Example #1
0
def uri_put_file(creds, uri, fp, content_encoding=None):
    assert fp.tell() == 0
    assert uri.startswith("wabs://")

    def log_upload_failures_on_error(exc_tup, exc_processor_cxt):
        def standard_detail_message(prefix=""):
            return prefix + "  There have been {n} attempts to upload  " "file {url} so far.".format(
                n=exc_processor_cxt, url=uri
            )

        typ, value, tb = exc_tup
        del exc_tup

        # Screen for certain kinds of known-errors to retry from
        if issubclass(typ, socket.error):
            socketmsg = value[1] if isinstance(value, tuple) else value

            logger.info(
                msg="Retrying upload because of a socket error",
                detail=standard_detail_message("The socket error's message is '{0}'.".format(socketmsg)),
            )
        else:
            # For all otherwise untreated exceptions, report them as a
            # warning and retry anyway -- all exceptions that can be
            # justified should be treated and have error messages
            # listed.
            logger.warning(
                msg="retrying file upload from unexpected exception",
                detail=standard_detail_message(
                    "The exception type is {etype} and its value is "
                    "{evalue} and its traceback is {etraceback}".format(
                        etype=typ, evalue=value, etraceback="".join(traceback.format_tb(tb))
                    )
                ),
            )

        # Help Python GC by resolving possible cycles
        del tb

    # Because we're uploading in chunks, catch rate limiting and
    # connection errors which occur for each individual chunk instead of
    # failing the whole file and restarting.
    @retry(retry_with_count(log_upload_failures_on_error))
    def upload_chunk(chunk, block_id):
        check_sum = base64.encodestring(md5(chunk).digest()).strip("\n")
        conn.put_block(url_tup.netloc, url_tup.path, chunk, block_id, content_md5=check_sum)

    url_tup = urlparse(uri)
    kwargs = dict(x_ms_blob_type="BlockBlob")
    if content_encoding is not None:
        kwargs["x_ms_blob_content_encoding"] = content_encoding

    conn = BlobService(creds.account_name, creds.account_key, protocol="https")
    conn.put_blob(url_tup.netloc, url_tup.path, "", **kwargs)

    # WABS requires large files to be uploaded in 4MB chunks
    block_ids = []
    length, index = 0, 0
    pool_size = os.getenv("WABS_UPLOAD_POOL_SIZE", 5)
    p = gevent.pool.Pool(size=pool_size)
    while True:
        data = fp.read(WABS_CHUNK_SIZE)
        if data:
            length += len(data)
            block_id = base64.b64encode(str(index))
            p.wait_available()
            p.spawn(upload_chunk, data, block_id)
            block_ids.append(block_id)
            index += 1
        else:
            p.join()
            break

    conn.put_block_list(url_tup.netloc, url_tup.path, block_ids)

    # To maintain consistency with the S3 version of this function we must
    # return an object with a certain set of attributes.  Currently, that set
    # of attributes consists of only 'size'
    return _Key(size=len(data))
Example #2
0
class AzureFS(LoggingMixIn, Operations):
    """Azure Blob Storage filesystem"""

    blobs = None
    containers = dict()  # <cname, dict(stat:dict,
    #files:None|dict<fname, stat>)
    fds = dict()  # <fd, (path, bytes, dirty)>
    fd = 0

    def __init__(self, account, key):
        self.blobs = BlobService(account, key)
        self.rebuild_container_list()

    def convert_to_epoch(self, date):
        """Converts Tue, 31 Jul 2012 07:17:34 GMT format to epoch"""
        return int(time.mktime(time.strptime(date, TIME_FORMAT)))

    def rebuild_container_list(self):
        cmap = dict()
        cnames = set()
        for c in self.blobs.list_containers():
            date = c.properties.last_modified
            cstat = dict(st_mode=(S_IFDIR | 0755),
                         st_uid=getuid(),
                         st_size=0,
                         st_mtime=self.convert_to_epoch(date))
            cname = c.name
            cmap['/' + cname] = dict(stat=cstat, files=None)
            cnames.add(cname)

        cmap['/'] = dict(files={},
                         stat=dict(st_mode=(S_IFDIR | 0755),
                                   st_uid=getuid(),
                                   st_size=0,
                                   st_mtime=int(time.time())))

