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Filename : name_expansion.py
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# -*- coding: utf-8 -*-
# Copyright 2012 Google Inc. 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.
"""Name expansion iterator and result classes.

Name expansion support for the various ways gsutil lets users refer to
collections of data (via explicit wildcarding as well as directory,
bucket, and bucket subdir implicit wildcarding). This class encapsulates
the various rules for determining how these expansions are done.
"""

from __future__ import absolute_import
from __future__ import print_function
from __future__ import division
from __future__ import unicode_literals

import collections
import logging
import os
import sys

import six

from apitools.base.py import encoding
import gslib
from gslib.exception import CommandException
from gslib.exception import NO_URLS_MATCHED_GENERIC
from gslib.exception import NO_URLS_MATCHED_TARGET
from gslib.plurality_checkable_iterator import PluralityCheckableIterator
from gslib.seek_ahead_thread import SeekAheadResult
from gslib.third_party.storage_apitools import storage_v1_messages as apitools_messages
import gslib.wildcard_iterator
from gslib.wildcard_iterator import StorageUrlFromString


class NameExpansionResult(object):
  """Holds one fully expanded result from iterating over NameExpansionIterator.

  The member data in this class need to be pickleable because
  NameExpansionResult instances are passed through Multiprocessing.Queue. In
  particular, don't include any boto state like StorageUri, since that pulls
  in a big tree of objects, some of which aren't pickleable (and even if
  they were, pickling/unpickling such a large object tree would result in
  significant overhead).

  The state held in this object is needed for handling the various naming cases
  (e.g., copying from a single source URL to a directory generates different
  dest URL names than copying multiple URLs to a directory, to be consistent
  with naming rules used by the Unix cp command). For more details see comments
  in _NameExpansionIterator.
  """

  def __init__(self, source_storage_url, is_multi_source_request,
               is_multi_top_level_source_request, names_container,
               expanded_storage_url, expanded_result):
    """Instantiates a result from name expansion.

    Args:
      source_storage_url: StorageUrl that was being expanded.
      is_multi_source_request: bool indicator whether multiple input URLs or
          src_url_str expanded to more than one BucketListingRef.
      is_multi_top_level_source_request: same as is_multi_source_request but
          measured before recursion.
      names_container: Bool indicator whether src_url names a container.
      expanded_storage_url: StorageUrl that was expanded.
      expanded_result: cloud object metadata in MessageToJson form (for
          pickleability), if any was iterated; None otherwise.
          Consumers must call JsonToMessage to get an apitools Object.
    """
    self.source_storage_url = source_storage_url
    self.is_multi_source_request = is_multi_source_request
    self.is_multi_top_level_source_request = is_multi_top_level_source_request
    self.names_container = names_container
    self.expanded_storage_url = expanded_storage_url
    self.expanded_result = encoding.MessageToJson(
        expanded_result) if expanded_result else None

  def __repr__(self):
    return '%s' % self.expanded_storage_url


class _NameExpansionIterator(object):
  """Class that iterates over all source URLs passed to the iterator.

  See details in __iter__ function doc.
  """

  def __init__(self,
               command_name,
               debug,
               logger,
               gsutil_api,
               url_strs,
               recursion_requested,
               all_versions=False,
               cmd_supports_recursion=True,
               project_id=None,
               ignore_symlinks=False,
               continue_on_error=False,
               bucket_listing_fields=None):
    """Creates a NameExpansionIterator.

