[Libguestfs] [libnbd PATCH 4/4] api: Add DF flag support for pread

[Richard W.M. Jones]

On Tue, May 28, 2019 at 05:00:22PM +0100, Richard W.M. Jones wrote:
> On Tue, May 28, 2019 at 10:23:13AM -0500, Eric Blake wrote:
> > But having some sort of callback that gets invoked
> > on each chunk would allow the client to add in such checking; we could
> > even provide a utility function in the library to serve as such a
> > callback if our checking is sufficient for the client's purposes.
> > Something like:
> > 
> > nbd_pread_callback(nbd, buf, count, offset, opaque, callback, flags)
> > 
> > where we still read off the wire into buf (to avoid ping-ponging a read
> > into a temporary buf that the callback then has to memcpy() into the
> > real buf), but call the callback as soon as each chunk is received:
> > 
> > callback(opaque, buf, count, offset, status)
> 
> TBH this looks like the most simple & sensible API to me.
> 
> > telling the user which subset of buf was just populated, and where
> > status is data, hole, or error. The signature may still need tweaking.
> > Or we may even want to let the user register a set of callbacks, where a
> > different callback is invoked for different chunk types:
> > 
> > set = nbd_callback_set_create(nbd, NBD_CMD_READ, opaque, default_cb,
> > default_error_cb);
> > nbd_callback_set_add(nbd, set, NBD_REPLY_TYPE_OFFSET_DATA, opaque, cb);
> > nbd_callback_set_add(nbd, set, NBD_REPLY_TYPE_OFFSET_HOLE, opaque, cb);
> > nbd_pread_callback(nbd, buf, count, offset, set, flags);
> 
> This would also work, with the caveat that the Python bindings will
> currently crash if you try to do this.  I'm trying to fix that now.

This problem in the generator should be fixed now.

Rich.

> > The idea of registering a set of callbacks to handle a particular
> > integer command id may work well for other extensions as well;
> > particularly if we encounter a case where an app wants to use libnbd for
> > the groundwork (setting up a tls connection handshake) but still
> > implement their own non-standard NBD extensions that the server will
> > understand (where the libnbd state machine parses the chunks off the
> > wire, but the client then interprets those chunks).  Which means we may
> > also need hooks for a client to inject other NBD_OPT sequences into the
> > state machine beyond what we know natively.
> 
> This could be interesting too.  I wonder if there are really going to
> be such extensions?
> 
> > Various ideas still floating around, it may be a while before we
> > actually have code to match those ideas in order to pick what works best.
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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