import cStringIO as StringIO
import os
import logging
import copy

from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import padding

from cachetools.func import lru_cache
from itertools import chain

from datetime import datetime, timedelta

from botocore.signers import CloudFrontSigner
from boto.exception import S3ResponseError
import boto.s3.connection
import boto.s3.multipart
import boto.gs.connection
import boto.s3.key
import boto.gs.key

from io import BufferedIOBase
from uuid import uuid4
from collections import namedtuple

from util.registry import filelike
from storage.basestorage import BaseStorageV2


logger = logging.getLogger(__name__)

_PartUploadMetadata = namedtuple('_PartUploadMetadata', ['path', 'offset', 'length'])
_CHUNKS_KEY = 'chunks'


class StreamReadKeyAsFile(BufferedIOBase):
  def __init__(self, key):
    self._key = key

  def read(self, amt=None):
    if self.closed:
      return None

    resp = self._key.read(amt)
    return resp

  def readable(self):
    return True

  @property
  def closed(self):
    return self._key.closed

  def close(self):
    self._key.close(fast=True)


class _CloudStorage(BaseStorageV2):
  def __init__(self, context, connection_class, key_class, connect_kwargs, upload_params,
               storage_path, bucket_name, access_key=None, secret_key=None):
    super(_CloudStorage, self).__init__()

    self.minimum_chunk_size = 5 * 1024 * 1024
    self.maximum_chunk_size = None

    self._initialized = False
    self._bucket_name = bucket_name
    self._access_key = access_key
    self._secret_key = secret_key
    self._root_path = storage_path
    self._connection_class = connection_class
    self._key_class = key_class
    self._upload_params = upload_params
    self._connect_kwargs = connect_kwargs
    self._cloud_conn = None
    self._cloud_bucket = None
    self._context = context

  def _initialize_cloud_conn(self):
    if not self._initialized:
      self._cloud_conn = self._connection_class(self._access_key, self._secret_key,
                                                **self._connect_kwargs)
      self._cloud_bucket = self._cloud_conn.get_bucket(self._bucket_name, validate=False)
      self._initialized = True

  def _debug_key(self, key):
    """Used for debugging only."""
    orig_meth = key.bucket.connection.make_request

    def new_meth(*args, **kwargs):
      print '#' * 16
      print args
      print kwargs
      print '#' * 16
      return orig_meth(*args, **kwargs)
    key.bucket.connection.make_request = new_meth

  def _init_path(self, path=None):
    path = os.path.join(self._root_path, path) if path else self._root_path
    if path and path[0] == '/':
      return path[1:]
    return path

  def get_cloud_conn(self):
    self._initialize_cloud_conn()
    return self._cloud_conn

  def get_cloud_bucket(self):
    return self._cloud_bucket

  def get_content(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    try:
      return key.get_contents_as_string()
    except S3ResponseError as s3r:
      # Raise an IOError in case the key was not found, to maintain the current
      # interface.
      if s3r.error_code == 'NoSuchKey':
        raise IOError('No such key: \'{0}\''.format(path))

      raise

  def put_content(self, path, content):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    key.set_contents_from_string(content, **self._upload_params)
    return path

  def get_supports_resumable_downloads(self):
    return True

  def get_direct_download_url(self, path, request_ip=None, expires_in=60, requires_cors=False, head=False):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    k = self._key_class(self._cloud_bucket, path)
    if head:
      return k.generate_url(expires_in, 'HEAD')
    return k.generate_url(expires_in)

  def get_direct_upload_url(self, path, mime_type, requires_cors=True):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    url = key.generate_url(300, 'PUT', headers={'Content-Type': mime_type}, encrypt_key=True)
    return url

  def stream_read(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    if not key.exists():
      raise IOError('No such key: \'{0}\''.format(path))
    while True:
      buf = key.read(self.buffer_size)
      if not buf:
        break
      yield buf

  def stream_read_file(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    if not key.exists():
      raise IOError('No such key: \'{0}\''.format(path))
    return StreamReadKeyAsFile(key)

  def __initiate_multipart_upload(self, path, content_type, content_encoding):
    # Minimum size of upload part size on S3 is 5MB
    self._initialize_cloud_conn()
    path = self._init_path(path)

