilesystemencoding()) from None
        except OSError:
            pass
        mode = item[b'mode']
        if stat.S_ISREG(mode):
            if not os.path.exists(os.path.dirname(path)):
                os.makedirs(os.path.dirname(path))
            # Hard link?
            if b'source' in item:
                source = os.path.join(dest, item[b'source'])
                if os.path.exists(path):
                    os.unlink(path)
                os.link(source, path)
            else:
                with open(path, 'wb') as fd:
                    ids = [c[0] for c in item[b'chunks']]
                    for data in self.pipeline.fetch_many(ids, is_preloaded=True):
                        if sparse and self.zeros.startswith(data):
                            # all-zero chunk: create a hole in a sparse file
                            fd.seek(len(data), 1)
                        else:
                            fd.write(data)
                    pos = fd.tell()
                    fd.truncate(pos)
                    fd.flush()
                    self.restore_attrs(path, item, fd=fd.fileno())
        elif stat.S_ISDIR(mode):
            if not os.path.exists(path):
                os.makedirs(path)
            if restore_attrs:
                self.restore_attrs(path, item)
        elif stat.S_ISLNK(mode):
            if not os.path.exists(os.path.dirname(path)):
                os.makedirs(os.path.dirname(path))
            source = item[b'source']
            if os.path.exists(path):
                os.unlink(path)
            try:
                os.symlink(source, path)
            except UnicodeEncodeError:
                raise self.IncompatibleFilesystemEncodingError(source, sys.getfilesystemencoding()) from None
            self.restore_attrs(path, item, symlink=True)
        elif stat.S_ISFIFO(mode):
            if not os.path.exists(os.path.dirname(path)):
                os.makedirs(os.path.dirname(path))
            os.mkfifo(path)
            self.restore_attrs(path, item)
        elif stat.S_ISCHR(mode) or stat.S_ISBLK(mode):
            os.mknod(path, item[b'mode'], item[b'rdev'])
            self.restore_attrs(path, item)
        else:
            raise Exception('Unknown archive item type %r' % item[b'mode'])

    def restore_attrs(self, path, item, symlink=False, fd=None):
        xattrs = item.get(b'xattrs', {})
        for k, v in xattrs.items():
            try:
                xattr.setxattr(fd or path, k, v, follow_symlinks=False)
            except OSError as e:
                if e.errno not in (errno.ENOTSUP, errno.EACCES, ):
                    # only raise if the errno is not on our ignore list:
                    # ENOTSUP == xattrs not supported here
                    # EACCES == permission denied to set this specific xattr
                    #           (this may happen related to security.* keys)
                    raise
        uid = gid = None
        if not self.numeric_owner:
            uid = user2uid(item[b'user'])
            gid = group2gid(item[b'group'])
        uid = item[b'uid'] if uid is None else uid
        gid = item[b'gid'] if gid is None else gid
        # This code is a bit of a mess due to os specific differences
        try:
            if fd:
                os.fchown(fd, uid, gid)
            else:
                os.lchown(path, uid, gid)
        except OSError:
            pass
        if fd:
            os.fchmod(fd, item[b'mode'])
        elif not symlink:
            os.chmod(path, item[b'mode'])
        elif has_lchmod:  # Not available on Linux
            os.lchmod(path, item[b'mode'])
        mtime = bigint_to_int(item[b'mtime'])
        if b'atime' in item:
            atime = bigint_to_int(item[b'atime'])
        else:
            # old archives only had mtime in item metadata
            atime = mtime
        if fd:
            os.utime(fd, None, ns=(atime, mtime))
        else:
            os.utime(path, None, ns=(atime, mtime), follow_symlinks=False)
        acl_set(path, item, self.numeric_owner)
        # Only available on OS X and FreeBSD
        if has_lchflags and b'bsdflags' in item:
            try:
                os.lchflags(path, item[b'bsdflags'])
            except OSError:
                pass

    def rename(self, name):
        if name in self.manifest.archives:
            raise self.AlreadyExists(name)
        metadata = StableDict(self._load_meta(self.id))
        metadata[b'name'] = name
        data = msgpack.packb(metadata, unicode_errors='surrogateescape')
        new_id = self.key.id_hash(data)
        self.cache.add_chunk(new_id, data, self.stats)
        self.manifest.archives[name] = {'id': new_id, 'time': metadata[b'time']}
        self.cache.chunk_decref(self.id, self.stats)
        del self.manifest.archives[self.name]

