[zeromq-dev] Extending zmq_msg_t API

[Thomas Rodgers]

zmq_msg_get() could be extended to give either the refcount or an indicator
on whether a message was share; based on other refcounted designs I'm
hesitant to promote surfacing the actual count.  Similarly, zmq_msg_set()
could allow 'unsharing' by adding a ZMQ_SHARED property #define and setting
it's value to 0 (no effect on non-shared messages).

So the only API surface area change is an additional message property.
 This seems the cleanest to me.

On Wednesday, July 9, 2014, Goswin von Brederlow <goswin-v-b at web.de> wrote:

> On Tue, Jul 08, 2014 at 10:42:41AM -0500, Thomas Rodgers wrote:
> > tl;dr; Is there any objection to adding some sort of accessor to the API
> to
> > determine if a given zmq_msg_t is_shared()?
> >
> > Background/Rationale:
> >
> > Something I encountered while writing a "high level" C++ wrapper for
> > zmq_msg_t and it's API is the following set of behaviors -
> >
> > zmq_msg_init(&msg_vsm, 20);
> >
> > Results in a type_vsm message, the body of which is held entirely within
> > the space allocated to zmq_msg_t
> >
> > zmq_msg_init(&msg_lmsg, 1024);
> >
> > Results in a type_lmsg message, the body is held as a reference to a
> block
> > of size bytes.
> >
> > memcpy(zmq_msg_data(&msg_vsm), "VSM", 3);
> > memcpy(zmq_msg_data(&msg_lmsg), "LMSG", 4);
> >
> > So far so good.  Now copy -
> >
> > zmq_msg_copy(&msg_vsm2, &msg_vsm);
> > zmq_msg_copy(&msg_lmsg2, &msg_lmsg);
> >
> > Now change contents -
> >
> > memcpy(zmq_msg_data(&msg_vsm2), "vsm", 3);
> > memcpy(zmq_msg_data(&msg_lmsg2), "lmsg", 4);
> >
> > assert(memcmp(&msg_vsm, &msg_vsm2, 3) != 0); // ok
> > assert(memcmp(&msg_lmsg, &msg_lmsg2, 4) != 0); // fail
> >
> > This happens by design (lmsg's are refcounted on copy, not deep copied).
> > But it results in a situation where a zmq_msg_t is sometimes a Value and
> > sometimes a Reference.  This could lead to astonishment for the unwary.
> >
> > >From the perspective of a wrapper (particularly one that takes a strong
> > stand on value semantics and local reasoning), this behavior is ungood.
>  So
> > my options are deep copy always or implement copy-on-write.
> >
> > For efficiency I prefer the latter approach in the case of type_lmsg
> > messages.  I have implemented the copy-on-write logic through a horrible
> > brittle hack that examines the last byte of zmq_msg_t.  I would prefer a
> > less brittle solution.
>
> "lmsg's are refcounted on copy" Can't you access the refcount?
> Or is that the API call you want to add?
>
> Maybe instead of is_shared() an unshare() call would be more usefull,
> which would copy the message payload if it is shared. Or both?
>
> MfG
>         Goswin
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