Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxx>
> + *
> + * 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);
Callbacks should come with a contract.
> + 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@xxxxxxxxxxxxx>
> + *
> + * 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;
> + }
> + 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;
> + }
> +
> + if (!found_child) {
> + error_setg(errp, "{%s, %s} parent-child pair not found", parent_id,
> + child_name);
> + return NULL;
> + }
> +
> + return found_child;
> +}
Especially when the callback can return NULL with and without setting an
error!
> 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',