    def delete(self, stats, progress=False):
        unpacker = msgpack.Unpacker(use_list=False)
        items_ids = self.metadata[b'items']
        pi = ProgressIndicatorPercent(total=len(items_ids), msg="Decrementing references %3.0f%%", same_line=True)
        for (i, (items_id, data)) in enumerate(zip(items_ids, self.repository.get_many(items_ids))):
            if progress:
                pi.show(i)
            unpacker.feed(self.key.decrypt(items_id, data))
            self.cache.chunk_decref(items_id, stats)
            for item in unpacker:
                if b'chunks' in item:
                    for chunk_id, size, csize in item[b'chunks']:
                        self.cache.chunk_decref(chunk_id, stats)
        if progress:
            pi.finish()
        self.cache.chunk_decref(self.id, stats)
        del self.manifest.archives[self.name]

    def stat_attrs(self, st, path):
        item = {
            b'mode': st.st_mode,
            b'uid': st.st_uid, b'user': uid2user(st.st_uid),
            b'gid': st.st_gid, b'group': gid2group(st.st_gid),
            b'atime': int_to_bigint(st.st_atime_ns),
            b'ctime': int_to_bigint(st.st_ctime_ns),
            b'mtime': int_to_bigint(st.st_mtime_ns),
        }
        if self.numeric_owner:
            item[b'user'] = item[b'group'] = None
        xattrs = xattr.get_all(path, follow_symlinks=False)
        if xattrs:
            item[b'xattrs'] = StableDict(xattrs)
        if has_lchflags and st.st_flags:
            item[b'bsdflags'] = st.st_flags
        acl_get(path, item, st, self.numeric_owner)
        return item

    def process_dir(self, path, st):
        item = {b'path': make_path_safe(path)}
        item.update(self.stat_attrs(st, path))
        self.add_item(item)
        return 'd'  # directory

    def process_fifo(self, path, st):
        item = {b'path': make_path_safe(path)}
        item.update(self.stat_attrs(st, path))
        self.add_item(item)
        return 'f'  # fifo

    def process_dev(self, path, st):
        item = {b'path': make_path_safe(path), b'rdev': st.st_rdev}
        item.update(self.stat_attrs(st, path))
        self.add_item(item)
        if stat.S_ISCHR(st.st_mode):
            return 'c'  # char device
        elif stat.S_ISBLK(st.st_mode):
            return 'b'  # block device

    def process_symlink(self, path, st):
        source = os.readlink(path)
        item = {b'path': make_path_safe(path), b'source': source}
        item.update(self.stat_attrs(st, path))
        self.add_item(item)
        return 's'  # symlink

    def process_stdin(self, path, cache):
        uid, gid = 0, 0
        fd = sys.stdin.buffer  # binary
        chunks = []
        for chunk in self.chunker.chunkify(fd):
            chunks.append(cache.add_chunk(self.key.id_hash(chunk), chunk, self.stats))
        self.stats.nfiles += 1
        t = int_to_bigint(int(time.time()) * 1000000000)
        item = {
            b'path': path,
            b'chunks': chunks,
            b'mode': 0o100660,  # regular file, ug=rw
            b'uid': uid, b'user': uid2user(uid),
            b'gid': gid, b'group': gid2group(gid),
            b'mtime': t, b'atime': t, b'ctime': t,
        }
        self.add_item(item)
        return 'i'  # stdin

