bug#31465: Automake 1.16 breaks custom dependency handling

[Eric Blake]

On 05/15/2018 11:56 AM, Sander Niemeijer wrote:
> I have a software package that uses SWIG to dynamically generates .c files.
> With automake 1.15 and earlier I was able to add my own dependency rules for 
> this by adding some custom entries to the Makefile.am.
>
> A small reduced example of the approach I am using:
>
> @AMDEP_TRUE@@am__include@ @address@hidden(DEPDIR)/address@hidden@
>
> foo.c: $(srcdir)/foo.i
> @AMDEP_TRUE@    source='$(srcdir)/foo.i' object='foo.c' libtool=no 
> @AMDEPBACKSLASH@
> @AMDEP_TRUE@    depfile='$(DEPDIR)/foo.Pc' tmpdepfile='$(DEPDIR)/foo.TPc' 
> @AMDEPBACKSLASH@
> @AMDEP_TRUE@    depmode='none' $(depcomp) @AMDEPBACKSLASH@
> @AMDEP_TRUE@    $(SWIG) $(SWIGFLAGS) -M $(srcdir)/foo.i > $(DEPDIR)/foo.Pc
>         $(SWIG) $(SWIGFLAGS) -o foo.c $(srcdir)/foo.i

Are you actually writing ALL of that in your Makefile.am?  Don't. You're 
abusing automake's internals, and when automake changes how its 
internals work, things fall apart fast.  You should only use documented 
entries into automake; any use of @AMDEP_TRUE@, @am__include@, 
@am__quote@, @AMDEPBACKSLASH@, and $(DEPDIR) falls in the realm of using 
undocumented internals.

Also, the fact that you are calling $(SWIG) two separate times per file 
may defeat the purpose of fast dependency tracking, which says that 
dependencies should be computed as a side-effect of the compilation, 
rather than as a second run of a tool.

The automake manual mentions that you can instead use the hammer of 
declaring foo.c as part of BUILT_SOURCES (so that it is guaranteed to be 
built before dependency rules kick in), or to list manual additional 
dependencies as needed, as in:

foo.$(OBJEXT): $(srcdir)/foo.i
foo.c: $(srcdir)/foo.i
        $(SWIG) $(SWIGFLAGS) -o foo.c $(srcdir)/foo.i


> I have been able to work around this by overriding am--depfiles to:
>
> am--depfiles: $(am__depfiles_remade) $(local_depfiles)

No, that's still mucking around with automake's internals, and a bad idea.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org