download: fix default permission of installed files

zc.buildout.download.Download may return temporary files
(only readable/writable by the current user) and by default
we'd like installed files to respect umask.

In the case of SlapOS, we usually want installed files to be readable
by members of 'slapsoft' group.

slapos/recipe/download.py

@@ -24,6 +24,7 @@
 # Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 #
 ##############################################################################
+import errno
 import os
 import shutil
 import zc.buildout
@@ -61,16 +62,18 @@ class Recipe(object):
       os.mkdir(parts)
       result.append(parts)
     path, is_temp = self._downloader(self._url, md5sum=self._md5sum)
-    try:
-      if os.path.exists(destination):
-        os.unlink(destination)
-      shutil.copy(path, destination)
-    finally:
+    with open(path, 'rb') as fsrc:
       if is_temp:
-        os.unlink(path)
-    mode = self._mode
-    if mode is not None:
-      os.chmod(destination, mode)
+        os.remove(path)
+      try:
+        os.remove(destination)
+      except OSError as e:
+        if e.errno != errno.ENOENT:
+          raise
+      with open(destination, 'wb') as fdst:
+        if self._mode is not None:
+          os.fchmod(fdst.fileno(), self._mode)
+        shutil.copyfileobj(fsrc, fdst)
     return result
 
   update = install