    def process_file(self, path, st, cache, ignore_inode=False):
        status = None
        safe_path = make_path_safe(path)
        # Is it a hard link?
        if st.st_nlink > 1:
            source = self.hard_links.get((st.st_ino, st.st_dev))
            if (st.st_ino, st.st_dev) in self.hard_links:
                item = self.stat_attrs(st, path)
                item.update({b'path': safe_path, b'source': source})
                self.add_item(item)
                status = 'h'  # regular file, hardlink (to already seen inodes)
                return status
            else:
                self.hard_links[st.st_ino, st.st_dev] = safe_path
        path_hash = self.key.id_hash(os.path.join(self.cwd, path).encode('utf-8', 'surrogateescape'))
        first_run = not cache.files
        ids = cache.file_known_and_unchanged(path_hash, st, ignore_inode)
        if first_run:
            logger.info('processing files')
        chunks = None
        if ids is not None:
            # Make sure all ids are available
            for id_ in ids:
                if not cache.seen_chunk(id_):
                    break
            else:
                chunks = [cache.chunk_incref(id_, self.stats) for id_ in ids]
                status = 'U'  # regular file, unchanged
        else:
            status = 'A'  # regular file, added
        item = {b'path': safe_path}
        # Only chunkify the file if needed
        if chunks is None:
            fh = Archive._open_rb(path)
            with os.fdopen(fh, 'rb') as fd:
                chunks = []
                for chunk in self.chunker.chunkify(fd, fh):
                    chunks.append(cache.add_chunk(self.key.id_hash(chunk), chunk, self.stats))
                    if self.show_progress:
                        self.stats.show_progress(item=item, dt=0.2)
            cache.memorize_file(path_hash, st, [c[0] for c in chunks])
            status = status or 'M'  # regular file, modified (if not 'A' already)
        item[b'chunks'] = chunks
        item.update(self.stat_attrs(st, path))
        self.stats.nfiles += 1
        self.add_item(item)
        return status

    @staticmethod
    def list_archives(repository, key, manifest, cache=None):
        # expensive! see also Manifest.list_archive_infos.
        for name, info in manifest.archives.items():
            yield Archive(repository, key, manifest, name, cache=cache)

    @staticmethod
    def _open_rb(path):
        try:
            # if we have O_NOATIME, this likely will succeed if we are root or owner of file:
            return os.open(path, flags_noatime)
        except PermissionError:
            if flags_noatime == flags_normal:
                # we do not have O_NOATIME, no need to try again:
                raise
            # Was this EPERM due to the O_NOATIME flag? Try again without it:
            return os.open(path, flags_normal)


# this set must be kept complete, otherwise the RobustUnpacker might malfunction:
ITEM_KEYS = set([b'path', b'source', b'rdev', b'chunks',
                 b'mode', b'user', b'group', b'uid', b'gid', b'mtime', b'atime', b'ctime',
                 b'xattrs', b'bsdflags', b'acl_nfs4', b'acl_access', b'acl_default', b'acl_extended', ])


class RobustUnpacker:
    """A restartable/robust version of the streaming msgpack unpacker
    """
    def __init__(self, validator):
        super().__init__()
        self.item_keys = [msgpack.packb(name) for name in ITEM_KEYS]
        self.validator = validator
        self._buffered_data = []
        self._resync = False
        self._unpacker = msgpack.Unpacker(object_hook=StableDict)

    def resync(self):
        self._buffered_data = []
        self._resync = True

    def feed(self, data):
        if self._resync:
            self._buffered_data.append(data)
        else:
            self._unpacker.feed(data)

    def __iter__(self):
        return self

    def __next__(self):
        if self._resync:
            data = b''.join(self._buffered_data)
            while self._resync:
                if not data:
                    raise StopIteration
                # Abort early if the data does not look like a serialized dict
                if len(data) < 2 or ((data[0] & 0xf0) != 0x80) or ((data[1] & 0xe0) != 0xa0):
                    data = data[1:]
                    continue
                # Make sure it looks like an item dict
                for pattern in self.item_keys:
                    if data[1:].startswith(pattern):
                        break
                else:
                    data = data[1:]
                    continue

                self._unpacker = msgpack.Unpacker(object_hook=StableDict)
                self._unpacker.feed(data)
                try:
                    item = next(self._unpacker)
                    if self.validator(item):
                        self._resync = False
                        return item
                # Ignore exceptions that might be raised when feeding
                # msgpack with invalid data
                except (TypeError, ValueError, StopIteration):
                    pass
                data = data[1:]
        else:
            return next(self._unpacker)


class ArchiveChecker:

    def __init__(self):
        self.error_found = False
        self.possibly_superseded = set()

