Add zgenhostid utility script

Turning the multihost property on requires that a hostid be set to allow
ZFS to determine when a foreign system is attemping to import a pool.
The error message instructing the user to set a hostid refers to
genhostid(1).

Genhostid(1) is not available on SUSE Linux.  This commit adds a script
modeled after genhostid(1) for those users.

Zgenhostid checks for an /etc/hostid file; if it does not exist, it
creates one and stores a value.  If the user has provided a hostid as an
argument, that value is used.  Otherwise, a random hostid is generated
and stored.

This differs from the CENTOS 6/7 versions of genhostid, which overwrite
the /etc/hostid file even though their manpages state otherwise.

A man page for zgenhostid is added. The one for genhostid is in (1), but
I put zgenhostid in (8) because I believe it's more appropriate.

The mmp tests are modified to use zgenhostid to set the hostid instead
of using the spl_hostid module parameter.  zgenhostid will not replace
an existing /etc/hostid file, so new mmp_clear_hostid calls are
required.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #6358
Closes #6379

cmd/Makefile.am

@@ -1,3 +1,3 @@
 SUBDIRS  = zfs zpool zdb zhack zinject zstreamdump ztest zpios
 SUBDIRS += mount_zfs fsck_zfs zvol_id vdev_id arcstat dbufstat zed
-SUBDIRS += arc_summary raidz_test
+SUBDIRS += arc_summary raidz_test zgenhostid

cmd/zgenhostid/Makefile.am

@@ -0,0 +1 @@
+dist_bin_SCRIPTS = zgenhostid

cmd/zgenhostid/zgenhostid

@@ -0,0 +1,61 @@
+#!/bin/bash
+
+# Emulate genhostid(1) available on RHEL/CENTOS, for use on distros
+# which do not provide that utility.
+#
+# Usage:
+#    zgenhostid
+#    zgenhostid <value>
+#
+# If /etc/hostid already exists and is size > 0, the script exits immediately
+# and changes nothing.  Unlike genhostid, this generates an error message.
+#
+# The first form generates a random hostid and stores it in /etc/hostid.
+# The second form checks that the provided value is between 0x1 and 0xFFFFFFFF
+# and if so, stores it in /etc/hostid.  This form is not supported by
+# genhostid(1).
+
+hostid_file=/etc/hostid
+
+function usage {
+	echo "$0 [value]"
+	echo "If $hostid_file is not present, store a hostid in it." >&2
+	echo "The optional value must be an 8-digit hex number between" >&2
+	echo "1 and 2^32-1.  If no value is provided, a random one will" >&2
+	echo "be generated.  The value must be unique among your systems." >&2
+}
+
+# hostid(1) ignores contents of /etc/hostid if size < 4 bytes.  It would
+# be better if this checked size >= 4 bytes but it the method must be
+# widely portable.
+if [ -s $hostid_file ]; then
+	echo "$hostid_file already exists.  No change made." >&2
+	exit 1
+fi
+
+if [ -n "$1" ]; then
+	host_id=$1
+else
+	# $RANDOM goes from 0..32k-1
+	number=$((((RANDOM % 4) * 32768 + RANDOM) * 32768 + RANDOM))
+	host_id=$(printf "%08x" $number)
+fi
+
+if egrep -o '^0{8}$' <<< $host_id >/dev/null 2>&1; then
+	usage
+	exit 2
+fi
+
+if ! egrep -o '^[a-fA-F0-9]{8}$' <<< $host_id >/dev/null 2>&1; then
+	usage
+	exit 3
+fi
+
+a=${host_id:6:2}
+b=${host_id:4:2}
+c=${host_id:2:2}
+d=${host_id:0:2}
+
+echo -ne \\x$a\\x$b\\x$c\\x$d > $hostid_file
+
+exit 0

cmd/zpool/zpool_main.c

@@ -2177,7 +2177,7 @@ show_import(nvlist_t *config)
 			break;
 		case ZPOOL_STATUS_HOSTID_REQUIRED:
 			(void) printf(gettext(" action: Set a unique system "
-			    "hostid with the genhostid(1) command.\n"));
+			    "hostid with the zgenhostid(8) command.\n"));
 			break;
 		default:
 			(void) printf(gettext(" action: The pool cannot be "
@@ -2304,7 +2304,7 @@ do_import(nvlist_t *config, const char *newname, const char *mntopts,
 			(void) fprintf(stderr, gettext("Cannot import '%s': "
 			    "pool has the multihost property on and the\n"
 			    "system's hostid is not set. Set a unique hostid "
-			    "with the genhostid(1) command.\n"), name);
+			    "with the zgenhostid(8) command.\n"), name);
 		} else {
 			char *hostname = "<unknown>";
 			uint64_t timestamp = 0;