Libguestfs - Apr 2020 - Re: [PATCH nbdkit] golang: Compile against the local nbdkit build, not installed.

On Thu, Apr 23, 2020 at 05:58:25PM +0100, Daniel P. Berrangé
wrote:
> On Thu, Apr 23, 2020 at 05:55:14PM +0100, Richard W.M. Jones wrote:
> > When compiling when an older nbdkit is installed, the build would fail
> > because certain symbols such as .get_ready were not defined:
> > 
> >   ../../src/libguestfs.org/nbdkit/nbdkit.go:541:8: plugin.get_ready
undefined (type _Ctype_struct_nbdkit_plugin has no field or method get_ready)
> > 
> > This happens because we were using the installed
<nbdkit-plugin.h>
> > rather than the local copy.
> > 
> > We don't want to modify the *.go files themselves as they might be
> > copied into other projects.  Instead we can set PKG_CONFIG to point to
> > a fake pkg-config binary which will return the correct CFLAGS.
> > 
> > Fixes: commit 1ff44288ae1cf95428283e252edd9474c3fe3b55
> > Thanks: Dan Berrangé, Eric Blake
> 
> > +# This fake pkg-config program is used to trick cgo so that
> > +# "#cgo pkg-config nbdkit" lines are processed relative to
the local
> > +# directory and not the installed nbdkit.
> > +
> > +case "$1" in
> > +    --cflags*) echo "-I@abs_top_builddir@/include" ;;
> > +    *) ;;
> > +esac
> 
> Don't you need something like this too:
> 
>    --libs)  echo "-L@abs_top_builddir@/lib -lnbdkit"
We don't actually have "libnbdkit".  It was proposed a while back
but
we didn't get sufficient review feedback, and the code was very
complex and unmaintainable so it never made it upstream.

cgo currently issues the following two commands:

  $PKG_CONFIG --cflags -- nbdkit nbdkit nbdkit
  $PKG_CONFIG --libs -- nbdkit nbdkit nbdkit

and yes I also don't know why it prints the package name 3 times.
> The installed nbdkit.pc would do this I presume. Or are you happy relying
> on the allow undefined symbols LDFLAGS
A "local" nbdkit.pc might indeed be better because we'd be using
pkg-config itself to parse the command line.

Rich.

