glance_store/glance_store/common/utils.py

# Copyright 2010 United States Government as represented by the
# Administrator of the National Aeronautics and Space Administration.
# All Rights Reserved.
#
#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

"""
System-level utilities and helper functions.
"""

import hashlib
import logging
import uuid

from oslo_concurrency import lockutils
from oslo_utils.secretutils import md5

try:
    from eventlet import sleep
except ImportError:
    from time import sleep

from glance_store.i18n import _


LOG = logging.getLogger(__name__)

synchronized = lockutils.synchronized_with_prefix('glance_store-')


def is_uuid_like(val):
    """Returns validation of a value as a UUID.

    For our purposes, a UUID is a canonical form string:
    aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa
    """

    try:
        return str(uuid.UUID(val)) == val
    except (TypeError, ValueError, AttributeError):
        return False


def chunkreadable(iter, chunk_size=65536):
    """
    Wrap a readable iterator with a reader yielding chunks of
    a preferred size, otherwise leave iterator unchanged.

    :param iter: an iter which may also be readable
    :param chunk_size: maximum size of chunk
    """
    return chunkiter(iter, chunk_size) if hasattr(iter, 'read') else iter


def chunkiter(fp, chunk_size=65536):
    """
    Return an iterator to a file-like obj which yields fixed size chunks

    :param fp: a file-like object
    :param chunk_size: maximum size of chunk
    """
    while True:
        chunk = fp.read(chunk_size)
        if chunk:
            yield chunk
        else:
            break


def cooperative_iter(iter):
    """
    Return an iterator which schedules after each
    iteration. This can prevent eventlet thread starvation.

    :param iter: an iterator to wrap
    """
    try:
        for chunk in iter:
            sleep(0)
            yield chunk
    except Exception as err:
        msg = _("Error: cooperative_iter exception %s") % err
        LOG.error(msg)
        raise


def cooperative_read(fd):
    """
    Wrap a file descriptor's read with a partial function which schedules
    after each read. This can prevent eventlet thread starvation.

    :param fd: a file descriptor to wrap
    """
    def readfn(*args):
        result = fd.read(*args)
        sleep(0)
        return result
    return readfn


def get_hasher(hash_algo, usedforsecurity=True):
    """
    Returns the required hasher, given the hashing algorithm.
    This is primarily to ensure that the hash algorithm is correctly
    chosen when executed on a FIPS enabled system

    :param hash_algo: hash algorithm requested
    :param usedforsecurity: whether the hashes are used in a security context
    """
    if str(hash_algo) == 'md5':
        return md5(usedforsecurity=usedforsecurity)
    else:
        return hashlib.new(str(hash_algo))


class CooperativeReader(object):
    """
    An eventlet thread friendly class for reading in image data.

    When accessing data either through the iterator or the read method
    we perform a sleep to allow a co-operative yield. When there is more than
    one image being uploaded/downloaded this prevents eventlet thread
    starvation, ie allows all threads to be scheduled periodically rather than
    having the same thread be continuously active.
    """
    def __init__(self, fd):
        """
        :param fd: Underlying image file object
        """
        self.fd = fd
        self.iterator = None
        # NOTE(markwash): if the underlying supports read(), overwrite the
        # default iterator-based implementation with cooperative_read which
        # is more straightforward
        if hasattr(fd, 'read'):
            self.read = cooperative_read(fd)

    def read(self, length=None):
        """Return the next chunk of the underlying iterator.

        This is replaced with cooperative_read in __init__ if the underlying
        fd already supports read().
        """
        if self.iterator is None:
            self.iterator = self.__iter__()
        try:
            return next(self.iterator)
        except StopIteration:
            return b''

    def __iter__(self):
        return cooperative_iter(self.fd.__iter__())