        self.containers = cmap  # destroys fs tree cache resistant to misses

    def _parse_path(self, path):  # returns </dir, file(=None)>
        if path.count('/') > 1:  # file
            return str(path[:path.rfind('/')]), str(path[path.rfind('/') + 1:])
        else:  # dir
            pos = path.rfind('/', 1)
            if pos == -1:
                return path, None
            else:
                return str(path[:pos]), None

    def parse_container(self, path):
        base_container = path[1:]  # /abc/def/g --> abc
        if base_container.find('/') > -1:
            base_container = base_container[:base_container.find('/')]
        return str(base_container)

    def _get_dir(self, path, contents_required=False):
        if not self.containers:
            self.rebuild_container_list()

        if path in self.containers and not (contents_required and \
                self.containers[path]['files'] is None):
            return self.containers[path]

        cname = self.parse_container(path)

        if '/' + cname not in self.containers:
            raise FuseOSError(ENOENT)
        else:
            if self.containers['/' + cname]['files'] is None:
                # fetch contents of container
                log.info("------> CONTENTS NOT FOUND: %s" % cname)

                blobs = self.blobs.list_blobs(cname)

                dirstat = dict(st_mode=(S_IFDIR | 0755),
                               st_size=0,
                               st_uid=getuid(),
                               st_mtime=time.time())

                if self.containers['/' + cname]['files'] is None:
                    self.containers['/' + cname]['files'] = dict()

                for f in blobs:
                    blob_name = f.name
                    blob_date = f.properties.last_modified
                    blob_size = long(f.properties.content_length)

                    node = dict(st_mode=(S_IFREG | 0644),
                                st_size=blob_size,
                                st_mtime=self.convert_to_epoch(blob_date),
                                st_uid=getuid())

                    if blob_name.find('/') == -1:  # file just under container
                        self.containers['/' + cname]['files'][blob_name] = node

            return self.containers['/' + cname]
        return None

    def _get_file(self, path):
        d, f = self._parse_path(path)
        dir = self._get_dir(d, True)
        if dir is not None and f in dir['files']:
            return dir['files'][f]

    def getattr(self, path, fh=None):
        d, f = self._parse_path(path)

        if f is None:
            dir = self._get_dir(d)
            return dir['stat']
        else:
            file = self._get_file(path)

            if file:
                return file

        raise FuseOSError(ENOENT)

    # FUSE
    def mkdir(self, path, mode):
        if path.count('/') <= 1:  # create on root
            name = path[1:]

            if not 3 <= len(name) <= 63:
                log.error("Container names can be 3 through 63 chars long.")
                raise FuseOSError(ENAMETOOLONG)
            if name is not name.lower():
                log.error("Container names cannot contain uppercase \
                        characters.")
                raise FuseOSError(EACCES)
            if name.count('--') > 0:
                log.error('Container names cannot contain consecutive \
                        dashes (-).')
                raise FuseOSError(EAGAIN)
            #TODO handle all "-"s must be preceded by letter or numbers
            #TODO starts with only letter or number, can contain letter, nr,'-'

            resp = self.blobs.create_container(name)

            if resp:
                self.rebuild_container_list()
                log.info("CONTAINER %s CREATED" % name)
            else:
                raise FuseOSError(EACCES)
                log.error("Invalid container name or container already \
                        exists.")
        else:
            raise FuseOSError(ENOSYS)  # TODO support 2nd+ level mkdirs

    def rmdir(self, path):
        if path.count('/') == 1:
            c_name = path[1:]
            resp = self.blobs.delete_container(c_name)

            if resp:
                if path in self.containers:
                    del self.containers[path]
            else:
                raise FuseOSError(EACCES)
        else:
            raise FuseOSError(ENOSYS)  # TODO support 2nd+ level mkdirs

    def create(self, path, mode):
        node = dict(st_mode=(S_IFREG | mode),
                    st_size=0,
                    st_nlink=1,
                    st_uid=getuid(),
                    st_mtime=time.time())
        d, f = self._parse_path(path)

        if not f:
            log.error("Cannot create files on root level: /")
            raise FuseOSError(ENOSYS)

        dir = self._get_dir(d, True)
        if not dir:
            raise FuseOSError(EIO)
        dir['files'][f] = node

        return self.open(path, data='')  # reusing handler provider

    def open(self, path, flags=0, data=None):
        if data == None:  # download contents
            c_name = self.parse_container(path)
            f_name = path[path.find('/', 1) + 1:]