    Args:
      command_name: name of command being run.
      debug: Debug level to pass to underlying iterators (range 0..3).
      logger: logging.Logger object.
      gsutil_api: Cloud storage interface.  Settable for testing/mocking.
      url_strs: PluralityCheckableIterator of URL strings needing expansion.
      recursion_requested: True if -r specified on command-line.  If so,
          listings will be flattened so mapped-to results contain objects
          spanning subdirectories.
      all_versions: Bool indicating whether to iterate over all object versions.
      cmd_supports_recursion: Bool indicating whether this command supports a
          '-r' flag. Useful for printing helpful error messages.
      project_id: Project id to use for bucket retrieval.
      ignore_symlinks: If True, ignore symlinks during iteration.
      continue_on_error: If true, yield no-match exceptions encountered during
                         iteration instead of raising them.
      bucket_listing_fields: Iterable fields to include in expanded results.
          Ex. ['name', 'acl']. Underyling iterator is responsible for converting
          these to list-style format ['items/name', 'items/acl']. If this is
          None, only the object name is included in the result.

    Examples of _NameExpansionIterator with recursion_requested=True:
      - Calling with one of the url_strs being 'gs://bucket' will enumerate all
        top-level objects, as will 'gs://bucket/' and 'gs://bucket/*'.
      - 'gs://bucket/**' will enumerate all objects in the bucket.
      - 'gs://bucket/abc' will enumerate either the single object abc or, if
         abc is a subdirectory, all objects under abc and any of its
         subdirectories.
      - 'gs://bucket/abc/**' will enumerate all objects under abc or any of its
        subdirectories.
      - 'file:///tmp' will enumerate all files under /tmp, as will
        'file:///tmp/*'
      - 'file:///tmp/**' will enumerate all files under /tmp or any of its
        subdirectories.

    Example if recursion_requested=False:
      calling with gs://bucket/abc/* lists matching objects
      or subdirs, but not sub-subdirs or objects beneath subdirs.

    Note: In step-by-step comments below we give examples assuming there's a
    gs://bucket with object paths:
      abcd/o1.txt
      abcd/o2.txt
      xyz/o1.txt
      xyz/o2.txt
    and a directory file://dir with file paths:
      dir/a.txt
      dir/b.txt
      dir/c/
    """
    self.command_name = command_name
    self.debug = debug
    self.logger = logger
    self.gsutil_api = gsutil_api
    self.url_strs = url_strs
    self.recursion_requested = recursion_requested
    self.all_versions = all_versions
    # Check self.url_strs.HasPlurality() at start because its value can change
    # if url_strs is itself an iterator.
    self.url_strs.has_plurality = self.url_strs.HasPlurality()
    self.cmd_supports_recursion = cmd_supports_recursion
    self.project_id = project_id
    self.ignore_symlinks = ignore_symlinks
    self.continue_on_error = continue_on_error
    self.bucket_listing_fields = (set(['name']) if not bucket_listing_fields
                                  else bucket_listing_fields)

    # Map holding wildcard strings to use for flat vs subdir-by-subdir listings.
    # (A flat listing means show all objects expanded all the way down.)
    self._flatness_wildcard = {True: '**', False: '*'}

  def __iter__(self):
    """Iterates over all source URLs passed to the iterator.

    For each src url, expands wildcards, object-less bucket names,
    subdir bucket names, and directory names, and generates a flat listing of
    all the matching objects/files.

    You should instantiate this object using the static factory function
    NameExpansionIterator, because consumers of this iterator need the
    PluralityCheckableIterator wrapper built by that function.

    Yields:
      gslib.name_expansion.NameExpansionResult.

    Raises:
      CommandException: if errors encountered.
    """
    for url_str in self.url_strs:
      storage_url = StorageUrlFromString(url_str)

      if (storage_url.IsFileUrl() and
          (storage_url.IsStream() or storage_url.IsFifo())):
        if self.url_strs.has_plurality:
          raise CommandException('Multiple URL strings are not supported '
                                 'with streaming ("-") URLs or named pipes.')
        yield NameExpansionResult(source_storage_url=storage_url,
                                  is_multi_source_request=False,
                                  is_multi_top_level_source_request=False,
                                  names_container=False,
                                  expanded_storage_url=storage_url,
                                  expanded_result=None)
        continue