    def check(self, repository, repair=False, archive=None, last=None, prefix=None, save_space=False):
        logger.info('Starting archive consistency check...')
        self.check_all = archive is None and last is None and prefix is None
        self.repair = repair
        self.repository = repository
        self.init_chunks()
        self.key = self.identify_key(repository)
        if Manifest.MANIFEST_ID not in self.chunks:
            logger.error("Repository manifest not found!")
            self.error_found = True
            self.manifest = self.rebuild_manifest()
        else:
            self.manifest, _ = Manifest.load(repository, key=self.key)
        self.rebuild_refcounts(archive=archive, last=last, prefix=prefix)
        self.orphan_chunks_check()
        self.finish(save_space=save_space)
        if self.error_found:
            logger.error('Archive consistency check complete, problems found.')
        else:
            logger.info('Archive consistency check complete, no problems found.')
        return self.repair or not self.error_found

    def init_chunks(self):
        """Fetch a list of all object keys from repository
        """
        # Explicitly set the initial hash table capacity to avoid performance issues
        # due to hash table "resonance"
        capacity = int(len(self.repository) * 1.2)
        self.chunks = ChunkIndex(capacity)
        marker = None
        while True:
            result = self.repository.list(limit=10000, marker=marker)
            if not result:
                break
            marker = result[-1]
            for id_ in result:
                self.chunks[id_] = (0, 0, 0)

    def identify_key(self, repository):
        cdata = repository.get(next(self.chunks.iteritems())[0])
        return key_factory(repository, cdata)

    def rebuild_manifest(self):
        """Rebuild the manifest object if it is missing

        Iterates through all objects in the repository looking for archive metadata blocks.
        """
        logger.info('Rebuilding missing manifest, this might take some time...')
        manifest = Manifest(self.key, self.repository)
        for chunk_id, _ in self.chunks.iteritems():
            cdata = self.repository.get(chunk_id)
            data = self.key.decrypt(chunk_id, cdata)
            # Some basic sanity checks of the payload before feeding it into msgpack
            if len(data) < 2 or ((data[0] & 0xf0) != 0x80) or ((data[1] & 0xe0) != 0xa0):
                continue
            if b'cmdline' not in data or b'\xa7version\x01' not in data:
                continue
            try:
                archive = msgpack.unpackb(data)
            # Ignore exceptions that might be raised when feeding
            # msgpack with invalid data
            except (TypeError, ValueError, StopIteration):
                continue
            if isinstance(archive, dict) and b'items' in archive and b'cmdline' in archive:
                logger.info('Found archive %s', archive[b'name'].decode('utf-8'))
                manifest.archives[archive[b'name'].decode('utf-8')] = {b'id': chunk_id, b'time': archive[b'time']}
        logger.info('Manifest rebuild complete.')
        return manifest

    def rebuild_refcounts(self, archive=None, last=None, prefix=None):
        """Rebuild object reference counts by walking the metadata

        Missing and/or incorrect data is repaired when detected
        """
        # Exclude the manifest from chunks
        del self.chunks[Manifest.MANIFEST_ID]

        def mark_as_possibly_superseded(id_):
            if self.chunks.get(id_, (0,))[0] == 0:
                self.possibly_superseded.add(id_)

        def add_callback(chunk):
            id_ = self.key.id_hash(chunk)
            cdata = self.key.encrypt(chunk)
            add_reference(id_, len(chunk), len(cdata), cdata)
            return id_

        def add_reference(id_, size, csize, cdata=None):
            try:
                count, _, _ = self.chunks[id_]
                self.chunks[id_] = count + 1, size, csize
            except KeyError:
                assert cdata is not None
                self.chunks[id_] = 1, size, csize
                if self.repair:
                    self.repository.put(id_, cdata)

        def verify_file_chunks(item):
            """Verifies that all file chunks are present

            Missing file chunks will be replaced with new chunks of the same
            length containing all zeros.
            """
            offset = 0
            chunk_list = []
            for chunk_id, size, csize in item[b'chunks']:
                if chunk_id not in self.chunks:
                    # If a file chunk is missing, create an all empty replacement chunk
                    logger.error('{}: Missing file chunk detected (Byte {}-{})'.format(item[b'path'].decode('utf-8', 'surrogateescape'), offset, offset + size))
                    self.error_found = True
                    data = bytes(size)
                    chunk_id = self.key.id_hash(data)
                    cdata = self.key.encrypt(data)
                    csize = len(cdata)
                    add_reference(chunk_id, size, csize, cdata)
                else:
                    add_reference(chunk_id, size, csize)
                chunk_list.append((chunk_id, size, csize))
                offset += size
            item[b'chunks'] = chunk_list