    metadata = {}
    if content_type is not None:
      metadata['Content-Type'] = content_type

    if content_encoding is not None:
      metadata['Content-Encoding'] = content_encoding

    if self._context.metric_queue is not None:
      self._context.metric_queue.multipart_upload_start.Inc()

    return self._cloud_bucket.initiate_multipart_upload(path, metadata=metadata,
                                                        **self._upload_params)

  def stream_write(self, path, fp, content_type=None, content_encoding=None):
    """ Writes the data found in the file-like stream to the given path. Raises an IOError
        if the write fails.
    """
    _, write_error = self._stream_write_internal(path, fp, content_type, content_encoding)
    if write_error is not None:
      logger.error('Error when trying to stream_write path `%s`: %s', path, write_error)
      raise IOError('Exception when trying to stream_write path')

  def _stream_write_internal(self, path, fp, content_type=None, content_encoding=None,
                             cancel_on_error=True, size=filelike.READ_UNTIL_END):
    """ Writes the data found in the file-like stream to the given path, with optional limit
        on size. Note that this method returns a *tuple* of (bytes_written, write_error) and should
        *not* raise an exception (such as IOError) if a problem uploading occurred. ALWAYS check
        the returned tuple on calls to this method.
    """
    write_error = None

    try:
      mp = self.__initiate_multipart_upload(path, content_type, content_encoding)
    except S3ResponseError as e:
      logger.exception('Exception when initiating multipart upload')
      return 0, e

    # We are going to reuse this but be VERY careful to only read the number of bytes written to it
    buf = StringIO.StringIO()

    num_part = 1
    total_bytes_written = 0
    while size == filelike.READ_UNTIL_END or total_bytes_written < size:
      bytes_to_copy = self.minimum_chunk_size
      if size != filelike.READ_UNTIL_END:
        # We never want to ask for more bytes than our caller has indicated to copy
        bytes_to_copy = min(bytes_to_copy, size - total_bytes_written)

      buf.seek(0)
      try:
        # Stage the bytes into the buffer for use with the multipart upload file API
        bytes_staged = self.stream_write_to_fp(fp, buf, bytes_to_copy)
        if bytes_staged == 0:
          break

        buf.seek(0)
        mp.upload_part_from_file(buf, num_part, size=bytes_staged)
        total_bytes_written += bytes_staged
        num_part += 1
      except (S3ResponseError, IOError) as e:
        logger.warn('Error when writing to stream in stream_write_internal at path %s: %s', path, e)
        write_error = e

        if self._context.metric_queue is not None:
          self._context.metric_queue.multipart_upload_end.Inc(labelvalues=['failure'])

        if cancel_on_error:
          try:
            mp.cancel_upload()
          except (S3ResponseError, IOError):
            logger.exception('Could not cancel upload')

          return 0, write_error
        else:
          break

    if total_bytes_written > 0:
      if self._context.metric_queue is not None:
        self._context.metric_queue.multipart_upload_end.Inc(labelvalues=['success'])

      self._perform_action_with_retry(mp.complete_upload)

    return total_bytes_written, write_error

  def exists(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    return key.exists()

  def remove(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    if key.exists():
      # It's a file
      key.delete()
      return
    # We assume it's a directory
    if not path.endswith('/'):
      path += '/'
    for key in self._cloud_bucket.list(prefix=path):
      key.delete()

  def get_checksum(self, path):
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)
    k = self._cloud_bucket.lookup(key)
    if k is None:
      raise IOError('No such key: \'{0}\''.format(path))

    return k.etag[1:-1][:7]

  def copy_to(self, destination, path):
    """ Copies the given path from this storage to the destination storage. """
    self._initialize_cloud_conn()

    # First try to copy directly via boto, but only if the storages are the
    # same type, with the same access information.
    if (self.__class__ == destination.__class__ and
        self._access_key and self._secret_key and
        self._access_key == destination._access_key and
        self._secret_key == destination._secret_key and
        self._connect_kwargs == destination._connect_kwargs):