      # Step 1: Expand any explicitly specified wildcards. The output from this
      # step is an iterator of BucketListingRef.
      # Starting with gs://buck*/abc* this step would expand to gs://bucket/abcd

      src_names_bucket = False
      if (storage_url.IsCloudUrl() and storage_url.IsBucket() and
          not self.recursion_requested):
        # UNIX commands like rm and cp will omit directory references.
        # If url_str refers only to buckets and we are not recursing,
        # then produce references of type BUCKET, because they are guaranteed
        # to pass through Step 2 and be omitted in Step 3.
        post_step1_iter = PluralityCheckableIterator(
            self.WildcardIterator(url_str).IterBuckets(bucket_fields=['id']))
      else:
        # Get a list of objects and prefixes, expanding the top level for
        # any listed buckets.  If our source is a bucket, however, we need
        # to treat all of the top level expansions as names_container=True.
        post_step1_iter = PluralityCheckableIterator(
            self.WildcardIterator(url_str).IterAll(
                bucket_listing_fields=self.bucket_listing_fields,
                expand_top_level_buckets=True))
        if storage_url.IsCloudUrl() and storage_url.IsBucket():
          src_names_bucket = True

      src_url_expands_to_multi = post_step1_iter.HasPlurality()
      is_multi_top_level_source_request = (self.url_strs.has_plurality or
                                           src_url_expands_to_multi)

      # Step 2: Expand bucket subdirs. The output from this
      # step is an iterator of (names_container, BucketListingRef).
      # Starting with gs://bucket/abcd this step would expand to:
      #   iter([(True, abcd/o1.txt), (True, abcd/o2.txt)]).
      subdir_exp_wildcard = self._flatness_wildcard[self.recursion_requested]
      if self.recursion_requested:
        post_step2_iter = _ImplicitBucketSubdirIterator(
            self, post_step1_iter, subdir_exp_wildcard,
            self.bucket_listing_fields)
      else:
        post_step2_iter = _NonContainerTuplifyIterator(post_step1_iter)
      post_step2_iter = PluralityCheckableIterator(post_step2_iter)

      # Because we actually perform and check object listings here, this will
      # raise if url_args includes a non-existent object.  However,
      # plurality_checkable_iterator will buffer the exception for us, not
      # raising it until the iterator is actually asked to yield the first
      # result.
      if post_step2_iter.IsEmpty():
        if self.continue_on_error:
          try:
            raise CommandException(NO_URLS_MATCHED_TARGET % url_str)
          except CommandException as e:
            # Yield a specialized tuple of (exception, stack_trace) to
            # the wrapping PluralityCheckableIterator.
            yield (e, sys.exc_info()[2])
        else:
          raise CommandException(NO_URLS_MATCHED_TARGET % url_str)

      # Step 3. Omit any directories, buckets, or bucket subdirectories for
      # non-recursive expansions.
      post_step3_iter = PluralityCheckableIterator(
          _OmitNonRecursiveIterator(post_step2_iter, self.recursion_requested,
                                    self.command_name,
                                    self.cmd_supports_recursion, self.logger))

      src_url_expands_to_multi = post_step3_iter.HasPlurality()
      is_multi_source_request = (self.url_strs.has_plurality or
                                 src_url_expands_to_multi)

      # Step 4. Expand directories and buckets. This step yields the iterated
      # values. Starting with gs://bucket this step would expand to:
      #  [abcd/o1.txt, abcd/o2.txt, xyz/o1.txt, xyz/o2.txt]
      # Starting with file://dir this step would expand to:
      #  [dir/a.txt, dir/b.txt, dir/c/]
      for (names_container, blr) in post_step3_iter:
        src_names_container = src_names_bucket or names_container
        if blr.IsObject():
          yield NameExpansionResult(
              source_storage_url=storage_url,
              is_multi_source_request=is_multi_source_request,
              is_multi_top_level_source_request=
              is_multi_top_level_source_request,
              names_container=src_names_container,
              expanded_storage_url=blr.storage_url,
              expanded_result=blr.root_object)
        else:
          # Use implicit wildcarding to do the enumeration.
          # At this point we are guaranteed that:
          # - Recursion has been requested because non-object entries are
          #   filtered in step 3 otherwise.
          # - This is a prefix or bucket subdirectory because only
          #   non-recursive iterations product bucket references.
          expanded_url = StorageUrlFromString(blr.url_string)
          if expanded_url.IsFileUrl():
            # Convert dir to implicit recursive wildcard.
            url_to_iterate = '%s%s%s' % (blr, os.sep, subdir_exp_wildcard)
          else:
            # Convert subdir to implicit recursive wildcard.
            url_to_iterate = expanded_url.CreatePrefixUrl(
                wildcard_suffix=subdir_exp_wildcard)