            try:
                data = self.blobs.get_blob(c_name, f_name)
            except AzureMissingResourceHttpError:
                dir = self._get_dir('/' + c_name, True)
                if f_name in dir['files']:
                    del dir['files'][f_name]
                raise FuseOSError(ENOENT)
            except AzureException as e:
                log.error("Read blob failed HTTP %d" % e.code)
                raise FuseOSError(EAGAIN)

        self.fd += 1
        self.fds[self.fd] = (path, data, False)

        return self.fd

    def flush(self, path, fh=None):
        if not fh:
            raise FuseOSError(EIO)
        else:
            if fh not in self.fds:
                raise FuseOSError(EIO)
            path = self.fds[fh][0]
            data = self.fds[fh][1]
            dirty = self.fds[fh][2]

            if not dirty:
                return 0  # avoid redundant write

            d, f = self._parse_path(path)
            c_name = self.parse_container(path)

            if data is None:
                data = ''

            try:
                if len(data) < 64 * 1024 * 1024:  # 64 mb
                    self.blobs.put_blob(c_name, f, data, 'BlockBlob')
                else:
                    # divide file by blocks and upload
                    block_size = 8 * 1024 * 1024
                    num_blocks = int(math.ceil(len(data) * 1.0 / block_size))
                    rd = str(random.randint(1, 1e8))
                    block_ids = list()

                    for i in range(num_blocks):
                        part = data[i * block_size:min((i + 1) *
                                                       block_size, len(data))]
                        block_id = base64.encodestring(
                            '%s_%s' % (rd, (8 - len(str(i))) * '0' + str(i)))
                        self.blobs.put_block(c_name, f, part, block_id)
                        block_ids.append(block_id)

                    self.blobs.put_block_list(c_name, f, block_ids)
            except AzureException:
                raise FuseOSError(EAGAIN)

            dir = self._get_dir(d, True)
            if not dir or f not in dir['files']:
                raise FuseOSError(EIO)

            # update local data
            dir['files'][f]['st_size'] = len(data)
            dir['files'][f]['st_mtime'] = time.time()
            self.fds[fh] = (path, data, False)  # mark as not dirty
            return 0

    def release(self, path, fh=None):
        if fh is not None and fh in self.fds:
            del self.fds[fh]

    def truncate(self, path, length, fh=None):
        return 0  # assume done, no need

    def write(self, path, data, offset, fh=None):
        if not fh or fh not in self.fds:
            raise FuseOSError(ENOENT)
        else:
            d = self.fds[fh][1]
            if d is None:
                d = ""
            self.fds[fh] = (self.fds[fh][0], d[:offset] + data, True)
            return len(data)

    def unlink(self, path):
        c_name = self.parse_container(path)
        d, f = self._parse_path(path)

        try:
            self.blobs.delete_blob(c_name, f)

            _dir = self._get_dir(path, True)
            if _dir and f in _dir['files']:
                del _dir['files'][f]
            return 0
        except AzureMissingResourceHttpError:
            raise FuseOSError(ENOENT)
        except Exception as e:
            raise FuseOSError(EAGAIN)

    def readdir(self, path, fh):
        if path == '/':
            return ['.', '..'] + [x[1:] for x in self.containers.keys() \
                    if x is not '/']

        dir = self._get_dir(path, True)
        if not dir:
            raise FuseOSError(ENOENT)
        return ['.', '..'] + dir['files'].keys()

    def read(self, path, size, offset, fh):
        if not fh or fh not in self.fds:
            raise FuseOSError(ENOENT)

        f_name = path[path.find('/', 1) + 1:]
        c_name = path[1:path.find('/', 1)]

        try:
            data = self.blobs.get_blob(c_name, f_name)
            self.fds[fh] = (self.fds[fh][0], data, False)
            return data[offset:offset + size]
        except URLError, e:
            if e.code == 404:
                raise FuseOSError(ENOENT)
            elif e.code == 403:
                raise FUSEOSError(EPERM)
            else:
                log.error("Read blob failed HTTP %d" % e.code)
                raise FuseOSError(EAGAIN)
        data = self.fds[fh][1]
        if data is None:
            data = ""
        return data[offset:offset + size]
Example #3
0
class AzureFS(LoggingMixIn, Operations):
    """Azure Blob Storage filesystem"""