      # Initialize the cloud connection on the destination as well.
      destination._initialize_cloud_conn()

      # Check the buckets for both the source and destination locations.
      if self._cloud_bucket is None:
        logger.error('Cloud bucket not found for location %s; Configuration is probably invalid!',
                     self._bucket_name)
        return

      if destination._cloud_bucket is None:
        logger.error('Cloud bucket not found for location %s; Configuration is probably invalid!',
                     destination._bucket_name)
        return

      # Perform the copy.
      logger.debug('Copying file from %s to %s via a direct boto copy', self._cloud_bucket,
                   destination._cloud_bucket)

      source_path = self._init_path(path)
      source_key = self._key_class(self._cloud_bucket, source_path)

      dest_path = destination._init_path(path)
      source_key.copy(destination._cloud_bucket, dest_path)
      return

    # Fallback to a slower, default copy.
    logger.debug('Copying file from %s to %s via a streamed copy', self._cloud_bucket,
                 destination)
    with self.stream_read_file(path) as fp:
      destination.stream_write(path, fp)

  def _rel_upload_path(self, uuid):
    return 'uploads/{0}'.format(uuid)

  def initiate_chunked_upload(self):
    self._initialize_cloud_conn()
    random_uuid = str(uuid4())

    metadata = {
      _CHUNKS_KEY: [],
    }

    return random_uuid, metadata

  def stream_upload_chunk(self, uuid, offset, length, in_fp, storage_metadata, content_type=None):
    self._initialize_cloud_conn()

    # We are going to upload each chunk to a separate key
    chunk_path = self._rel_upload_path(str(uuid4()))
    bytes_written, write_error = self._stream_write_internal(chunk_path, in_fp,
                                                             cancel_on_error=False, size=length,
                                                             content_type=content_type)

    new_metadata = copy.deepcopy(storage_metadata)

    # We are only going to track keys to which data was confirmed written
    if bytes_written > 0:
      new_metadata[_CHUNKS_KEY].append(_PartUploadMetadata(chunk_path, offset, bytes_written))

    return bytes_written, new_metadata, write_error

  def _chunk_generator(self, chunk_list):
    for chunk in chunk_list:
      yield filelike.StreamSlice(self.stream_read_file(chunk.path), 0, chunk.length)

  @staticmethod
  def _chunk_list_from_metadata(storage_metadata):
    return [_PartUploadMetadata(*chunk_args) for chunk_args in storage_metadata[_CHUNKS_KEY]]

  def _client_side_chunk_join(self, final_path, chunk_list):
    # If there's only one chunk, just "move" (copy and delete) the key and call it a day.
    if len(chunk_list) == 1:
      chunk_path = self._init_path(chunk_list[0].path)
      abs_final_path = self._init_path(final_path)

      # Let the copy raise an exception if it fails.
      self._cloud_bucket.copy_key(abs_final_path, self._bucket_name, chunk_path)

      # Attempt to clean up the old chunk.
      try:
        self._cloud_bucket.delete_key(chunk_path)
      except IOError:
        # We failed to delete a chunk. This sucks, but we shouldn't fail the push.
        msg = 'Failed to clean up chunk %s for move of %s'
        logger.exception(msg, chunk_path, abs_final_path)
    else:
      # Concatenate and write all the chunks as one key.
      concatenated = filelike.FilelikeStreamConcat(self._chunk_generator(chunk_list))
      self.stream_write(final_path, concatenated)

      # Attempt to clean up all the chunks.
      for chunk in chunk_list:
        try:
          self._cloud_bucket.delete_key(self._init_path(chunk.path))
        except IOError:
          # We failed to delete a chunk. This sucks, but we shouldn't fail the push.
          msg = 'Failed to clean up chunk %s for reupload of %s'
          logger.exception(msg, chunk.path, final_path)

  @staticmethod
  def _perform_action_with_retry(action, *args, **kwargs):
    # Note: Sometimes Amazon S3 simply raises an internal error when trying to complete a
    # an action. The recommendation is to simply try calling the action again.
    for remaining_retries in range(2, -1, -1):
      try:
        action(*args, **kwargs)
        break
      except S3ResponseError as s3re:
        if remaining_retries and s3re.status == 200 and s3re.error_code == 'InternalError':
          # Weird internal error case. Retry.
          continue