        def robust_iterator(archive):
            """Iterates through all archive items

            Missing item chunks will be skipped and the msgpack stream will be restarted
            """
            unpacker = RobustUnpacker(lambda item: isinstance(item, dict) and b'path' in item)
            _state = 0

            def missing_chunk_detector(chunk_id):
                nonlocal _state
                if _state % 2 != int(chunk_id not in self.chunks):
                    _state += 1
                return _state

            def report(msg, chunk_id, chunk_no):
                cid = hexlify(chunk_id).decode('ascii')
                msg += ' [chunk: %06d_%s]' % (chunk_no, cid)  # see debug-dump-archive-items
                self.error_found = True
                logger.error(msg)

            i = 0
            for state, items in groupby(archive[b'items'], missing_chunk_detector):
                items = list(items)
                if state % 2:
                    for chunk_id in items:
                        report('item metadata chunk missing', chunk_id, i)
                        i += 1
                    continue
                if state > 0:
                    unpacker.resync()
                for chunk_id, cdata in zip(items, repository.get_many(items)):
                    unpacker.feed(self.key.decrypt(chunk_id, cdata))
                    try:
                        for item in unpacker:
                            if isinstance(item, dict):
                                yield item
                            else:
                                report('Did not get expected metadata dict when unpacking item metadata', chunk_id, i)
                    except Exception:
                        report('Exception while unpacking item metadata', chunk_id, i)
                        raise
                    i += 1

        if archive is None:
            # we need last N or all archives
            archive_items = sorted(self.manifest.archives.items(), reverse=True,
                                   key=lambda name_info: name_info[1][b'time'])
            if prefix is not None:
                archive_items = [item for item in archive_items if item[0].startswith(prefix)]
            num_archives = len(archive_items)
            end = None if last is None else min(num_archives, last)
        else:
            # we only want one specific archive
            archive_items = [item for item in self.manifest.archives.items() if item[0] == archive]
            num_archives = 1
            end = 1

        with cache_if_remote(self.repository) as repository:
            for i, (name, info) in enumerate(archive_items[:end]):
                logger.info('Analyzing archive {} ({}/{})'.format(name, num_archives - i, num_archives))
                archive_id = info[b'id']
                if archive_id not in self.chunks:
                    logger.error('Archive metadata block is missing!')
                    self.error_found = True
                    del self.manifest.archives[name]
                    continue
                mark_as_possibly_superseded(archive_id)
                cdata = self.repository.get(archive_id)
                data = self.key.decrypt(archive_id, cdata)
                archive = StableDict(msgpack.unpackb(data))
                if archive[b'version'] != 1:
                    raise Exception('Unknown archive metadata version')
                decode_dict(archive, (b'name', b'hostname', b'username', b'time', b'time_end'))
                archive[b'cmdline'] = [arg.decode('utf-8', 'surrogateescape') for arg in archive[b'cmdline']]
                items_buffer = ChunkBuffer(self.key)
                items_buffer.write_chunk = add_callback
                for item in robust_iterator(archive):
                    if b'chunks' in item:
                        verify_file_chunks(item)
                    items_buffer.add(item)
                items_buffer.flush(flush=True)
                for previous_item_id in archive[b'items']:
                    mark_as_possibly_superseded(previous_item_id)
                archive[b'items'] = items_buffer.chunks
                data = msgpack.packb(archive, unicode_errors='surrogateescape')
                new_archive_id = self.key.id_hash(data)
                cdata = self.key.encrypt(data)
                add_reference(new_archive_id, len(data), len(cdata), cdata)
                info[b'id'] = new_archive_id

    def orphan_chunks_check(self):
        if self.check_all:
            unused = set()
            for id_, (count, size, csize) in self.chunks.iteritems():
                if count == 0:
                    unused.add(id_)
            orphaned = unused - self.possibly_superseded
            if orphaned:
                logger.error('{} orphaned objects found!'.format(len(orphaned)))
                self.error_found = True
            if self.repair:
                for id_ in unused:
                    self.repository.delete(id_)
        else:
            logger.warning('Orphaned objects check skipped (needs all archives checked).')

    def finish(self, save_space=False):
        if self.repair:
            self.manifest.write()
            self.repository.commit(save_space=save_space)