Chroot: allow multiple simultaneous entry.

Chroot: allow multiple simultaneous entry.

Change enter_chroot.sh to keep a list of open PIDs (enter_chroot.sh
instances) in chroot/var/lock, and use flock on that file to support
atomic operations to it.

When entering chroot, append the PID to the lock file.

When exiting chroot, clear out current PID and any dead
PIDs/duplicates. If the lock file is empty, only then clean up the
mounts.

Note that I have a little bit of code duplication: the number 200,
which is a manually numbered fd in bash, seemed impossible to place
inside a variable. If you know how to do it, please let me know.

[tedbo, 2010-02-09 04:01:54]

This is a cool change. Thanks! One comment below:

http://codereview.chromium.org/584010/diff/1/2
File src/scripts/enter_chroot.sh (right):

http://codereview.chromium.org/584010/diff/1/2#newcode122
src/scripts/enter_chroot.sh:122: if [ -s "$LOCKFILE" ]; then
Why isn't this check done while the lock is held?

[adlr, 2010-02-09 16:43:33]

oh, good catch. no idea how i missed it... ready for another look.

[tedbo, 2010-02-10 08:44:29]

LGTM

Thanks! A number of people have been wanting this.