The download method must return a file descriptor.

It should return the file descriptor instead of file content as string.

This implementation does not consume RAM Memory, so it is safer to
download big files.

When you call file_descriptor.close, it will remove the file from tmp
folder automatically.

slapos/libnetworkcache.py

@@ -18,6 +18,7 @@ import hashlib
 import httplib
 import json
 import os
+import tempfile
 from urllib import urlretrieve
 from urlparse import urlparse
 
@@ -129,11 +130,12 @@ class NetworkcacheClient(object):
   def download(self, sha512sum):
     ''' Download the file.
     It uses http GET request method.
-    Returns downloaded file absolute location
     '''
     sha_cache_url = os.path.join(self.shacache_url, sha512sum)
-    path, headers = urlretrieve(sha_cache_url)
-    return path
+    file_descriptor = tempfile.NamedTemporaryFile()
+    path, headers = urlretrieve(sha_cache_url, file_descriptor.name)
+    file_descriptor.seek(0)
+    return file_descriptor
 
   def select(self, directory_key=None):
     ''' Download a file from shacache by selecting the entry in shadir