    blobs = None
    containers = dict()  # <cname, dict(stat:dict,
                                    #files:None|dict<fname, stat>)
    fds = dict()  # <fd, (path, bytes, dirty)>
    fd = 0

    def __init__(self, account, key):
        self.blobs = BlobService(account, key)
        self.rebuild_container_list()

    def convert_to_epoch(self, date):
        """Converts Tue, 31 Jul 2012 07:17:34 GMT format to epoch"""
        return int(time.mktime(time.strptime(date, TIME_FORMAT)))

    def rebuild_container_list(self):
        cmap = dict()
        cnames = set()
        for c in self.blobs.list_containers():
            date = c.properties.last_modified
            cstat = dict(st_mode=(S_IFDIR | 0755), st_uid=getuid(), st_size=0,
                         st_mtime=self.convert_to_epoch(date))
            cname = c.name
            cmap['/' + cname] = dict(stat=cstat, files=None)
            cnames.add(cname)

        cmap['/'] = dict(files={},
                         stat=dict(st_mode=(S_IFDIR | 0755),
                                     st_uid=getuid(), st_size=0,
                                     st_mtime=int(time.time())))

        self.containers = cmap   # destroys fs tree cache resistant to misses

    def _parse_path(self, path):    # returns </dir, file(=None)>
        if path.count('/') > 1:     # file
            return str(path[:path.rfind('/')]), str(path[path.rfind('/') + 1:])
        else:                       # dir
            pos = path.rfind('/', 1)
            if pos == -1:
                return path, None
            else:
                return str(path[:pos]), None

    def parse_container(self, path):
        base_container = path[1:]   # /abc/def/g --> abc
        if base_container.find('/') > -1:
            base_container = base_container[:base_container.find('/')]
        return str(base_container)

    def _get_dir(self, path, contents_required=False):
        if not self.containers:
            self.rebuild_container_list()

        if path in self.containers and not (contents_required and \
                self.containers[path]['files'] is None):
            return self.containers[path]

        cname = self.parse_container(path)

        if '/' + cname not in self.containers:
            raise FuseOSError(ENOENT)
        else:
            if self.containers['/' + cname]['files'] is None:
                # fetch contents of container
                log.info("------> CONTENTS NOT FOUND: %s" % cname)

                blobs = self.blobs.list_blobs(cname)

                dirstat = dict(st_mode=(S_IFDIR | 0755), st_size=0,
                               st_uid=getuid(), st_mtime=time.time())

                if self.containers['/' + cname]['files'] is None:
                    self.containers['/' + cname]['files'] = dict()

                for f in blobs:
                    blob_name = f.name
                    blob_date = f.properties.last_modified
                    blob_size = long(f.properties.content_length)

                    node = dict(st_mode=(S_IFREG | 0644), st_size=blob_size,
                                st_mtime=self.convert_to_epoch(blob_date),
                                st_uid=getuid())

                    if blob_name.find('/') == -1:  # file just under container
                        self.containers['/' + cname]['files'][blob_name] = node

            return self.containers['/' + cname]
        return None

    def _get_file(self, path):
        d, f = self._parse_path(path)
        dir = self._get_dir(d, True)
        if dir is not None and f in dir['files']:
            return dir['files'][f]

    def getattr(self, path, fh=None):
        d, f = self._parse_path(path)

        if f is None:
            dir = self._get_dir(d)
            return dir['stat']
        else:
            file = self._get_file(path)

            if file:
                return file

        raise FuseOSError(ENOENT)

    # FUSE
    def mkdir(self, path, mode):
        if path.count('/') <= 1:    # create on root
            name = path[1:]

            if not 3 <= len(name) <= 63:
                log.error("Container names can be 3 through 63 chars long.")
                raise FuseOSError(ENAMETOOLONG)
            if name is not name.lower():
                log.error("Container names cannot contain uppercase \
                        characters.")
                raise FuseOSError(EACCES)
            if name.count('--') > 0:
                log.error('Container names cannot contain consecutive \
                        dashes (-).')
                raise FuseOSError(EAGAIN)
            #TODO handle all "-"s must be preceded by letter or numbers
            #TODO starts with only letter or number, can contain letter, nr,'-'

            resp = self.blobs.create_container(name)