          wc_iter = PluralityCheckableIterator(
              self.WildcardIterator(url_to_iterate).IterObjects(
                  bucket_listing_fields=self.bucket_listing_fields))
          src_url_expands_to_multi = (src_url_expands_to_multi or
                                      wc_iter.HasPlurality())
          is_multi_source_request = (self.url_strs.has_plurality or
                                     src_url_expands_to_multi)
          # This will be a flattened listing of all underlying objects in the
          # subdir.
          for blr in wc_iter:
            yield NameExpansionResult(
                source_storage_url=storage_url,
                is_multi_source_request=is_multi_source_request,
                is_multi_top_level_source_request=(
                    is_multi_top_level_source_request),
                names_container=True,
                expanded_storage_url=blr.storage_url,
                expanded_result=blr.root_object)

  def WildcardIterator(self, url_string):
    """Helper to instantiate gslib.WildcardIterator.

    Args are same as gslib.WildcardIterator interface, but this method fills
    in most of the values from instance state.

    Args:
      url_string: URL string naming wildcard objects to iterate.

    Returns:
      Wildcard iterator over URL string.
    """
    return gslib.wildcard_iterator.CreateWildcardIterator(
        url_string,
        self.gsutil_api,
        all_versions=self.all_versions,
        project_id=self.project_id,
        ignore_symlinks=self.ignore_symlinks,
        logger=self.logger)


class SeekAheadNameExpansionIterator(object):
  """Creates and wraps a _NameExpansionIterator and yields SeekAheadResults.

  Unlike the NameExpansionIterator, which can make API calls upon __init__
  to check for plurality, this iterator does no work until the first iteration
  occurs.
  """

  def __init__(self,
               command_name,
               debug,
               gsutil_api,
               url_strs,
               recursion_requested,
               all_versions=False,
               cmd_supports_recursion=True,
               project_id=None,
               ignore_symlinks=False,
               file_size_will_change=False):
    """Initializes a _NameExpansionIterator with the inputs."""

    # Count data bytes only will be transferred/rewritten.
    # Note that the rsync command uses a different iterator, thus it is not
    # included here.
    self.count_data_bytes = (command_name in ('cp', 'mv', 'rewrite') and
                             not file_size_will_change)

    # Only query the file size if we are counting data bytes, as this may
    # result in stat'ing files, which is more expensive.
    bucket_listing_fields = ['size'] if self.count_data_bytes else None

    self.name_expansion_iterator = _NameExpansionIterator(
        command_name,
        debug,
        logging.getLogger('dummy'),
        gsutil_api,
        PluralityCheckableIterator(url_strs),
        recursion_requested,
        all_versions=all_versions,
        cmd_supports_recursion=cmd_supports_recursion,
        project_id=project_id,
        ignore_symlinks=ignore_symlinks,
        continue_on_error=True,
        bucket_listing_fields=bucket_listing_fields)

  def __iter__(self):
    for name_expansion_result in self.name_expansion_iterator:
      if self.count_data_bytes and name_expansion_result.expanded_result:
        iterated_metadata = encoding.JsonToMessage(
            apitools_messages.Object, name_expansion_result.expanded_result)
        iterated_size = iterated_metadata.size or 0
        yield SeekAheadResult(data_bytes=iterated_size)
      else:
        yield SeekAheadResult()


def NameExpansionIterator(command_name,
                          debug,
                          logger,
                          gsutil_api,
                          url_strs,
                          recursion_requested,
                          all_versions=False,
                          cmd_supports_recursion=True,
                          project_id=None,
                          ignore_symlinks=False,
                          continue_on_error=False,
                          bucket_listing_fields=None):
  """Static factory function for instantiating _NameExpansionIterator.

