Re: [PATCH 22/22] iotests: Mirror must not attempt to create loops

[Vladimir Sementsov-Ogievskiy]

20.09.2019 18:28, Max Reitz wrote:
> Signed-off-by: Max Reitz <address@hidden>
> ---
>   tests/qemu-iotests/041     | 152 +++++++++++++++++++++++++++++++++++++
>   tests/qemu-iotests/041.out |   4 +-
>   2 files changed, 154 insertions(+), 2 deletions(-)
> 
> diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
> index e4cc829fe2..6ea4764ae8 100755
> --- a/tests/qemu-iotests/041
> +++ b/tests/qemu-iotests/041
> @@ -1265,6 +1265,158 @@ class TestReplaces(iotests.QMPTestCase):
>   
>           self.vm.assert_block_path('filter0/file', 'target')
>   
> +    '''
> +    See what happens when the @sync/@replaces configuration dictates
> +    creating a loop.
> +    '''
> +    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[qcow2] --file--> ...

or qed, actually

> +
> +        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[qcow2] --file--> ...
> +          target[qcow2] --file--> ...
> +
> +        Then, because of sync=none and drive-mirror in absolute-paths mode,
> +        the source is attached to the target:
> +          source[throttle] --file--> filtered[qcow2] --file--> ...
> +                 ^
                     |
> +              backing
> +                 |
> +            target[qcow2] --file--> ...
> +
> +        Replacing filtered by target would yield:
> +          source[throttle] --file--> target[qcow2] --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.
> +    '''
> +    def test_loop_during_mirror(self):
> +        qemu_img('create', '-f', iotests.imgfmt, test_img, str(1 * 1024 * 
> 1024))
> +
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'null-co',
> +                                 'node-name': 'common-base',
> +                                 'read-zeroes': True,

why do you need read-zeroes?

> +                                 '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', {})

Hmm, why don't you create them both in one query?

> +
> +        '''

the following is hard to read without some hint like, "We are going to ..."

> +        x-blockdev-change can only add children to a quorum node that
> +        have no parent yet, so we need an intermediate node between
> +        target and common-base that has no parents other than target.
> +        We cannot use any parent that would forward the RESIZE
> +        permission (because the job takes it, but unshares it on the
> +        source), so we make it a backing child of a qcow2 node.
> +        Unfortunately, we cannot add backing files to Quorum nodes
> +        (because of an op blocker), so we put another raw node between
> +        the qcow2 node and common-base.
> +        '''
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'qcow2',
> +                                 'node-name': 'base-parent',
> +                                 'file': {
> +                                     'driver': 'file',
> +                                     'filename': test_img
> +                                 },
> +                                 'backing': {
> +                                     'driver': 'raw',
> +                                     'file': 'common-base'
> +                                 }
> +                             })
> +
> +        # Add a quorum node with a single child, we will add
> +        # base-parent to prepare a loop later
> +        result = self.vm.qmp('blockdev-add', **{
> +                                 'driver': 'quorum',

It would be good to skip test-cases if quorum unsupported, like other test-cases
with quorum.

> +                                 'node-name': 'target',
> +                                 'vote-threshold': 1,
> +                                 'children': [
> +                                     {
> +                                         'driver': 'null-co',
> +                                         'read-zeroes': True
> +                                     }
> +                                 ]
> +                             })
> +        self.assert_qmp(result, 'return', {})

It would be nice to comment out current block graph here...

> +
> +        result = self.vm.qmp('blockdev-mirror', job_id='mirror',
> +                             device='source', target='target', sync='full',
> +                             replaces='common-base')
> +        self.assert_qmp(result, 'return', {})
> +
> +        result = self.vm.qmp('x-blockdev-change',
> +                             parent='target', node='base-parent')
> +        self.assert_qmp(result, 'return', {})
> +
> +        '''

and here, like you do in previous test-case. And here it even nicer, as this 
test-case
is more complex.

> +        This asks for this configuration post-mirror:
> +
> +        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')
> +
>   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
> 


-- 
Best regards,
Vladimir