Re: [PATCH for-5.0 v2 23/23] iotests: Mirror must not attempt to create loops

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

11.11.2019 19:02, Max Reitz wrote:
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   tests/qemu-iotests/041     | 235 +++++++++++++++++++++++++++++++++++++
>   tests/qemu-iotests/041.out |   4 +-
>   2 files changed, 237 insertions(+), 2 deletions(-)
> 
> diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
> index 9a00cf6f7b..0e43bb699d 100755
> --- a/tests/qemu-iotests/041
> +++ b/tests/qemu-iotests/041
> @@ -1246,6 +1246,241 @@ class TestReplaces(iotests.QMPTestCase):
>   
>           self.vm.assert_block_path('filter0', '/file', 'target')
>   
> +    """
> +    See what happens when the @sync/@replaces configuration dictates
> +    creating a loop.
> +    """
> +    @iotests.skip_if_unsupported(['throttle'])
> +    def test_loop(self):
> +        qemu_img('create', '-f', iotests.imgfmt, test_img, str(1 * 1024 * 1024))
> +
> +        # Dummy group so we can create a NOP filter
> +        result = self.vm.qmp('object-add', qom_type='throttle-group', id='tg0')
> +        self.assert_qmp(result, 'return', {})
> +
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'throttle',
> +                                 'node-name': 'source',
> +                                 'throttle-group': 'tg0',
> +                                 'file': {
> +                                     'driver': iotests.imgfmt,
> +                                     'node-name': 'filtered',
> +                                     'file': {
> +                                         'driver': 'file',
> +                                         'filename': test_img
> +                                     }
> +                                 }
> +                             })
> +        self.assert_qmp(result, 'return', {})
> +
> +        # Block graph is now:
> +        #   source[throttle] --file--> filtered[imgfmt] --file--> ...
> +
> +        result = self.vm.qmp('drive-mirror', job_id='mirror', device='source',
> +                             target=target_img, format=iotests.imgfmt,
> +                             node_name='target', sync='none',
> +                             replaces='filtered')
> +
> +        """
> +        Block graph before mirror exits would be (ignoring mirror_top):
> +          source[throttle] --file--> filtered[imgfmt] --file--> ...
> +          target[imgfmt] --file--> ...
> +
> +        Then, because of sync=none and drive-mirror in absolute-paths mode,
> +        the source is attached to the target:
> +          source[throttle] --file--> filtered[imgfmt] --file--> ...
> +                 ^
> +              backing
> +                 |
> +            target[imgfmt] --file--> ...
> +
> +        Replacing filtered by target would yield:
> +          source[throttle] --file--> target[imgfmt] --file--> ...
> +                 ^                        |
> +                 +------- backing --------+
> +
> +        I.e., a loop.  bdrv_replace_node() detects this and simply
> +        does not let source's file link point to target.  However,
> +        that means that target cannot really replace source.
> +
> +        drive-mirror should detect this and not allow this case.
> +        """
> +
> +        self.assert_qmp(result, 'error/desc',
> +                        "Replacing 'filtered' by 'target' with this sync " + \
> +                        "mode would result in a loop, because the former " + \
> +                        "would be a child of the latter's backing file " + \
> +                        "('source') after the mirror job")
> +
> +    """
> +    Test what happens when there would be no loop with the pre-mirror
> +    configuration, but something changes during the mirror job that asks
> +    for a loop to be created during completion.
> +    """
> +    @iotests.skip_if_unsupported(['copy-on-read', 'quorum'])
> +    def test_loop_during_mirror(self):
> +        qemu_img('create', '-f', iotests.imgfmt, test_img, str(1 * 1024 * 1024))
> +
> +        """
> +        In this test, we are going to mirror from a node that is a
> +        filter above some file "common-base".  The target is a quorum
> +        node (with just an unrelated null-co child).
> +
> +        We will ask the mirror job to replace common-base by the
> +        target upon completion.  That is a completely valid
> +        configuration so far.
> +
> +        However, while the job is running, we add common-base as an
> +        (indirect[1]) child to the target quorum node.  This way,
> +        completing the job as requested would yield a loop, because
> +        the target would be supposed to replace common-base -- which
> +        is its own (indirect) child.
> +
> +        [1] It needs to be an indirect child, because if it were a
> +        direct child, the mirror job would simply end by effectively
> +        injecting the target above common-base.  This is the same
> +        effect as when using sync=none: The target ends up above the
> +        source.
> +
> +        So only loops that have a length of more than one node are
> +        forbidden, which means common-base must be an indirect child
> +        of the target.
> +
> +        (Furthermore, we are going to use x-blockdev-change to add
> +        common-base as a child to the target.  This command only
> +        allows doing so for nodes that have no parent yet.
> +        common-base will have a parent already, though, namely the
> +        source node.  Therefore, this is another reason why we need at
> +        least one node above common-base, so this parent can become
> +        target's child during the mirror.)
> +        """
> +
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'null-co',
> +                                 'node-name': 'common-base',
> +                                 'read-zeroes': True,
> +                                 'size': 1 * 1024 * 1024
> +                             })
> +        self.assert_qmp(result, 'return', {})
> +
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'copy-on-read',
> +                                 'node-name': 'source',
> +                                 'file': 'common-base'
> +                             })
> +        self.assert_qmp(result, 'return', {})
> +
> +        """
> +        As explained above, we have to create a parent above
> +        common-base.
> +
> +        We cannot use any parent that would forward the RESIZE
> +        permission, because the job takes it on the target, but
> +        unshares it on the source: After the x-blockdev-change
> +        operation during the mirror job, this parent will be a child
> +        of the target, so common-base will be an (indirect) child of
> +        both the mirror's source and target.  Thus, the job would
> +        conflict with itself.
> +
> +        Therefore, we make common-base a backing child of a $imgfmt
> +        node.  Unfortunately, we cannot let the mirror job replace a
> +        node that acts as a backing child somewhere (because of an op
> +        blocker), so we put another raw node between the $imgfmt node
> +        and common-base.
> +        """
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': iotests.imgfmt,
> +                                 'node-name': 'base-parent',
> +                                 'file': {
> +                                     'driver': 'file',
> +                                     'filename': test_img
> +                                 },
> +                                 'backing': {
> +                                     'driver': 'raw',
> +                                     'file': 'common-base'
> +                                 }
> +                             })

