text: #
  #    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.
  
  """Utility for fetching a resource (e.g. a template) from a URL."""
  
  from oslo.config import cfg
  import requests
  from requests import exceptions
  
  from six.moves import urllib
  
  from heat.common import exception
  from heat.common.i18n import _
  from heat.common.i18n import _LI
  from heat.openstack.common import log as logging
  
  cfg.CONF.import_opt('max_template_size', 'heat.common.config')
  
  LOG = logging.getLogger(__name__)
  
  
  class URLFetchError(exception.Error, IOError):
      pass
  
  
  def get(url, allowed_schemes=('http', 'https')):
      """Get the data at the specified URL.
  
      The URL must use the http: or https: schemes.
      The file: scheme is also supported if you override
      the allowed_schemes argument.
      Raise an IOError if getting the data fails.
      """
      LOG.info(_LI('Fetching data from %s'), url)
  
      components = urllib.parse.urlparse(url)
  
      if components.scheme not in allowed_schemes:
          raise URLFetchError(_('Invalid URL scheme %s') % components.scheme)
  
      if components.scheme == 'file':
          try:
              return urllib.request.urlopen(url).read()
          except urllib.error.URLError as uex:
              raise URLFetchError(_('Failed to retrieve template: %s') % uex)
  
      try:
          resp = requests.get(url, stream=True)
          resp.raise_for_status()
  
          # We cannot use resp.text here because it would download the
          # entire file, and a large enough file would bring down the
          # engine.  The 'Content-Length' header could be faked, so it's
          # necessary to download the content in chunks to until
          # max_template_size is reached.  The chunk_size we use needs
          # to balance CPU-intensive string concatenation with accuracy
          # (eg. it's possible to fetch 1000 bytes greater than
          # max_template_size with a chunk_size of 1000).
          reader = resp.iter_content(chunk_size=1000)
          result = ""
          for chunk in reader:
              result += chunk
              if len(result) > cfg.CONF.max_template_size:
                  raise URLFetchError("Template exceeds maximum allowed size (%s"
                                      " bytes)" % cfg.CONF.max_template_size)
          return result
  
      except exceptions.RequestException as ex:
          raise URLFetchError(_('Failed to retrieve template: %s') % ex)
  