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Re: [Qemu-block] [Qemu-devel] [RFC PATCH 1/2] util - add automated ID ge
From: |
John Snow |
Subject: |
Re: [Qemu-block] [Qemu-devel] [RFC PATCH 1/2] util - add automated ID generation utility |
Date: |
Tue, 1 Sep 2015 15:13:52 -0400 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 |
On 09/01/2015 01:23 PM, Jeff Cody wrote:
> Multiple sub-systems in QEMU may find it useful to generated IDs
generate
> 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
predictability
> 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 -----| | |
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:
>
"take this form" or "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 <address@hidden>
> ---
> include/qemu-common.h | 8 ++++++++
> util/id.c | 35 +++++++++++++++++++++++++++++++++++
> 2 files changed, 43 insertions(+)
>
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index bbaffd1..f6b0105 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -237,6 +237,14 @@ int64_t strtosz_suffix_unit(const char *nptr, char **end,
> #define STR_OR_NULL(str) ((str) ? (str) : "null")
>
> /* id.c */
> +
> +typedef enum IdSubSystems {
> + ID_QDEV,
> + ID_BLOCK,
> + ID_MAX /* last element, used as array size */
> +} IdSubSystems;
> +
> +char *id_generate(IdSubSystems);
> bool id_wellformed(const char *id);
>
> /* path.c */
> diff --git a/util/id.c b/util/id.c
> index 09b22fb..48e2935 100644
> --- a/util/id.c
> +++ b/util/id.c
> @@ -26,3 +26,38 @@ bool id_wellformed(const char *id)
> }
> return true;
> }
> +
> +#define ID_SPECIAL_CHAR '#'
> +
> +/* Generates an ID of the form:
> + *
> + * "#block146",
> + *
> + * where:
> + * - "#" is always the reserved character '#'
> + * - "block" refers to the subsystem identifed via IdSubSystems
> + * and id_subsys_str[]
> + * - "1" is a unique number (up to a uint64_t) for the subsystem,
> + * - "46" is a pseudo-random numer to create uniqueness
> + *
> + * The caller is responsible for freeing the returned string with g_free()
> + */
> +char *id_generate(IdSubSystems id)
> +{
> + const char *id_subsys_str[] = {
> + [ID_QDEV] = "qdev",
> + [ID_BLOCK] = "block",
> + };
> +
Do we want this local to this function? A lookup table may be useful for
utilities at some point.
> + static uint64_t id_counters[ID_MAX];
> + uint32_t rnd;
> +
> + assert(id < ID_MAX);
> +
> + 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);
> +}
>
So basically, it's #<sys><counter><rnd>
So we could see:
|block|1|32|
For the block subsystem, 1st device, salt is 3.
But we could also see:
|block|13|2|
Block subsys, 13th device, salt is 2.
Forcing a zero-pad on the salt should be enough to disambiguate in all
cases:
block132
block1302
This way, the last two digits are *always* salt, making the ID
unambiguous and, I think, impossible to collide against regardless of
that the rng returns in the future for new IDs.