self.assert_qmp(result, 'return', {})

> +
> +        """
> +        Add a quorum node with a single child, we will add base-parent
> +        to prepare a loop later.
> +        (We do not care about this single child at all, but it is
> +        impossible to create a quorum node without any children.  We
> +        will ignore this child from now on.)
> +        """
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'quorum',
> +                                 'node-name': 'target',
> +                                 'vote-threshold': 1,
> +                                 'children': [
> +                                     {
> +                                         'driver': 'null-co',
> +                                         'read-zeroes': True,
> +                                         'size': 1 * 1024 * 1024
> +                                     }
> +                                 ]
> +                             })
> +        self.assert_qmp(result, 'return', {})
> +
> +        """
> +        Current block graph:
> +
> +        base-parent[$imgfmt] --backing--> [raw]
> +                                            |
> +                                           file
> +                                            v
> +              source[COR] --file--> common-base[null-co]
> +
> +        target[quorum]
> +
> +
> +        The following blockdev-mirror asks for this graph post-mirror:
> +
> +        base-parent[$imgfmt] --backing--> [raw]
> +                                            |
> +                                           file
> +                                            v
> +                source[COR] --file--> target[quorum]
> +
> +        That would be a valid configuration without any loops.
> +        """
> +
> +        result = self.vm.qmp('blockdev-mirror', job_id='mirror',
> +                             device='source', target='target', sync='full',
> +                             replaces='common-base')
> +        self.assert_qmp(result, 'return', {})
> +
> +        """
> +        However, now we will make base-parent a child of target.
> +        Before the mirror job completes, that is still completely
> +        valid:
> +
> +                                             source
> +                                               |
> +                                               v
> +        target -> base-parent -> [raw] -> common-base
> +        """
> +
> +        result = self.vm.qmp('x-blockdev-change',
> +                             parent='target', node='base-parent')
> +        self.assert_qmp(result, 'return', {})
> +
> +        """
> +        However, post-mirror, we thus ask for a loop:
> +
> +        source -> target (replaced common-base) -> base-parent
> +                                  ^                    |
> +                                  |                    v
> +                                  +----------------- [raw]
> +
> +        bdrv_replace_node() would not allow such a configuration, but
> +        we should not pretend we can create it, so the mirror job
> +        should fail during completion.
> +        """
> +
> +        self.complete_and_wait('mirror',
> +                               completion_error='Operation not permitted')

Thanks for exhaustive comments!

> +
>   if __name__ == '__main__':
>       iotests.main(supported_fmts=['qcow2', 'qed'],
>                    supported_protocols=['file'])
> diff --git a/tests/qemu-iotests/041.out b/tests/qemu-iotests/041.out
> index 877b76fd31..20a8158b99 100644
> --- a/tests/qemu-iotests/041.out
> +++ b/tests/qemu-iotests/041.out
> @@ -1,5 +1,5 @@
> -..............................................................................................
> +................................................................................................
>   ----------------------------------------------------------------------
> -Ran 94 tests
> +Ran 96 tests
>   
>   OK
> 


With forgotten assertion added:

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

-- 
Best regards,
Vladimir