  This wraps the resulting iterator in a PluralityCheckableIterator and checks
  that it is non-empty. Also, allows url_strs to be either an array or an
  iterator.

  Args:
    command_name: name of command being run.
    debug: Debug level to pass to underlying iterators (range 0..3).
    logger: logging.Logger object.
    gsutil_api: Cloud storage interface.  Settable for testing/mocking.
    url_strs: Iterable URL strings needing expansion.
    recursion_requested: True if -r specified on command-line.  If so,
        listings will be flattened so mapped-to results contain objects
        spanning subdirectories.
    all_versions: Bool indicating whether to iterate over all object versions.
    cmd_supports_recursion: Bool indicating whether this command supports a '-r'
        flag. Useful for printing helpful error messages.
    project_id: Project id to use for the current command.
    ignore_symlinks: If True, ignore symlinks during iteration.
    continue_on_error: If true, yield no-match exceptions encountered during
                       iteration instead of raising them.
    bucket_listing_fields: Iterable fields to include in expanded results.
        Ex. ['name', 'acl']. Underyling iterator is responsible for converting
        these to list-style format ['items/name', 'items/acl']. If this is
        None, only the object name is included in the result.

  Raises:
    CommandException if underlying iterator is empty.

  Returns:
    Name expansion iterator instance.

  For example semantics, see comments in NameExpansionIterator.__init__.
  """
  url_strs = PluralityCheckableIterator(url_strs)
  name_expansion_iterator = _NameExpansionIterator(
      command_name,
      debug,
      logger,
      gsutil_api,
      url_strs,
      recursion_requested,
      all_versions=all_versions,
      cmd_supports_recursion=cmd_supports_recursion,
      project_id=project_id,
      ignore_symlinks=ignore_symlinks,
      continue_on_error=continue_on_error,
      bucket_listing_fields=bucket_listing_fields)
  name_expansion_iterator = PluralityCheckableIterator(name_expansion_iterator)
  if name_expansion_iterator.IsEmpty():
    raise CommandException(NO_URLS_MATCHED_GENERIC)
  return name_expansion_iterator


class _NonContainerTuplifyIterator(object):
  """Iterator that produces the tuple (False, blr) for each iterated value.

  Used for cases where blr_iter iterates over a set of
  BucketListingRefs known not to name containers.
  """

  def __init__(self, blr_iter):
    """Instantiates iterator.

    Args:
      blr_iter: iterator of BucketListingRef.
    """
    self.blr_iter = blr_iter

  def __iter__(self):
    for blr in self.blr_iter:
      yield (False, blr)


class _OmitNonRecursiveIterator(object):
  """Iterator wrapper for that omits certain values for non-recursive requests.

  This iterates over tuples of (names_container, BucketListingReference) and
  omits directories, prefixes, and buckets from non-recurisve requests
  so that we can properly calculate whether the source URL expands to multiple
  URLs.

  For example, if we have a bucket containing two objects: bucket/foo and
  bucket/foo/bar and we do a non-recursive iteration, only bucket/foo will be
  yielded.
  """

  def __init__(self, tuple_iter, recursion_requested, command_name,
               cmd_supports_recursion, logger):
    """Instanties the iterator.

