Re: [PATCH 2/8] block: add BlockParentClass class

[Markus Armbruster <armbru@redhat.com>]

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

> Add a class that will unify block parents for blockdev-replace
> functionality we are going to add.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  include/block/block-parent.h | 32 +++++++++++++++++
>  block/block-parent.c         | 66 ++++++++++++++++++++++++++++++++++++
>  block/meson.build            |  1 +
>  3 files changed, 99 insertions(+)
>  create mode 100644 include/block/block-parent.h
>  create mode 100644 block/block-parent.c
>
> diff --git a/include/block/block-parent.h b/include/block/block-parent.h
> new file mode 100644
> index 0000000000..bd9c9d91e6
> --- /dev/null
> +++ b/include/block/block-parent.h
> @@ -0,0 +1,32 @@
> +/*
> + * Block Parent class
> + *
> + * Copyright (c) 2021 Virtuozzo International GmbH.
> + *
> + * Authors:
> + *  Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#ifndef BLOCK_PARENT_H
> +#define BLOCK_PARENT_H
> +
> +#include "block/block.h"
> +
> +typedef struct BlockParentClass {
> +    const char *name;
> +
> +    int (*find_child)(const char *parent_id, const char *child_name,
> +                      BlockDriverState *child_bs, BdrvChild **child,
> +                      Error **errp);
> +    QTAILQ_ENTRY(BlockParentClass) next;
> +} BlockParentClass;
> +
> +void block_parent_class_register(BlockParentClass *cls);
> +
> +BdrvChild *block_find_child(const char *parent_id, const char *child_name,
> +                            BlockDriverState *child_bs, Error **errp);
> +
> +#endif /* BLOCK_PARENT_H */
> diff --git a/block/block-parent.c b/block/block-parent.c
> new file mode 100644
> index 0000000000..73b6026c42
> --- /dev/null
> +++ b/block/block-parent.c
> @@ -0,0 +1,66 @@
> +/*
> + * Block Parent class
> + *
> + * Copyright (c) 2021 Virtuozzo International GmbH.
> + *
> + * Authors:
> + *  Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "block/block-parent.h"
> +#include "qapi/error.h"
> +
> +static QTAILQ_HEAD(, BlockParentClass) block_parent_classes =
> +    QTAILQ_HEAD_INITIALIZER(block_parent_classes);
> +
> +void block_parent_class_register(BlockParentClass *cls)
> +{
> +    QTAILQ_INSERT_HEAD(&block_parent_classes, cls, next);
> +}
> +
> +BdrvChild *block_find_child(const char *parent_id, const char *child_name,
> +                            BlockDriverState *child_bs, Error **errp)
> +{
> +    BdrvChild *found_child = NULL;
> +    BlockParentClass *found_cls = NULL, *cls;
> +
> +    QTAILQ_FOREACH(cls, &block_parent_classes, next) {
> +        int ret;
> +        BdrvChild *c;
> +
> +        /*
> +         * Note that .find_child must fail if parent is found but doesn't have
> +         * corresponding child.
> +         */
> +        ret = cls->find_child(parent_id, child_name, child_bs, &c, errp);
> +        if (ret < 0) {
> +            return NULL;

Interesting: when one method fails, the entire function fails, even when
other methods succeed.  The contract should probably spell this out.

> +        }
> +        if (ret == 0) {
> +            continue;
> +        }
> +
> +        if (!found_child) {
> +            found_cls = cls;
> +            found_child = c;
> +            continue;
> +        }
> +
> +        error_setg(errp, "{%s, %s} parent-child pair is ambiguous: it match "
> +                   "both %s and %s", parent_id, child_name, found_cls->name,
> +                   cls->name);
> +        return NULL;
> +    }

Style recommendation: if very much prefer

           if bad
               error out
           normal

over

           if ok
               normal
           bad

In this case:

           if (found_child) {
               error_setg(...);
               return 0;
           }

           found_cls = cls;
           found_child = c;
       }

> +
> +    if (!found_child) {
> +        error_setg(errp, "{%s, %s} parent-child pair not found", parent_id,
> +                   child_name);
> +        return NULL;
> +    }
> +
> +    return found_child;
> +}
> diff --git a/block/meson.build b/block/meson.build
> index 0450914c7a..5200e0ffce 100644
> --- a/block/meson.build
> +++ b/block/meson.build
> @@ -10,6 +10,7 @@ block_ss.add(files(
>    'blkverify.c',
>    'block-backend.c',
>    'block-copy.c',
> +  'block-parent.c',
>    'commit.c',
>    'copy-on-read.c',
>    'preallocate.c',