#!/bin/sh

# The hostname and hostid of the last system to access a ZFS pool are stored in
# the ZFS pool itself.  A pool is foreign if, during `zfs import`, the current
# hostname and hostid are different than the stored values thereof.
#
# The hostname and hostid on Solaris are intrinsic, but they are not on Linux,
# so the spl kernel module invokes /bin/hostname and /usr/bin/hostid from the
# userland in its initialization routine.
#
# However, these two indentifiers are usually undefined in the Linux initramfs
# environment, so the /etc/hostname and /etc/hostid files must be added to the
# initrd.  Things like a DHCP lease change can affect the hostid too.
#
# ZFS requires stable values for hostname and hostid, but basic Linux systems
# do not.  The hostid is therefore stabilized by creating the /etc/hostid file
# in the regular environment if it does not already exist.  An undefined
# hostname is usuallly stable.
#
# Neither /etc/hostname nor /etc/hostid are controlled configuration files in
# Debian distributions, but the spl package nevertheless installs a dummy
# /etc/hostid file that contains the HW_INVALID_HOSTID sentinal value so that
# the package manager will track it.

# This result is always an eight-character hexadecimal number sans the 0x
# prefix.  Remember that /usr/bin/hostid generates a value if the /etc/hostid
# file doesn't exist or is malformed.
HOSTID=$(hostid)

if [ -f /etc/hostid -a "0x$HOSTID" != "0xffffffff" ]
then
	# This system already has a stable hostid.
	exit 0
fi

# Truncate the dummy file and generate the actual system hostid.
: >/etc/hostid
HOSTID=$(hostid)

# @TODO: Check whether this method is appropriate for gethostid(2) on big
# endian systems.  The /etc/hostname file on i386 and amd64 systems must be
# a little endian integer of exacly four bytes.  Regardless, a consistent
# hostid is more important than a correct byte order here.

# Conveniences like a ${HOSTID:$ii:2} substring range or a `sed` one-liner
# are prohibited here because this file must be dash-compatible by policy.
AA=$(echo $HOSTID | cut -b 1,2)
BB=$(echo $HOSTID | cut -b 3,4)
CC=$(echo $HOSTID | cut -b 5,6)
DD=$(echo $HOSTID | cut -b 7,8)

# Invoke the external printf because the dash builtin lacks the byte format.
/usr/bin/printf "\x$DD\x$CC\x$BB\x$AA" >"/etc/hostid"

# @ASSERT: [ "$HOSTID" = "$(hostid)" ]