        # Otherwise, raise it.
        logger.exception('Exception trying to perform action %s', action)
        raise s3re

  @staticmethod
  def _rechunk(chunk, max_chunk_size):
    """ Rechunks the chunk list to meet maximum chunk size restrictions for the storage engine. """
    if max_chunk_size is None or chunk.length <= max_chunk_size:
      yield chunk
    else:
      newchunk_length = chunk.length / 2
      first_subchunk = _PartUploadMetadata(chunk.path, chunk.offset, newchunk_length)
      second_subchunk = _PartUploadMetadata(chunk.path,
                                            chunk.offset + newchunk_length,
                                            chunk.length - newchunk_length)
      for subchunk in chain(_CloudStorage._rechunk(first_subchunk, max_chunk_size),
                            _CloudStorage._rechunk(second_subchunk, max_chunk_size)):
        yield subchunk


  def complete_chunked_upload(self, uuid, final_path, storage_metadata, force_client_side=False):
    self._initialize_cloud_conn()
    chunk_list = self._chunk_list_from_metadata(storage_metadata)

    # Here is where things get interesting: we are going to try to assemble this server side
    # In order to be a candidate all parts (after offsets have been computed) must be at least 5MB
    server_side_assembly = False
    if not force_client_side:
      server_side_assembly = True
      for chunk_offset, chunk in enumerate(chunk_list):
        # If the chunk is both too small, and not the last chunk, we rule out server side assembly
        if chunk.length < self.minimum_chunk_size and (chunk_offset + 1) < len(chunk_list):
          server_side_assembly = False
          break

    if server_side_assembly:
      logger.debug('Performing server side assembly of multi-part upload for: %s', final_path)
      try:
        # Awesome, we can do this completely server side, now we have to start a new multipart
        # upload and use copy_part_from_key to set all of the chunks.
        mpu = self.__initiate_multipart_upload(final_path, content_type=None, content_encoding=None)
        updated_chunks = chain.from_iterable([_CloudStorage._rechunk(c, self.maximum_chunk_size)
                                              for c in chunk_list])

        for index, chunk in enumerate(updated_chunks):
          abs_chunk_path = self._init_path(chunk.path)
          self._perform_action_with_retry(mpu.copy_part_from_key, self.get_cloud_bucket().name,
                                          abs_chunk_path, index + 1, start=chunk.offset,
                                          end=chunk.length + chunk.offset - 1)

        self._perform_action_with_retry(mpu.complete_upload)
      except IOError as ioe:
        # Something bad happened, log it and then give up
        msg = 'Exception when attempting server-side assembly for: %s'
        logger.exception(msg, final_path)
        mpu.cancel_upload()
        raise ioe

    else:
      # We are going to turn all of the server side objects into a single file-like stream, and
      # pass that to stream_write to chunk and upload the final object.
      self._client_side_chunk_join(final_path, chunk_list)


  def cancel_chunked_upload(self, uuid, storage_metadata):
    self._initialize_cloud_conn()

    # We have to go through and delete all of the uploaded chunks
    for chunk in self._chunk_list_from_metadata(storage_metadata):
      self.remove(chunk.path)


class S3Storage(_CloudStorage):
  def __init__(self, context, storage_path, s3_bucket, s3_access_key=None,
               s3_secret_key=None, host=None, port=None):
    upload_params = {
      'encrypt_key': True,
    }
    connect_kwargs = {}
    if host:
      if host.startswith('http:') or host.startswith('https:'):
        raise ValueError('host name must not start with http:// or https://')

      connect_kwargs['host'] = host

    if port:
      connect_kwargs['port'] = int(port)

    super(S3Storage, self).__init__(context, boto.s3.connection.S3Connection, boto.s3.key.Key,
                                    connect_kwargs, upload_params, storage_path, s3_bucket,
                                    access_key=s3_access_key or None,
                                    secret_key=s3_secret_key or None)

    self.maximum_chunk_size = 5 * 1024 * 1024 * 1024 # 5GB.

