Mark the LevelDB storage backend as deprecated

docs/topics/downloader-middleware.rst

@@ -348,7 +348,6 @@ HttpCacheMiddleware
 
         * :ref:`httpcache-storage-fs`
         * :ref:`httpcache-storage-dbm`
-        * :ref:`httpcache-storage-leveldb`
 
     You can change the HTTP cache storage backend with the :setting:`HTTPCACHE_STORAGE`
     setting. Or you can also :ref:`implement your own storage backend. <httpcache-storage-custom>`
@@ -478,27 +477,6 @@ DBM storage backend
     By default, it uses the anydbm_ module, but you can change it with the
     :setting:`HTTPCACHE_DBM_MODULE` setting.
 
-.. _httpcache-storage-leveldb:
-
-LevelDB storage backend
-~~~~~~~~~~~~~~~~~~~~~~~
-
-.. class:: LeveldbCacheStorage
-
-    .. versionadded:: 0.23
-
-    A LevelDB_ storage backend is also available for the HTTP cache middleware.
-
-    This backend is not recommended for development because only one process
-    can access LevelDB databases at the same time, so you can't run a crawl and
-    open the scrapy shell in parallel for the same spider.
-
-    In order to use this storage backend, install the `LevelDB python
-    bindings`_ (e.g. ``pip install leveldb``).
-
-    .. _LevelDB: https://github.com/google/leveldb
-    .. _leveldb python bindings: https://pypi.python.org/pypi/leveldb
-
 .. _httpcache-storage-custom:
 
 Writing your own storage backend

scrapy/extensions/httpcache.py

@@ -1,19 +1,24 @@
 from __future__ import print_function
-import os
+
 import gzip
 import logging
-from six.moves import cPickle as pickle
+import os
+from email.utils import mktime_tz, parsedate_tz
 from importlib import import_module
 from time import time
+from warnings import warn
 from weakref import WeakKeyDictionary
-from email.utils import mktime_tz, parsedate_tz
+
+from six.moves import cPickle as pickle
 from w3lib.http import headers_raw_to_dict, headers_dict_to_raw
+
+from scrapy.exceptions import ScrapyDeprecationWarning
 from scrapy.http import Headers, Response
 from scrapy.responsetypes import responsetypes
-from scrapy.utils.request import request_fingerprint
-from scrapy.utils.project import data_path
 from scrapy.utils.httpobj import urlparse_cached
+from scrapy.utils.project import data_path
 from scrapy.utils.python import to_bytes, to_unicode, garbage_collect
+from scrapy.utils.request import request_fingerprint
 
 
 logger = logging.getLogger(__name__)
@@ -345,6 +350,8 @@ class FilesystemCacheStorage(object):
 class LeveldbCacheStorage(object):
 
     def __init__(self, settings):
+        warn("The LevelDB storage backend is deprecated.",
+             ScrapyDeprecationWarning, stacklevel=2)
         import leveldb
         self._leveldb = leveldb
         self.cachedir = data_path(settings['HTTPCACHE_DIR'], createdir=True)