Re: [Qemu-devel] [PULL 04/16] Makefile: Rules for docker testing

[Eduardo Habkost]

On Mon, Jun 06, 2016 at 04:01:53PM +0200, Paolo Bonzini wrote:
> 
> 
> On 06/06/2016 14:50, Eduardo Habkost wrote:
> >   # Makes the definition constant after the first expansion
> >   DOCKER_SRC_COPY = $(eval DOCKER_SRC_COPY := 
> > docker-src.$(CUR_TIME))$(DOCKER_SRC_COPY)
> >   
> >   $(shell echo DEBUG: src_copy: $(DOCKER_SRC_COPY) >&2)
> >   
> >   $(DOCKER_SRC_COPY):
> >     @mkdir $@
> >     $(call make-archive-maybe, $(SRC_PATH), $@/qemu.tgz)
> > 
> > and I still get a failure: 
> > https://travis-ci.org/ehabkost/qemu/jobs/135574871
> > 
> >   $ make -j3 && ${TEST_CMD}
> >   DEBUG: cur_time: 2016.13278
> >   /home/travis/build/ehabkost/qemu/tests/docker/Makefile.include:37: *** 
> > unterminated variable reference.  Stop.
> > 
> > I am very confused by that code: using "=" to make a variable
> > reference itself is not allowed, but in this case no error is
> > reported because of the $(eval) trick. I have no idea what will
> > happen the second time the variable is expanded. Isn't $(eval)
> > going to be expanded again?
> 
> No, the second time you'll get the expansion of
> 
> DOCKER_SRC_COPY := docker-src.$(CUR_TIME)

And why it isn't enough to simply write the above line in the
Makefile without any eval trick?

> 
> > Why did you use this trick instead of just relying on ":=" to
> > expand the shell command only once?
> 
> I think initially the $(mkdir) was in DOCKER_SRC_COPY too.  Also, too
> many $(shell) end up slowing down make.

And how exactly does the eval trick helps avoiding slowing down
make, that wouldn't happen if using just:
  DOCKER_SRC_COPY = docker-src.$(CUR_TIME)
?

-- 
Eduardo