Fix caching in python3 (apache#806)

* caravel: fix visualization cache for python3

python3 wants bytes and not strings:

2016-07-22 10:36:09,474:INFO:root:Caching for the next 28800 seconds
2016-07-22 10:36:09,475:WARNING:root:Could not cache key 1eeb45f32960f0df0ad99a125bdaf199
2016-07-22 10:36:09,475:ERROR:root:'str' does not support the buffer interface
Traceback (most recent call last):
  File "/home/rm/caraveltest/venv/lib/python3.4/site-packages/caravel/viz.py", line 306, in get_json
    zlib.compress(self.json_dumps(payload)),
TypeError: 'str' does not support the buffer interface

Tested with memcached and pylibmc client library.

* docs: add note about using a proper memcached client library

caravel/viz.py

@@ -23,7 +23,7 @@
 from flask_babel import lazy_gettext as _
 from markdown import markdown
 import simplejson as json
-from six import string_types
+from six import string_types, PY3
 from werkzeug.datastructures import ImmutableMultiDict, MultiDict
 from werkzeug.urls import Href
 from dateutil import relativedelta as rdelta
@@ -279,7 +279,10 @@ def get_json(self):
         if payload:
             is_cached = True
             try:
-                payload = json.loads(zlib.decompress(payload))
+                cached_data = zlib.decompress(payload)
+                if PY3:
+                    cached_data = cached_data.decode('utf-8')
+                payload = json.loads(cached_data)
             except Exception as e:
                 logging.error("Error reading cache")
                 payload = None
@@ -302,9 +305,12 @@ def get_json(self):
             logging.info("Caching for the next {} seconds".format(
                 cache_timeout))
             try:
+                data = self.json_dumps(payload)
+                if PY3:
+                    data = bytes(data, 'utf-8')
                 cache.set(
                     cache_key,
-                    zlib.compress(self.json_dumps(payload)),
+                    zlib.compress(data),
                     timeout=cache_timeout)
             except Exception as e:
                 # cache.set call can fail if the backend is down or if

docs/installation.rst

@@ -177,8 +177,10 @@ caching purpose. Configuring your caching backend is as easy as providing
 a ``CACHE_CONFIG``, constant in your ``caravel_config.py`` that
 complies with the Flask-Cache specifications.
 
-Flask-Cache supports multiple caching backends (Redis, Memcache,
-SimpleCache (in-memory), or the local filesystem).
+Flask-Cache supports multiple caching backends (Redis, Memcached,
+SimpleCache (in-memory), or the local filesystem). If you are going to use
+Memcached please use the pylibmc client library as python-memcached does
+not handle storing binary data correctly.
 
 For setting your timeouts, this is done in the Caravel metadata and goes
 up the "timeout searchpath", from your slice configuration, to your