Re: [Qemu-devel] [PATCH v3 1/4] util - add automated ID generation utility

[Jeff Cody <jcody@redhat.com>]

On Tue, Oct 13, 2015 at 09:37:29AM +0200, Markus Armbruster wrote:
> Jeff Cody <jcody@redhat.com> 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().
> >
> > Reviewed-by: John Snow <jsnow@redhat.com>
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > Reviewed-by: Alberto Garcia <berto@igalia.com>
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > ---
> >  include/qemu-common.h |  8 ++++++++
> >  util/id.c             | 37 +++++++++++++++++++++++++++++++++++++
> >  2 files changed, 45 insertions(+)
> >
> > diff --git a/include/qemu-common.h b/include/qemu-common.h
> > index 0bd212b..2f74540 100644
> > --- a/include/qemu-common.h
> > +++ b/include/qemu-common.h
> > @@ -246,6 +246,14 @@ int64_t qemu_strtosz_suffix_unit(const char *nptr, char **end,
> >  #define STR_OR_NULL(str) ((str) ? (str) : "null")
> >  
> >  /* id.c */
> > +
> > +typedef enum IdSubSystems {
> > +    ID_QDEV,
> 
> ID_QDEV is not used in this series.  Do you intend to use it in a
> followup-series?  Can we reasonably expect that series will be accepted?
> 

John Arbuckle has a patch on list that uses it.  I haven't reviewed
it, however - but I guess it depends ultimately on whether qdev will
allow autogeneration for its IDs or not.

> You could sidestep these questions by making id_generate() take a string
> argument ;)
> 

I'd rather avoid having each system specifying a string inline in
their code.  It is cleaner to have the strings defined in a central
location, I think (not to mention, easier to reference).

> > +    ID_BLOCK,
> > +    ID_MAX      /* last element, used as array size */
> > +} IdSubSystems;
> > +
> > +char *id_generate(IdSubSystems id);
> >  bool id_wellformed(const char *id);
> >  
> >  /* path.c */
> [...]