Re: [Libguestfs] [PATCH libnbd v4] Add Go language bindings (golang) (RHBZ#1814538).
On Wed, Mar 25, 2020 at 10:14:30AM +0000, Daniel P. Berrangé wrote:
On Wed, Mar 25, 2020 at 10:01:02AM +0000, Richard W.M. Jones wrote:
> On Wed, Mar 25, 2020 at 09:48:02AM +0000, Daniel P. Berrangé wrote:
> > On Wed, Mar 25, 2020 at 09:42:19AM +0000, Richard W.M. Jones wrote:
> > > Now runs a complete set of tests, notably including the AIO test.
> > >
> > > File descriptors are passed in and out as plain ints (instead of
> > > *os.File) for a couple of reasons: (1) We have to pass the plain int
> > > to syscall.Select. (2) Turning an fd into an os.File causes golang to
> > > set the blocking flag which is deeply unhelpful.
> >
> > FWIW, using non-blocking FDs and syscall.Select is not normal best
> > practice for Go, where the expected model would be to use goroutines
> > with the file in blocking mode.
>
> Right, however I couldn't find any documentation about how to
> integrate a C library with that mechanism. Does libvirt-go do
> anything like this?
What's the intended usage of the FD ? I was thinking it was just for
doing plain read/write I/O from Go, but now I'm thinking this was wrong
interpretation
http://libguestfs.org/libnbd.3.html#Socket-and-direction
For example here's how it's integrated with glib:
https://github.com/libguestfs/libnbd/blob/master/examples/glib-main-loop.c
Integration with poll(2):
https://github.com/libguestfs/libnbd/blob/a5fe64ab5323d649d348e61c9126ef7...
It currently works from golang too, but it seems likely that
synchronous calls like nbd_pread are blocking the golang main loop
(because we make our own call to poll(2)). The asynch stuff also
works (see libnbd_590_aio_copy_test.go in the attached patch) but it's
rather awkward.
Rich.
--
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