[Buildroot] [RFC/PATCH] fs/common.mk: Fix wrong double dollar usage

[Ezequiel Garcia <ezequiel.garcia@free-electrons.com>]

Hi Thomas,

Thanks for your answer.

On Tue, Mar 12, 2013 at 10:05:04AM +0100, Thomas De Schampheleire wrote:
> Hi,
> 
> On Sat, Mar 9, 2013 at 10:04 PM, Ezequiel Garcia
> <ezequiel.garcia@free-electrons.com> wrote:
> > Double dollar sign is used to put an explicit dollar sign,
> > for instance, when writing a makefile rule.
> >
> > In this case, there are some makefile conditionals where
> > makefile variables are evaluated using double dollar signs
> > instead of single dollar, which is wrong.
> >
> > In particular, this fixes a buildroot 'make' stall
> > when building with an empty device table (empty BR2_ROOTFS_DEVICE_TABLE).
> >
> > Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
> > ---
> > I'm sending this as RFC because I'm new to buildroot
> > and because I'm not a makefile wizard.
> > AFAIK, this fixes a real bug in my compilation,
> > as explained in the commit message.
> 
> In fact you are reverting changes made by Arnout a while back, see git
> commit 847895d29524d81b64afb059b8649a77802a469b
> http://git.buildroot.org/buildroot/commit/?id=847895d29524d81b64afb059b8649a77802a469b
> There were specific reasons to make these changes, so I don't think we
> should revert them like this.
> 
> Be aware that these make targets are inside a 'define
> ROOTFS_TARGET_INTERNAL' statement, and later in the file this
> 'function' is executed with 'call'. This makes the rules for $ or $$ a
> little different than in standard make recipes.

Mmmm, I see. So it's not as easy as it seemed!
However, please note I'm *only* fixing the conditionals,
not the targets or the rules. See below.

> 
> You mention a 'stall', can you elaborate more? What happens exactly?
> Can you debug this further?
> 

Yes, I have. The problem is that this conditional

  ifneq ($$(ROOTFS_DEVICE_TABLES),)
         cat $$(ROOTFS_DEVICE_TABLES) > $$(FULL_DEVICE_TABLE)

is *never* false. However, if ROOTFS_DEVICE_TABLES is empty
the command will execute like this:

  $ cat > some_file

which simply stalls (well it doesn't stall, it's waiting for EOF at stdin).
I think this is easily reproducible, just set an empty value in your
BR2_ROOTFS_DEVICE_TABLE option.

As I said in the patch I'm far from a makefile wizard, but it seems
to me that $$(ROOTFS_DEVICE_TABLES) (double dollar) is not equivalent
to $(ROOTFS_DEVICE_TABLES) (single dollar) from the perspective of the
conditional.

Thanks,
-- 
Ezequiel Garc?a, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com