Re: [PATCH v10 4/8] qapi: add blockdev-replace command

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> Add a command that can replace bs in following BdrvChild structures:
>
>  - qdev blk root child
>  - block-export blk root child
>  - any child of BlockDriverState selected by child-name
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

[...]

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index b82af74256..9cc7c3d140 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -6334,3 +6334,27 @@
>  ##
>  { 'struct': 'DummyBlockCoreForceArrays',
>    'data': { 'unused-block-graph-info': ['BlockGraphInfo'] } }
> +
> +##
> +# @blockdev-replace:
> +#
> +# Replace a block-node associated with device (@parent should be
> +# QOM path, and @child should be "root") or with block-export (@parent
> +# should be export name, and @child should be "root") or any child
> +# of block-node (@parent should be node-name, and @child should be any
> +# if its children names) with @new-child block-node.

of its

> +#
> +# @parent: QOM path or block-export or node-name, which @child should
> +#     be repalced.  If several matching parents exist, the command

replaced

> +#     will fail

End the sentence with a period, please.

> +#
> +# @child: child to replace.  Must be "root" when parent is QOM path or
> +#     block-export

Likewise.

> +#
> +# @new-child: node-name of the block-node, which should become a
> +#    replacement for @child's current block-node

Likewise.

Indent one more, please.

> +#
> +# Since 11.0
> +##

Let's see whether I understand...

@parent determines which of the three cases mentioned in the commit
message it ids:

* If @parent is a QOM path, case 1.

* If @parent is a block export name (@id in BlockExportOptions and
  BlockExportInfo), case 2.

* If @parent is a block node name (@node-name in BlockdevOptions and
  BlockDeviceInfo), case 3.

Correct?

Problem: this is ambiguous.  A @parent "foo" could in fact be any of the
three cases.

3. If a block node named "foo" exists, it's case 3.

2. If a block export named "foo" exists, it's also case 2.

1. If exactly one QOM object named "foo" exists, it's also case 1.  Why?
   "foo" is a syntactically valid partial QOM path.  Partial QOM paths
   are a convenience feature that is virtually unknown (and possibly
   ill-advised): you can omit leading path components as long as there's
   no ambiguity.

Peeking ahead in the series...  PATCH 8 appears to deprecate the
ambiguity between case 2. and 3.

I think we need to do better.

More questions...

In case 1 and 2, @child "should be root".  What happens when it's
something else?

In case 3, @child "should be any if its children names".  I figure the
command fails when it isn't.

> +{ 'command': 'blockdev-replace',
> +  'data': { 'parent': 'str', 'child': 'str', 'new-child': 'str' } }

[...]