            if resp:
                self.rebuild_container_list()
                log.info("CONTAINER %s CREATED" % name)
            else:
                raise FuseOSError(EACCES)
                log.error("Invalid container name or container already \
                        exists.")
        else:
            raise FuseOSError(ENOSYS)  # TODO support 2nd+ level mkdirs

    def rmdir(self, path):
        if path.count('/') == 1:
            c_name = path[1:]
            resp = self.blobs.delete_container(c_name)

            if resp:
                if path in self.containers:
                    del self.containers[path]
            else:
                raise FuseOSError(EACCES)
        else:
            raise FuseOSError(ENOSYS)  # TODO support 2nd+ level mkdirs

    def create(self, path, mode):
        node = dict(st_mode=(S_IFREG | mode), st_size=0, st_nlink=1,
                     st_uid=getuid(), st_mtime=time.time())
        d, f = self._parse_path(path)

        if not f:
            log.error("Cannot create files on root level: /")
            raise FuseOSError(ENOSYS)

        dir = self._get_dir(d, True)
        if not dir:
            raise FuseOSError(EIO)
        dir['files'][f] = node

        return self.open(path, data='')     # reusing handler provider

    def open(self, path, flags=0, data=None):
        if data == None:                    # download contents
            c_name = self.parse_container(path)
            f_name = path[path.find('/', 1) + 1:]

            try:
                data = self.blobs.get_blob(c_name, f_name)
            except AzureMissingResourceHttpError:
                dir = self._get_dir('/' + c_name, True)
                if f_name in dir['files']:
                    del dir['files'][f_name]
                raise FuseOSError(ENOENT)
            except AzureException as e:
                log.error("Read blob failed HTTP %d" % e.code)
                raise FuseOSError(EAGAIN)

        self.fd += 1
        self.fds[self.fd] = (path, data, False)

        return self.fd

    def flush(self, path, fh=None):
        if not fh:
            raise FuseOSError(EIO)
        else:
            if fh not in self.fds:
                raise FuseOSError(EIO)
            path = self.fds[fh][0]
            data = self.fds[fh][1]
            dirty = self.fds[fh][2]

            if not dirty:
                return 0     # avoid redundant write

            d, f = self._parse_path(path)
            c_name = self.parse_container(path)

            if data is None:
                data = ''

            try:
                if len(data) < 64 * 1024 * 1024:   # 64 mb
                    self.blobs.put_blob(c_name, f, data, 'BlockBlob')
                else:
                    # divide file by blocks and upload
                    block_size = 8 * 1024 * 1024
                    num_blocks = int(math.ceil(len(data) * 1.0 / block_size))
                    rd = str(random.randint(1, 1e8))
                    block_ids = list()

                    for i in range(num_blocks):
                        part = data[i * block_size:min((i + 1) * block_size,
                            len(data))]
                        block_id = base64.encodestring('%s_%s' % (rd,
                            (8 - len(str(i))) * '0' + str(i)))
                        self.blobs.put_block(c_name, f, part, block_id)
                        block_ids.append(block_id)

                    self.blobs.put_block_list(c_name, f, block_ids)
            except AzureException:
                raise FuseOSError(EAGAIN)

            dir = self._get_dir(d, True)
            if not dir or f not in dir['files']:
                raise FuseOSError(EIO)

            # update local data
            dir['files'][f]['st_size'] = len(data)
            dir['files'][f]['st_mtime'] = time.time()
            self.fds[fh] = (path, data, False)  # mark as not dirty
            return 0

    def release(self, path, fh=None):
        if fh is not None and fh in self.fds:
            del self.fds[fh]

    def truncate(self, path, length, fh=None):
        return 0     # assume done, no need

    def write(self, path, data, offset, fh=None):
        if not fh or fh not in self.fds:
            raise FuseOSError(ENOENT)
        else:
            d = self.fds[fh][1]
            if d is None:
                d = ""
            self.fds[fh] = (self.fds[fh][0], d[:offset] + data, True)
            return len(data)

    def unlink(self, path):
        c_name = self.parse_container(path)
        d, f = self._parse_path(path)

        try:
            self.blobs.delete_blob(c_name, f)

