Use shutil to copy the file content.

When you use 'for line in file', it uses the next method under the hood.
And such method can load very big pieces of the binary file into the RAM
memory.

The shutil method will copy the file using smalls chunks, so the
RAM memory is gonna be safe.

src/zc/buildout/networkcache.py

@@ -17,6 +17,7 @@ import hashlib
 import os
 import posixpath
 import re
+import shutil
 
 try:
     from slapos.libnetworkcache import NetworkcacheClient, UploadError, \
@@ -73,13 +74,14 @@ def download_network_cached(dir_url, cache_url, path, url, logger, md5sum=None):
     try:
         nc = NetworkcacheClient(shacache=cache_url, shadir=dir_url)
         file_descriptor = nc.select(directory_key)
+
+        buffer_size = min(1024, os.path.getsize(file_descriptor.name))
         f = open(path, 'w+b')
         try:
-          for line in file_descriptor:
-            f.write(line)
+            shutil.copyfileobj(file_descriptor, f, buffer_size)
         finally:
-          f.close()
-          file_descriptor.close()
+            f.close()
+            file_descriptor.close()
 
         if not check_md5sum(path, md5sum):
             logger.info('MD5 checksum mismatch downloading %s' % file_name)