  def setup(self):
    self.get_cloud_bucket().set_cors_xml("""<?xml version="1.0" encoding="UTF-8"?>
      <CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
          <CORSRule>
              <AllowedOrigin>*</AllowedOrigin>
              <AllowedMethod>GET</AllowedMethod>
              <MaxAgeSeconds>3000</MaxAgeSeconds>
              <AllowedHeader>Authorization</AllowedHeader>
          </CORSRule>
          <CORSRule>
              <AllowedOrigin>*</AllowedOrigin>
              <AllowedMethod>PUT</AllowedMethod>
              <MaxAgeSeconds>3000</MaxAgeSeconds>
              <AllowedHeader>Content-Type</AllowedHeader>
              <AllowedHeader>x-amz-acl</AllowedHeader>
              <AllowedHeader>origin</AllowedHeader>
          </CORSRule>
      </CORSConfiguration>""")

class GoogleCloudStorage(_CloudStorage):
  def __init__(self, context, storage_path, access_key, secret_key, bucket_name):
    upload_params = {}
    connect_kwargs = {}
    super(GoogleCloudStorage, self).__init__(context, boto.gs.connection.GSConnection,
                                             boto.gs.key.Key, connect_kwargs, upload_params,
                                             storage_path, bucket_name, access_key, secret_key)

  def setup(self):
    self.get_cloud_bucket().set_cors_xml("""<?xml version="1.0" encoding="UTF-8"?>
      <CorsConfig>
        <Cors>
          <Origins>
            <Origin>*</Origin>
          </Origins>
          <Methods>
            <Method>GET</Method>
            <Method>PUT</Method>
          </Methods>
          <ResponseHeaders>
            <ResponseHeader>Content-Type</ResponseHeader>
          </ResponseHeaders>
          <MaxAgeSec>3000</MaxAgeSec>
        </Cors>
      </CorsConfig>""")

  def _stream_write_internal(self, path, fp, content_type=None, content_encoding=None,
                             cancel_on_error=True, size=filelike.READ_UNTIL_END):
    """ Writes the data found in the file-like stream to the given path, with optional limit
        on size. Note that this method returns a *tuple* of (bytes_written, write_error) and should
        *not* raise an exception (such as IOError) if a problem uploading occurred. ALWAYS check
        the returned tuple on calls to this method.
    """
    # Minimum size of upload part size on S3 is 5MB
    self._initialize_cloud_conn()
    path = self._init_path(path)
    key = self._key_class(self._cloud_bucket, path)

    if content_type is not None:
      key.set_metadata('Content-Type', content_type)

    if content_encoding is not None:
      key.set_metadata('Content-Encoding', content_encoding)

    if size != filelike.READ_UNTIL_END:
      fp = filelike.StreamSlice(fp, 0, size)

    # TODO figure out how to handle cancel_on_error=False
    try:
      key.set_contents_from_stream(fp)
    except IOError as ex:
      return 0, ex

    return key.size, None

  def complete_chunked_upload(self, uuid, final_path, storage_metadata):
    self._initialize_cloud_conn()

    # Boto does not support GCS's multipart upload API because it differs from S3, so
    # we are forced to join it all locally and then reupload.
    # See https://github.com/boto/boto/issues/3355
    chunk_list = self._chunk_list_from_metadata(storage_metadata)
    self._client_side_chunk_join(final_path, chunk_list)


class RadosGWStorage(_CloudStorage):
  def __init__(self, context, hostname, is_secure, storage_path, access_key, secret_key,
               bucket_name, port=None):
    upload_params = {}
    connect_kwargs = {
      'host': hostname,
      'is_secure': is_secure,
      'calling_format': boto.s3.connection.OrdinaryCallingFormat(),
    }

    if port:
      connect_kwargs['port'] = int(port)

    super(RadosGWStorage, self).__init__(context, boto.s3.connection.S3Connection,
                                         boto.s3.key.Key, connect_kwargs, upload_params,
                                         storage_path, bucket_name, access_key, secret_key)

  # TODO remove when radosgw supports cors: http://tracker.ceph.com/issues/8718#change-38624
  def get_direct_download_url(self, path, request_ip=None, expires_in=60, requires_cors=False,
                              head=False):
    if requires_cors:
      return None

    return super(RadosGWStorage, self).get_direct_download_url(path, request_ip, expires_in,
                                                               requires_cors, head)