            _dir = self._get_dir(path, True)
            if _dir and f in _dir['files']:
                del _dir['files'][f]
            return 0
        except AzureMissingResourceHttpError:
            raise FuseOSError(ENOENT)
        except Exception as e:
            raise FuseOSError(EAGAIN)

    def readdir(self, path, fh):
        if path == '/':
            return ['.', '..'] + [x[1:] for x in self.containers.keys() \
                    if x is not '/']

        dir = self._get_dir(path, True)
        if not dir:
            raise FuseOSError(ENOENT)
        return ['.', '..'] + dir['files'].keys()

    def read(self, path, size, offset, fh):
        if not fh or fh not in self.fds:
            raise FuseOSError(ENOENT)

        f_name = path[path.find('/', 1) + 1:]
        c_name = path[1:path.find('/', 1)]

        try:
            data = self.blobs.get_blob(c_name, f_name)
            self.fds[fh] = (self.fds[fh][0], data, False)
            return data[offset:offset + size]
        except URLError, e:
            if e.code == 404:
                raise FuseOSError(ENOENT)
            elif e.code == 403:
                raise FUSEOSError(EPERM)
            else:
                log.error("Read blob failed HTTP %d" % e.code)
                raise FuseOSError(EAGAIN)
        data = self.fds[fh][1]
        if data is None:
            data = ""
        return data[offset:offset + size]
Example #4
0
def uri_put_file(creds, uri, fp, content_type=None):
    assert fp.tell() == 0
    assert uri.startswith('wabs://')

    def log_upload_failures_on_error(exc_tup, exc_processor_cxt):
        def standard_detail_message(prefix=''):
            return (prefix + '  There have been {n} attempts to upload  '
                    'file {url} so far.'.format(n=exc_processor_cxt, url=uri))

        typ, value, tb = exc_tup
        del exc_tup

        # Screen for certain kinds of known-errors to retry from
        if issubclass(typ, socket.error):
            socketmsg = value[1] if isinstance(value, tuple) else value

            logger.info(
                msg='Retrying upload because of a socket error',
                detail=standard_detail_message(
                    "The socket error's message is '{0}'.".format(socketmsg)))
        else:
            # For all otherwise untreated exceptions, report them as a
            # warning and retry anyway -- all exceptions that can be
            # justified should be treated and have error messages
            # listed.
            logger.warning(
                msg='retrying file upload from unexpected exception',
                detail=standard_detail_message(
                    'The exception type is {etype} and its value is '
                    '{evalue} and its traceback is {etraceback}'.format(
                        etype=typ,
                        evalue=value,
                        etraceback=''.join(traceback.format_tb(tb)))))

        # Help Python GC by resolving possible cycles
        del tb

    # Because we're uploading in chunks, catch rate limiting and
    # connection errors which occur for each individual chunk instead of
    # failing the whole file and restarting.
    @retry(retry_with_count(log_upload_failures_on_error))
    def upload_chunk(chunk, block_id):
        check_sum = base64.b64encode(md5(chunk).digest()).decode('utf-8')
        conn.put_block(url_tup.netloc,
                       url_tup.path.lstrip('/'),
                       chunk,
                       block_id,
                       content_md5=check_sum)

    url_tup = urlparse(uri)
    kwargs = dict(x_ms_blob_type='BlockBlob')
    if content_type is not None:
        kwargs['x_ms_blob_content_type'] = content_type

    conn = BlobService(creds.account_name,
                       creds.account_key,
                       sas_token=creds.access_token,
                       protocol='https')
    conn.put_blob(url_tup.netloc, url_tup.path.lstrip('/'), b'', **kwargs)

    # WABS requires large files to be uploaded in 4MB chunks
    block_ids = []
    length, index = 0, 0
    pool_size = os.getenv('WABS_UPLOAD_POOL_SIZE', 5)
    p = gevent.pool.Pool(size=pool_size)
    while True:
        data = fp.read(WABS_CHUNK_SIZE)
        if data:
            length += len(data)
            block_id = base64.b64encode(
                str(index).encode('utf-8')).decode('utf-8')
            p.wait_available()
            p.spawn(upload_chunk, data, block_id)
            block_ids.append(block_id)
            index += 1
        else:
            p.join()
            break

    conn.put_block_list(url_tup.netloc, url_tup.path.lstrip('/'), block_ids)

    # To maintain consistency with the S3 version of this function we must
    # return an object with a certain set of attributes.  Currently, that set
    # of attributes consists of only 'size'
    return _Key(size=len(data))