[libvirt] [PATCH] build: Fix build with old automake

[Jiri Denemark]

On Tue, Mar 18, 2014 at 10:42:01 +0000, Daniel Berrange wrote:
> On Tue, Mar 18, 2014 at 11:36:51AM +0100, Jiri Denemark wrote:
> > On Mon, Mar 17, 2014 at 09:33:11 -0600, Eric Blake wrote:
> > > On 03/14/2014 12:55 PM, Jiri Denemark wrote:
> > > > On Fri, Mar 14, 2014 at 07:54:58 -0600, Eric Blake wrote:
> > > >> On 03/14/2014 04:43 AM, Jiri Denemark wrote:
> > > >>> Ancient automake (such as from RHEL5) does not provide abs_srcdir and
> > > >>> abs_builddir variables which are used by a recent commit of mine
> > > >>> (e562e82).
> > > >>>
> > > >>> Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
> > > >>> ---
> > > 
> > > >>> +# old automake does not provide abs_{src,build}dir variables
> > > >>> +abs_builddir = $(shell pwd)
> > > >>> +abs_srcdir = $(shell cd $(srcdir) && pwd)
> > > 
> > > Hmm, just noticed another thing - with NEWER automake, we are now less
> > > efficient (calling out to $(shell) overwrites the value that is already
> > > provided for free without a subprocess by newer automake).  Does it work
> > > if you use:
> > > 
> > > abs_builddir ?= $(shell pwd)
> > 
> > Automake does a fair amount of magic here so abs_builddir = $(shel ...)
> > does not actually overwrite the old value because there is no old value
> > :-) Automake just does not put it's own abs_builddir definition in that
> > case. If I switch to abs_builddir ?= $(shell ...), automake adds it's
> > own definition so it seems it could work except that it doesn't. With
> > "=", automake adds the definition above the other variable definition
> > and namely above libvirt_cpu_la_DEPENDENCIES which uses it:
> > 
> >     abs_builddir = $(shell ...)
> >     ...
> >     libvirt_cpu_la_DEPENDENCIES = $(abs_builddir)/...
> >     ...
> >     rules
> > 
> > When I switch to "?=", automake apparently does not recognize it as a
> > variable definition and puts it between the block with variable
> > definitions and rules. Thus the result is:
> > 
> >     abs_builddir = /some/build/path
> >     ...
> >     libvirt_cpu_la_DEPENDENCIES = $(abs_builddir)/...
> >     ...
> >     abs_builddir ?= $(shell ...)
> >     ...
> >     rules
> > 
> > which of course does not work with old automake which does not add the
> > first abs_builddir definition there and thus libvirt_cpu_la_DEPENDENCIES
> > sees $(abs_builddir) empty.
> > 
> > Anyway, making two extra shell commands once per build does not seem
> > like anything we should really care about so I'd just stick with
> > 
> >     abs_builddir = $(shell ...)
> 
> Why don't we just avoid the whole issue by removing use of abs_srcdir
> and abs_builddir.  Can this rule:
> 
> $(abs_builddir)/cpu/cpu_map.xml:
>         $(AM_V_GEN)ln -s $(abs_srcdir)/cpu/cpu_map.xml $@
> 
> be just changed to
> 
> cpu/cpu_map.xml:
>         $(AM_V_GEN)ln -s $(srcdir)/cpu/cpu_map.xml $@

That's what I tried first but it does not work at all. I don't
understand why but make thinks cpu/cpu_map.xml target is uptodate even
though the file does not exist in builddir. And $(srcdir) doesn't work
for relative VPATH. For example, if VPATH is .., then

    abs_srcdir = /some/path/src
    abs_builddir = /some/path/build/src
    srcdir = ../../src

and the /some/path/build/src/cpu/cpu_map.xml link will be
../../src/cpu/cpu_map.xml and thus will point to itself. And we can't
just blindly do

    ln -s ../$(srcdir)/cpu/cpu_map.xml $@

because this would not work for absolute VPATH, when srcdir is
/some/path/src.

Yeah, it's a mess.

Jirka