Libguestfs - Mar 2020 - Re: [PATCH libnbd v4] Add Go language bindings (golang) (RHBZ#1814538).

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?

Rich.

-- 
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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 


Regards,
Daniel
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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/a5fe64ab5323d649d348e61c9126ef7e7234c145/examples/batched-read-write.c#L69

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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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