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

[Jeff Cody <jcody@redhat.com>]

On Tue, Sep 01, 2015 at 03:13:52PM -0400, John Snow wrote:
> 
> 
> 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"
> 

All I can say is, sometimes my fingers don't obey my brain.


> >     #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             | 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.
> 

Possibly.  I'm neutral, we can move it out of the function and make it
static.

> > +    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.

Yes - that is actually what I meant to do.  We definitely want to 
enforce two digits for the random element.

Thanks,

Jeff