AU: Pass opened device fd from update engine to bspatch.

AU: Pass opened device fd from update engine to bspatch.

Whenever a device is written and closes, some processing happens
(involving udev, devicekit-disks, udev, and even
chrome). Historically, bspatch called open, write, close on the raw
device, thus causing much CPU usage every time bspatch was called,
which can be many times per second for delta updates.

This CL causes us to pass /dev/fd/## and the file descriptor to
bspatch. A corresponding CL causes bspatch to detect this and skip
open/close calls. Together, these CLs dramatically reduce resource
usage on clients performing delta updates.

BUG=7636
TEST=unittests; tested delta update on device

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=5a23283

[petkov, 2010-10-12 21:56:35]

LGTM

http://codereview.chromium.org/3723005/diff/1/2
File delta_performer.cc (right):

http://codereview.chromium.org/3723005/diff/1/2#newcode449
delta_performer.cc:449: const string& path = "/dev/fd/" + StringPrintf("%d",
fd);
... = StringPrintf("/dev/fd/%d"), fd);