Re: ideas towards requiring VPATH build

[Eric Blake]

On 10/11/19 4:28 PM, Eric Blake wrote:
> I know we've talked about enforcing a VPATH build, but haven't yet 
> flipped the switch.  This week, I've played with using a VPATH build (cd 
> qemu; mkdir -p build; cd ./build; ../configure ...; make ...), but find 
> my old habits of expecting an in-tree build to just work (cd qemu; make 
> ...) hard to overcome.  So this is what I've come up with: if you place 
> the following file in-tree, then any 'make ...' command you type in-tree 
> without using -C will have the same effect as if you had typed the same 
> command in the build directory, but without having to manually remember 
> to switch to the build directory.
>
> Perhaps this can be a starting point for a patch to actually include 
> this file in qemu.git as part of the larger effort to force VPATH 
> builds, while still having the convenience of in-tree make working for 
> those who were used to it.  (I places an echo and sleep in my file to 
> remind myself when I forgot to use the build directory, but that is not 
> mandatory if we want GNUmakefile stored in qemu.git).
>
> Presumably, any full switch to force a VPATH build would also include 
> creating the build directory as needed (my hack assumes that it already 
> exists).
>
> $ cat GNUmakefile
> # Hack for redirecting while reminding myself to use distinct builddir
> %: force
>      @echo 'changing directory to build...'
>      @sleep 2
>      @$(MAKE) -C build -f Makefile $(MAKECMDGOALS)
> force: ;
> GNUmakefile: ;

Works for 'make all' or 'make check', but doesn't quite work for 'make'. 
 For that, I had to add:

ifeq ($(MAKECMDGOALS),)
recurse: all
endif

prior to the %: line (the name 'recurse' is not special, it merely has 
to be something unlikely to be in the actual Makefile, and appear as the 
first rule with a dependency on the name of the real first rule in 
Makefile, so that when make is invoked without a target, we still end up 
invoking the actual Makefile rather than our GNUmakefile shim claiming 
that 'force' has nothing to do).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org