Add zimport.sh compatibility test script

Verify that an assortment of known good reference pools can be imported
using different versions of the ZoL code.

By default references pools for the major ZFS implementation will be
checked against the most recent ZoL tags and the master development branch.
Alternate tags or branches may be verified with the '-s <src-tag> option.
Passing the keyword "installed" will instruct the script to test whatever
version is installed.

Preferentially a reference pool is used for all tests.  However, if one
does not exist and the pool-tag matches one of the src-tags then a new
reference pool will be created using binaries from that source build.
This is particularly useful when you need to test your changes before
opening a pull request.

New reference pools may be added by placing a bzip2 compressed tarball
of the pool in the scripts/zpool-example directory and then passing
the -p <pool-tag> option.  To increase the test coverage reference pools
should be collected for all the major ZFS implementations.  Having these
pools easily available is also helpful to the developers.

Care should be taken to run these tests with a kernel supported by all
the listed tags.  Otherwise build failure will cause false positives.

EXAMPLES:

The following example will verify the zfs-0.6.2 tag, the master branch,
and the installed zfs version can correctly import the listed pools.
Note there is no reference pool available for master and installed but
because binaries are available one is automatically constructed.  The
working directory is also preserved between runs (-k) preventing the
need to rebuild from source for multiple runs.

 zimport.sh -k -f /var/tmp/zimport \
     -s "zfs-0.6.2 master installed" \
     -p "zevo-1.1.1 zol-0.6.2 zol-0.6.2-173 master installed"

--------------------- ZFS on Linux Source Versions --------------
                zfs-0.6.2       master          0.6.2-175_g36eb554
-----------------------------------------------------------------
Clone SPL       Local         Local           Skip
Clone ZFS       Local         Local           Skip
Build SPL       Pass          Pass            Skip
Build ZFS       Pass          Pass            Skip
-----------------------------------------------------------------
zevo-1.1.1      Pass          Pass            Pass
zol-0.6.2       Pass          Pass            Pass
zol-0.6.2-173   Fail          Pass            Pass
master          Pass          Pass            Pass
installed       Pass          Pass            Pass

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2094
Revert changes to zbookmark_t

Commit 1421c89142376bfd41e4de22ed7c7846b9e41f95 added a field to
zbookmark_t that unintentinoally caused a disk format change. This
negatively affected backward compatibility and platform portability.
Therefore, this field is being removed.

The function that field permitted is left unimplemented until a later
patch that will reimplement the field in a way that does not affect the
disk format.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2094
Workaround for pools affected by #2094 erratum

ZoL commit 1421c89 unintentionally changed the disk format in a forward-
compatible, but not backward compatible way. This was accomplished by
adding an entry to zbookmark_t, which is included in a couple of
on-disk structures. That lead to the creation of pools with incorrect
dsl_scan_phys_t objects that could only be imported by versions of ZoL
containing that commit.  Such pools cannot be imported by other versions
of ZFS or past versions of ZoL.

The additional field has been removed by the previous commit.  However,
affected pools must be imported and scrubbed using a version of ZoL with
this commit applied.  This will return the pools to a state in which they
may be imported by other implementations.

The 'zpool import' or 'zpool status' command can be used to determine if
a pool is impacted.  A message similar to one of the following means your
pool must be scrubbed to restore compatibility.

$ zpool import
   pool: zol-0.6.2-173
     id: 1165955789558693437
  state: ONLINE
 status: Errata #1 detected.
 action: The pool can be imported using its name or numeric identifier,
	however there is a compatibility issue which should be corrected
	by running 'zpool scrub'
   see: http://zfsonlinux.org/msg/ZFS-8000-ER
 config:
 ...

$ zpool status
  pool: zol-0.6.2-173
 state: ONLINE
  scan: pool compatibility issue detected.
   see: https://github.com/zfsonlinux/zfs/issues/2094
action: To correct the issue run 'zpool scrub'.
config:
...

If there was an async destroy in progress 'zpool import' will prevent
the pool from being imported.  Further advice on how to proceed will be
provided by the error message as follows.

$ zpool import
   pool: zol-0.6.2-173
     id: 1165955789558693437
  state: ONLINE
 status: Errata #1 detected.
 action: The pool can not be imported with this version of ZFS due to an
	active asynchronous destroy.  Revert to an earlier version and
	allow the destroy to complete before updating.
   see: http://zfsonlinux.org/msg/ZFS-8000-ER
 config:
 ...

Pools affected by the damaged dsl_scan_phys_t can be detected prior to
an upgrade by running the following command as root:

zdb -dddd poolname 1 | grep -P '^\t\tscan = ' | sed -e 's;scan = ;;' | wc -w

Note that `poolname` must be replaced with the name of the pool you wish
to check. A value of 25 indicates the dsl_scan_phys_t has been damaged.
A value of 24 indicates that the dsl_scan_phys_t is normal. A value of 0
indicates that there has never been a scrub run on the pool.

The regression caused by the change to zbookmark_t never made it into a
tagged release or any Gentoo backports. Only those using HEAD were
affected. However, this patch has a few limitations. There is no way to
detect a damaged dsl_scan_phys_t object when it has occurred on 32-bit
systems due to integer rounding that wrote incorrect information, but
avoided the overflow on them. Correcting such issues requires triggering
a scrub.

In addition, bptree_entry_phys_t objects are also affected. These
objects only exist during an asynchronous destroy and automating repair
of damaged bptree_entry_phys_t objects is non-trivial. Any pools that
have been imported by an affected version of ZoL must have all
asynchronous destroy operations finish before export and subsequent
import by a version containing this commit.  Failure to do that will
prevent pool import. The presence of any background destroys on any
imported pools can be checked by running `zpool get freeing` as root.
This will display a non-zero value for any pool with an active
asynchronous destroy.

Lastly, it is expected that no user data has been lost as a result of
this erratum.

Original-patch-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2094