[PATCH 2/8] block: add BlockParentClass class

[Vladimir Sementsov-Ogievskiy]

16.09.2021 11:34, Markus Armbruster wrote:
> Vladimir Sementsov-Ogievskiy <vsementsov at 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 at 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 at 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);
> 
> Callbacks should come with a contract.

will add

> 
>> +    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 at 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;
>> +        }
>> +        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!

Hmm. Callback returns int.

And this block_find_child() function return NULL only together with errp set.

> 
>> 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',
> 


-- 
Best regards,
Vladimir