  # TODO remove when radosgw supports cors: http://tracker.ceph.com/issues/8718#change-38624
  def get_direct_upload_url(self, path, mime_type, requires_cors=True):
    if requires_cors:
      return None

    return super(RadosGWStorage, self).get_direct_upload_url(path, mime_type, requires_cors)

  def complete_chunked_upload(self, uuid, final_path, storage_metadata):
    self._initialize_cloud_conn()

    # RadosGW does not support multipart copying from keys, so we are forced to join
    # it all locally and then reupload.
    # See https://github.com/ceph/ceph/pull/5139
    chunk_list = self._chunk_list_from_metadata(storage_metadata)
    self._client_side_chunk_join(final_path, chunk_list)


class RHOCSStorage(RadosGWStorage):
  """ RHOCSStorage implements storage explicitly via RHOCS. For now, this uses the same protocol
      as RadowsGW, but we create a distinct driver for future additional capabilities.
  """
  pass


class CloudFrontedS3Storage(S3Storage):
  """ An S3Storage engine that redirects to CloudFront for all requests outside of AWS. """
  def __init__(self, context, cloudfront_distribution_domain, cloudfront_key_id,
               cloudfront_privatekey_filename, storage_path, s3_bucket, *args, **kwargs):
    super(CloudFrontedS3Storage, self).__init__(context, storage_path, s3_bucket, *args, **kwargs)

    self.cloudfront_distribution_domain = cloudfront_distribution_domain
    self.cloudfront_key_id = cloudfront_key_id
    self.cloudfront_privatekey = self._load_private_key(cloudfront_privatekey_filename)

  def get_direct_download_url(self, path, request_ip=None, expires_in=60, requires_cors=False,
                              head=False):
    # If CloudFront could not be loaded, fall back to normal S3.
    if self.cloudfront_privatekey is None or request_ip is None:
      return super(CloudFrontedS3Storage, self).get_direct_download_url(path, request_ip,
                                                                        expires_in, requires_cors,
                                                                        head)

    resolved_ip_info = None
    logger.debug('Got direct download request for path "%s" with IP "%s"', path, request_ip)

    # Lookup the IP address in our resolution table and determine whether it is under AWS.
    # If it is, then return an S3 signed URL, since we are in-network.
    resolved_ip_info = self._context.ip_resolver.resolve_ip(request_ip)
    logger.debug('Resolved IP information for IP %s: %s', request_ip, resolved_ip_info)
    if resolved_ip_info and resolved_ip_info.provider == 'aws':
      return super(CloudFrontedS3Storage, self).get_direct_download_url(path, request_ip,
                                                                        expires_in, requires_cors,
                                                                        head)

    url = 'https://%s/%s' % (self.cloudfront_distribution_domain, path)
    expire_date = datetime.now() + timedelta(seconds=expires_in)
    signer = self._get_cloudfront_signer()
    signed_url = signer.generate_presigned_url(url, date_less_than=expire_date)
    logger.debug('Returning CloudFront URL for path "%s" with IP "%s": %s', path, resolved_ip_info,
                 signed_url)
    return signed_url

  @lru_cache(maxsize=1)
  def _get_cloudfront_signer(self):
    return CloudFrontSigner(self.cloudfront_key_id, self._get_rsa_signer())

  @lru_cache(maxsize=1)
  def _get_rsa_signer(self):
    private_key = self.cloudfront_privatekey
    def handler(message):
      return private_key.sign(message, padding.PKCS1v15(), hashes.SHA1())

    return handler

  @lru_cache(maxsize=1)
  def _load_private_key(self, cloudfront_privatekey_filename):
    """ Returns the private key, loaded from the config provider, used to sign direct
        download URLs to CloudFront.
    """
    if self._context.config_provider is None:
      return None

    with self._context.config_provider.get_volume_file(cloudfront_privatekey_filename) as key_file:
      return serialization.load_pem_private_key(
        key_file.read(),
        password=None,
        backend=default_backend()
      )