Add caching at HTTP level.

This uses Reqests-Cache, which should be installed on the PYTHONPATH or
provided as a wheel in blender_cache/requests_cache-*.whl. In the latter
case, it'll be automatically added to sys.path.

blender_cloud/cache.py

@@ -0,0 +1,66 @@
+"""Cache management."""
+
+import os
+import sys
+import logging
+
+from . import appdirs
+
+log = logging.getLogger(__name__)
+
+
+# Add our shipped Requests-Cache wheel to the Python path
+if not any('requests_cache' in path for path in sys.path):
+    import glob
+
+    # TODO: gracefully handle errors when the wheel cannot be found.
+    my_dir = os.path.dirname(__file__)
+    wheel = glob.glob(os.path.join(my_dir, 'requests_cache*.whl'))[0]
+    sys.path.append(wheel)
+
+import requests_cache
+
+
+def cache_directory() -> str:
+    """Returns an OS-specifc cache location, and ensures it exists.
+
+    Should be replaced with a call to bpy.utils.user_resource('CACHE', ...)
+    once https://developer.blender.org/T47684 is finished.
+    """
+
+    # TODO: just use bpy.utils.user_resource('CACHE', ...)
+
+    cache_dir = os.path.join(appdirs.user_cache_dir(appname='Blender', appauthor=False), 'blender-cloud')
+
+    os.makedirs(cache_dir, exist_ok=True)
+
+    return cache_dir
+
+
+def requests_session() -> requests_cache.CachedSession:
+    """Creates a Requests-Cache session object."""
+
+    cache_dir = cache_directory()
+    cache_name = os.path.join(cache_dir, 'blender_cloud_cache')
+    log.info('Storing cache in %s' % cache_name)
+
+    req_sess = requests_cache.CachedSession(backend='sqlite',
+                                            cache_name=cache_name)
+
+    return req_sess
+
+
+def debug_show_responses():
+
+    req_sess = requests_session()
+
+    log.info('Cache type: %s', type(req_sess.cache))
+    log.info('Cached URLs:')
+    for key in req_sess.cache.keys_map:
+        value = req_sess.cache.keys_map[key]
+        log.info('  %s = %s' % (key, value))
+
+    log.info('Cached responses:')
+    for key in req_sess.cache.responses:
+        response, timekey = req_sess.cache.get_response_and_time(key)
+        log.info('  %s = %s' % (key, response.content))