-- 
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On Thu, Apr 23, 2020 at 06:04:20PM +0100, Richard W.M. Jones
wrote:
> On Thu, Apr 23, 2020 at 05:58:25PM +0100, Daniel P. Berrangé wrote:
> > On Thu, Apr 23, 2020 at 05:55:14PM +0100, Richard W.M. Jones wrote:
> > > When compiling when an older nbdkit is installed, the build would
fail
> > > because certain symbols such as .get_ready were not defined:
> > > 
> > >   ../../src/libguestfs.org/nbdkit/nbdkit.go:541:8:
plugin.get_ready undefined (type _Ctype_struct_nbdkit_plugin has no field or
method get_ready)
> > > 
> > > This happens because we were using the installed
<nbdkit-plugin.h>
> > > rather than the local copy.
> > > 
> > > We don't want to modify the *.go files themselves as they
might be
> > > copied into other projects.  Instead we can set PKG_CONFIG to
point to
> > > a fake pkg-config binary which will return the correct CFLAGS.
> > > 
> > > Fixes: commit 1ff44288ae1cf95428283e252edd9474c3fe3b55
> > > Thanks: Dan Berrangé, Eric Blake
> > 
> > > +# This fake pkg-config program is used to trick cgo so that
> > > +# "#cgo pkg-config nbdkit" lines are processed
relative to the local
> > > +# directory and not the installed nbdkit.
> > > +
> > > +case "$1" in
> > > +    --cflags*) echo "-I@abs_top_builddir@/include" ;;
> > > +    *) ;;
> > > +esac
> > 
> > Don't you need something like this too:
> > 
> >    --libs)  echo "-L@abs_top_builddir@/lib -lnbdkit"
> 
> We don't actually have "libnbdkit".  It was proposed a while
back but
> we didn't get sufficient review feedback, and the code was very
> complex and unmaintainable so it never made it upstream.
Ah I see then.
> cgo currently issues the following two commands:
> 
>   $PKG_CONFIG --cflags -- nbdkit nbdkit nbdkit
>   $PKG_CONFIG --libs -- nbdkit nbdkit nbdkit
> 
> and yes I also don't know why it prints the package name 3 times.
You have three .go files which have the pkg-config stanza in them.
It is just combining them all and not bothering to eliminate duplicates
as they're harmless
> 
> > The installed nbdkit.pc would do this I presume. Or are you happy
relying
> > on the allow undefined symbols LDFLAGS
> 
> A "local" nbdkit.pc might indeed be better because we'd be
using
> pkg-config itself to parse the command line.
Regards,
Daniel
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On Thu, Apr 23, 2020 at 06:06:12PM +0100, Daniel P. Berrangé
wrote:
> On Thu, Apr 23, 2020 at 06:04:20PM +0100, Richard W.M. Jones wrote:
> > On Thu, Apr 23, 2020 at 05:58:25PM +0100, Daniel P. Berrangé wrote:
> > > On Thu, Apr 23, 2020 at 05:55:14PM +0100, Richard W.M. Jones
wrote:
> > > > When compiling when an older nbdkit is installed, the build
would fail
> > > > because certain symbols such as .get_ready were not defined:
> > > > 
> > > >   ../../src/libguestfs.org/nbdkit/nbdkit.go:541:8:
plugin.get_ready undefined (type _Ctype_struct_nbdkit_plugin has no field or
method get_ready)
> > > > 
> > > > This happens because we were using the installed
<nbdkit-plugin.h>
> > > > rather than the local copy.
> > > > 
> > > > We don't want to modify the *.go files themselves as
they might be
> > > > copied into other projects.  Instead we can set PKG_CONFIG
to point to
> > > > a fake pkg-config binary which will return the correct
CFLAGS.
> > > > 
> > > > Fixes: commit 1ff44288ae1cf95428283e252edd9474c3fe3b55
> > > > Thanks: Dan Berrangé, Eric Blake
> > > 
> > > > +# This fake pkg-config program is used to trick cgo so that
> > > > +# "#cgo pkg-config nbdkit" lines are processed
relative to the local
> > > > +# directory and not the installed nbdkit.
> > > > +
> > > > +case "$1" in
> > > > +    --cflags*) echo
"-I@abs_top_builddir@/include" ;;
> > > > +    *) ;;
> > > > +esac
> > > 
> > > Don't you need something like this too:
> > > 
> > >    --libs)  echo "-L@abs_top_builddir@/lib -lnbdkit"
> > 
> > We don't actually have "libnbdkit".  It was proposed a
while back but
> > we didn't get sufficient review feedback, and the code was very
> > complex and unmaintainable so it never made it upstream.
> 
> Ah I see then.
> 
> > cgo currently issues the following two commands:
> > 
> >   $PKG_CONFIG --cflags -- nbdkit nbdkit nbdkit
> >   $PKG_CONFIG --libs -- nbdkit nbdkit nbdkit
> > 
> > and yes I also don't know why it prints the package name 3 times.
> 
> You have three .go files which have the pkg-config stanza in them.
> It is just combining them all and not bothering to eliminate duplicates
> as they're harmless
Ah yes, good point!

Rich.
> > 
> > > The installed nbdkit.pc would do this I presume. Or are you happy
relying
> > > on the allow undefined symbols LDFLAGS
> > 
> > A "local" nbdkit.pc might indeed be better because we'd
be using
> > pkg-config itself to parse the command line.
> 
> Regards,
> Daniel
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange
:|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com
:|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange
:|
-- 
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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