    Args:
      tuple_iter: Iterator over names_container, BucketListingReference
                  from step 2 in the NameExpansionIterator
      recursion_requested: If false, omit buckets, dirs, and subdirs
      command_name: Command name for user messages
      cmd_supports_recursion: Command recursion support for user messages
      logger: Log object for user messages
    """
    self.tuple_iter = tuple_iter
    self.recursion_requested = recursion_requested
    self.command_name = command_name
    self.cmd_supports_recursion = cmd_supports_recursion
    self.logger = logger

  def __iter__(self):
    for (names_container, blr) in self.tuple_iter:
      if not self.recursion_requested and not blr.IsObject():
        # At this point we either have a bucket or a prefix,
        # so if recursion is not requested, we're going to omit it.
        expanded_url = StorageUrlFromString(blr.url_string)
        if expanded_url.IsFileUrl():
          desc = 'directory'
        else:
          desc = blr.type_name
        if self.cmd_supports_recursion:
          self.logger.info('Omitting %s "%s". (Did you mean to do %s -r?)',
                           desc, blr.url_string, self.command_name)
        else:
          self.logger.info('Omitting %s "%s".', desc, blr.url_string)
      else:
        yield (names_container, blr)


class _ImplicitBucketSubdirIterator(object):
  """Iterator wrapper that performs implicit bucket subdir expansion.

  Each iteration yields tuple (names_container, expanded BucketListingRefs)
    where names_container is true if URL names a directory, bucket,
    or bucket subdir.

  For example, iterating over [BucketListingRef("gs://abc")] would expand to:
    [BucketListingRef("gs://abc/o1"), BucketListingRef("gs://abc/o2")]
  if those subdir objects exist, and [BucketListingRef("gs://abc") otherwise.
  """

  def __init__(self, name_exp_instance, blr_iter, subdir_exp_wildcard,
               bucket_listing_fields):
    """Instantiates the iterator.

    Args:
      name_exp_instance: calling instance of NameExpansion class.
      blr_iter: iterator over BucketListingRef prefixes and objects.
      subdir_exp_wildcard: wildcard for expanding subdirectories;
          expected values are ** if the mapped-to results should contain
          objects spanning subdirectories, or * if only one level should
          be listed.
      bucket_listing_fields: Fields requested in enumerated results.
    """
    self.blr_iter = blr_iter
    self.name_exp_instance = name_exp_instance
    self.subdir_exp_wildcard = subdir_exp_wildcard
    self.bucket_listing_fields = bucket_listing_fields

  def __iter__(self):
    for blr in self.blr_iter:
      if blr.IsPrefix():
        # This is a bucket subdirectory, list objects according to the wildcard.
        prefix_url = StorageUrlFromString(blr.url_string).CreatePrefixUrl(
            wildcard_suffix=self.subdir_exp_wildcard)
        implicit_subdir_iterator = PluralityCheckableIterator(
            self.name_exp_instance.WildcardIterator(prefix_url).IterAll(
                bucket_listing_fields=self.bucket_listing_fields))
        if not implicit_subdir_iterator.IsEmpty():
          for exp_blr in implicit_subdir_iterator:
            yield (True, exp_blr)
        else:
          # Prefix that contains no objects, for example in the $folder$ case
          # or an empty filesystem directory.
          yield (False, blr)
      elif blr.IsObject():
        yield (False, blr)
      else:
        raise CommandException(
            '_ImplicitBucketSubdirIterator got a bucket reference %s' % blr)


class CopyObjectInfo(object):
  """Represents the information needed for copying a single object.
  """

  def __init__(self, name_expansion_result, exp_dst_url,
               have_existing_dst_container):
    """Instantiates the object info from name expansion result and destination.

