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

[Markus Armbruster <armbru@redhat.com>]

Jeff Cody <jcody@redhat.com> writes:

> 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.

Then that patch should add ID_QDEV.

>> 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).

Covered by your artistic license :)

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