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" > #block076 > > The caller of id_generate() is responsible for freeing the generated > node name string with g_free(). > > Signed-off-by: Jeff Cody <jc...@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. > + 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.