devpts: Always return a distinct instance when mounting
When devpts is mounted and the newinstance flag is not passed the code
first checks to see if the system devpts instance has been exported to
userspace. If it has not the system devpts instance is returned
otherwise a fresh instance of devpts is allocated and returned.
If newinstance is passed a fresh devpts instance is always returned.
Combined with the earlier work to cause mounts of devpts to fail
if devpts is mounted over itself, this ensures that the system devpts
is mounted on /dev/pts on all of the systems I have tested.
This has been verified to work properly on openwrt-15.05, centos5,
centos6, centos7, debian-6.0.2, debian-7.9, debian-8.2, ubuntu-14.04.3,
ubuntu-15.10, fedora23, magia-5, mint-17.3, opensuse-42.1, slackware-14.1,
gentoo-20151225 (13.0?), archlinux-2015-12-01
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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