    Args:
      name_expansion_result: StorageUrl that was being expanded.
      exp_dst_url: StorageUrl of the destination.
      have_existing_dst_container: Whether exp_url names an existing directory,
          bucket, or bucket subdirectory.
    """
    self.source_storage_url = name_expansion_result.source_storage_url
    self.is_multi_source_request = name_expansion_result.is_multi_source_request
    self.is_multi_top_level_source_request = (
        name_expansion_result.is_multi_top_level_source_request)
    self.names_container = name_expansion_result.names_container
    self.expanded_storage_url = name_expansion_result.expanded_storage_url
    self.expanded_result = name_expansion_result.expanded_result

    self.exp_dst_url = exp_dst_url
    self.have_existing_dst_container = have_existing_dst_container


# Describes the destination information resulted from ExpandUrlToSingleBlr.
DestinationInfo = collections.namedtuple(
    'DestinationInfo',
    [
        # The expanded destination StorageURL.
        'exp_dst_url',
        # Bool indicating whether the expanded destination names an existing
        # directory, bucket, or bucket subdirectory.
        'have_existing_dst_container',
    ])

# Describes (NameExpansionIterator, DestinationInfo) tuple.
NameExpansionIteratorDestinationTuple = collections.namedtuple(
    'NameExpansionIteratorDestinationTuple', [
        'name_expansion_iter',
        'destination',
    ])


class CopyObjectsIterator(six.Iterator):
  """Iterator wrapper for copying objects and keeping track of source URL types.

  This is used in the cp command for copying from multiple source to multiple
  destinations. It takes a list of NameExpansionIteratorDestinationTuple. It
  wraps them and return CopyObjectInfo objects that wraps NameExpansionResult
  with the destination. It's used also for collecting analytics
  PerformanceSummary info, because there may be multiple source URLs and we want
  to know if any of them are file URLs, if any of them are cloud URLs, if any of
  them require daisy chain operations, and if any use different providers. The
  source URL type information will be aggregated at the end of _SequentialApply
  or _ParallelApply.
  """

  def __init__(self, name_expansion_dest_iter, is_daisy_chain):
    """Instantiates the iterator.

    Args:
      name_expansion_dest_iter: NameExpansionIteratorDestinationTuple iterator.
      is_daisy_chain: The -D option in cp might have already been specified, in
          which case we do not need to check again for daisy chain operations.
    """
    self.is_daisy_chain = is_daisy_chain
    self.has_file_src = False
    self.has_cloud_src = False
    self.provider_types = []

    self.name_expansion_dest_iter = name_expansion_dest_iter
    name_expansion_dest_tuple = next(self.name_expansion_dest_iter)
    self.current_expansion_iter = name_expansion_dest_tuple.name_expansion_iter
    self.current_destination = name_expansion_dest_tuple.destination

  def __iter__(self):
    return self

  def __next__(self):
    """Keeps track of URL types as the command iterates over arguments."""
    try:
      name_expansion_result = next(self.current_expansion_iter)
    except StopIteration:
      name_expansion_dest_tuple = next(self.name_expansion_dest_iter)
      self.current_expansion_iter = (
          name_expansion_dest_tuple.name_expansion_iter)
      self.current_destination = name_expansion_dest_tuple.destination
      return self.__next__()

    elt = CopyObjectInfo(name_expansion_result,
                         self.current_destination.exp_dst_url,
                         self.current_destination.have_existing_dst_container)

    # Check if we've seen a file source.
    if not self.has_file_src and elt.source_storage_url.IsFileUrl():
      self.has_file_src = True
    # Check if we've seen a cloud source.
    if not self.has_cloud_src and elt.source_storage_url.IsCloudUrl():
      self.has_cloud_src = True

    # Check if we've seen a daisy-chain condition.
    if self.current_destination.exp_dst_url.IsCloudUrl():
      dst_url_scheme = self.current_destination.exp_dst_url.scheme
    else:
      dst_url_scheme = None

    if (not self.is_daisy_chain and dst_url_scheme is not None and
        elt.source_storage_url.IsCloudUrl() and
        elt.source_storage_url.scheme != dst_url_scheme):
      self.is_daisy_chain = True
    # Check if we've seen a new provider type.
    if elt.source_storage_url.scheme not in self.provider_types:
      self.provider_types.append(elt.source_storage_url.scheme)

    return elt