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

[Jeff Cody]

On Tue, Sep 01, 2015 at 12:55:15PM -0600, Eric Blake wrote:
> 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_/
> 

Good point.

> > +        [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])
> 

Yes, I think we do.  If one is missing, that is certainly a mistake,
and we run the risk of collisions as well.

> 
> > +
> > +    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.
>

Thanks

-Jeff