From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42063) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZcvFR-0003eZ-S5 for qemu-devel@nongnu.org; Fri, 18 Sep 2015 08:58:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZcvFQ-0003EW-Np for qemu-devel@nongnu.org; Fri, 18 Sep 2015 08:58:41 -0400 From: Markus Armbruster References: <45295c490f1d7c2c2209a19fba8c656967d03d5b.1441140367.git.jcody@redhat.com> Date: Fri, 18 Sep 2015 14:58:31 +0200 In-Reply-To: <45295c490f1d7c2c2209a19fba8c656967d03d5b.1441140367.git.jcody@redhat.com> (Jeff Cody's message of "Tue, 1 Sep 2015 18:30:15 -0400") Message-ID: <87twqr3mvc.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH v2 1/2] util - add automated ID generation utility List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jeff Cody Cc: kwolf@redhat.com, programmingkidx@gmail.com, jsnow@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org Jeff Cody 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(). > > Signed-off-by: Jeff Cody > --- > include/qemu-common.h | 8 ++++++++ > util/id.c | 36 ++++++++++++++++++++++++++++++++++++ > 2 files changed, 44 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..9457f2d 100644 > --- a/util/id.c > +++ b/util/id.c > @@ -26,3 +26,39 @@ bool id_wellformed(const char *id) > } > return true; > } > + > +#define ID_SPECIAL_CHAR '#' > + > +static const char * const id_subsys_str[] = { Like Eric, I'd prefer *const. > + [ID_QDEV] = "qdev", > + [ID_BLOCK] = "block", > +}; > + > +/* Generates an ID of the form: Style nit: wing you comments on both ends, please. > + * > + * "#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 zero-padded two digit pseudo-random number Recommend to note that the value does not satisfy id_wellformed(). I'd specify a bit more losely: /* * Generates an ID of the form PREFIX SUBSYSTEM NUMBER * where * - PREFIX is the reserved character '#' * - SUBSYSTEM identifies the subsystem creating the ID * - NUMBER is a decimal number unique within SUBSYSTEM. * Example: "#block146" * * Note that these IDs do not satisfy id_wellformed(). > + * > + * The caller is responsible for freeing the returned string with g_free() > + */ > +char *id_generate(IdSubSystems id) > +{ > + static uint64_t id_counters[ID_MAX]; > + uint32_t rnd; > + > + assert(id < ID_MAX); > + assert(id_subsys_str[id]); > + > + rnd = g_random_int_range(0, 99); 99 is off by one: "Returns a random gint32 equally distributed over the range [begin ..end -1]." > + > + return g_strdup_printf("%c%s%" PRIu64 "%02" PRId32, ID_SPECIAL_CHAR, > + id_subsys_str[id], > + id_counters[id]++, > + rnd); > +}