Correct caching of disk images

The cachecontrol library can't cache large images [1], which we
work around here by using our own serializer to allow msgpage
to load big files.

It's quite likely this caching is the wrong way to go, and
missing some important details, but it is a useful way to
speed up the experimentation.

[1] psf/cachecontrol#200

compute.py

@@ -13,21 +13,39 @@
 
 import cachecontrol
 from cachecontrol.caches import file_cache
+from cachecontrol import serialize
 import etcd3
 import libvirt
+import msgpack
 import psutil
 import requests
 import yaml
 
 from ecomp import clients
 from ecomp import conf
 
+
+# deal with size limitations in CacheControl
+class MySerializer(serialize.Serializer):
+
+    def _loads_v4(self, request, data):
+        try:
+            cached = msgpack.loads(
+                data, encoding="utf-8", max_bin_len=2147483647)
+        except ValueError:
+            return
+
+        return self.prepare_response(request, cached)
+
+
 KEY = '/hosts'
 SLEEP = 1
 COMPUTE_UUID = str(uuid.uuid4())
 CLIENT = None
 CACHED_SESSION = cachecontrol.CacheControl(
-    requests.Session(), cache=file_cache.FileCache('.web_cache'))
+    requests.Session(),
+    cache=file_cache.FileCache('.web_cache'),
+    serializer=MySerializer())
 
 
 # default config
@@ -196,6 +214,7 @@ def _copy_image(source, instance, size):
         os.unlink(source_file)
     except FileNotFoundError:
         pass
+    _print('Fetching image from %s' % source)
     source_refresh = CACHED_SESSION.get(source, stream=True)
     with open(source_file, 'wb') as sf:
         shutil.copyfileobj(source_refresh.raw, sf)