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

[Markus Armbruster]

Jeff Cody <address@hidden> writes:

> 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 <address@hidden>
> ---
>  include/qemu-common.h |  8 ++++++++
>  util/id.c             | 36 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 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..9457f2d 100644
> --- a/util/id.c
> +++ b/util/id.c
> @@ -26,3 +26,39 @@ bool id_wellformed(const char *id)
>      }
>      return true;
>  }
> +
> +#define ID_SPECIAL_CHAR '#'
> +
> +static const char * const id_subsys_str[] = {

Like Eric, I'd prefer *const.

> +    [ID_QDEV]  = "qdev",
> +    [ID_BLOCK] = "block",
> +};
> +
> +/* Generates an ID of the form:

Style nit: wing you comments on both ends, please.

> + *
> + * "#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 zero-padded two digit pseudo-random number

Recommend to note that the value does not satisfy id_wellformed().

I'd specify a bit more losely:

/*
 * Generates an ID of the form PREFIX SUBSYSTEM NUMBER
 * where
 * - PREFIX is the reserved character '#'
 * - SUBSYSTEM identifies the subsystem creating the ID
 * - NUMBER is a decimal number unique within SUBSYSTEM.
 * Example: "#block146"
 *
 * Note that these IDs do not satisfy id_wellformed().

> + *
> + * The caller is responsible for freeing the returned string with g_free()
> + */
> +char *id_generate(IdSubSystems id)
> +{
> +    static uint64_t id_counters[ID_MAX];
> +    uint32_t rnd;
> +
> +    assert(id < ID_MAX);
> +    assert(id_subsys_str[id]);
> +
> +    rnd = g_random_int_range(0, 99);

99 is off by one: "Returns a random gint32 equally distributed over the
range [begin ..end -1]."

> +
> +    return g_strdup_printf("%c%s%" PRIu64 "%02" PRId32, ID_SPECIAL_CHAR,
> +                                                        id_subsys_str[id],
> +                                                        id_counters[id]++,
> +                                                        rnd);
> +}