Re: [Qemu-block] [RFC PATCH 1/2] util - add automated ID generation utility

[Eric Blake]

On 09/01/2015 11:23 AM, Jeff Cody wrote:
> Multiple sub-systems in QEMU may find it useful to generated IDs
> for objects that a user may reference via QMP or HMP.  This patch
> presents a standardized way to do it, so that automatic ID generation
> follows the same rules.
> 
> This patch enforces the following rules when generating an ID:
> 
> 1.) Guarantee no collisions with a user-specified ID
> 2.) Identify the sub-system the ID belongs to
> 3.) Guarantee of uniqueness
> 4.) Spoiling predictibility, to avoid creating an assumption
>     of object ordering and parsing (i.e., we don't want users to think
>     they can guess the next ID based on prior behavior).
> 
> The scheme for this is as follows (no spaces):
> 
>                 # subsys D RR
> Reserved char --|    |   | |
> Subsytem String -----|   | |

s/Subsytem/Subsystem/

> Unique number (64-bit) --| |
> Two-digit random number ---|
> 
> For example, a generated node-name for the block sub-system may take the
> look like this:

s/take the//

> 
>     #block076
> 
> The caller of id_generate() is responsible for freeing the generated
> node name string with g_free().
> 
> Signed-off-by: Jeff Cody <address@hidden>
> ---
>  include/qemu-common.h |  8 ++++++++
>  util/id.c             | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 43 insertions(+)
> 

> +char *id_generate(IdSubSystems id)
> +{
> +    const char *id_subsys_str[] = {

s/id_/const id_/

> +        [ID_QDEV]  = "qdev",
> +        [ID_BLOCK] = "block",
> +    };

Do we want some sort of compile-time assertion that we have entries for
all id values?...

> +
> +    static uint64_t id_counters[ID_MAX];
> +    uint32_t rnd;
> +
> +    assert(id < ID_MAX);

...maybe in the form of assert(id_subsys_str[id])


> +
> +    rnd = g_random_int_range(0, 99);
> +
> +    return g_strdup_printf("%c%s%" PRIu64 "%" PRId32, ID_SPECIAL_CHAR,
> +                                                      id_subsys_str[id],
> +                                                      id_counters[id]++,
> +                                                      rnd);
> +}
> 

Looks reasonable to me.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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