Re: [Qemu-devel] [PATCH v2 1/2] util - add automated ID generation utility

[Eric Blake <eblake@redhat.com>]

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On 09/01/2015 04:30 PM, Jeff Cody wrote:
> Multiple sub-systems in QEMU may find it useful to generate 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 predictability, 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 --|    |   | |
> Subsystem String ----|   | |
> Unique number (64-bit) --| |
> Two-digit random number ---|
> 
> For example, a generated node-name for the block sub-system may look
> like this:
> 
>     #block076
> 
> The caller of id_generate() is responsible for freeing the generated
> node name string with g_free().
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  include/qemu-common.h |  8 ++++++++
>  util/id.c             | 36 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

> +
> +static const char * const id_subsys_str[] = {
> +    [ID_QDEV]  = "qdev",
> +    [ID_BLOCK] = "block",
> +};

We're inconsistent on whether there should be a space in '*const'
(compare commit a91e211 with b86f461, for example).  But since qemu.git
has both spellings in various contexts, it's probably not worth changing
here.

> +
> +    rnd = g_random_int_range(0, 99);

Thankfully, g_random_* is a PRNG that is not consuming system entropy
(auto-generated names should not be starving /dev/random).

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


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