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

[Richard W.M. Jones]

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 at abs_top_builddir@/include" ;;
> > > > +    *) ;;
> > > > +esac
> > > 
> > > Don't you need something like this too:
> > > 
> > >    --libs)  echo "-L at 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
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