[Buildroot] [RFCv3 08/15] package/pkg-generic: handle host-xz as an extract dependency

[Yann E. MORIN <yann.morin.1998@free.fr>]

On 2017-12-02 21:16 +0100, Thomas Petazzoni spake thusly:
> Hello,
> 
> On Sat, 2 Dec 2017 16:08:43 +0100, Yann E. MORIN wrote:
> 
> > > +ifeq ($(filter host-tar host-skeleton host-xz,$(1)),)
> > > +$(2)_EXTRACT_DEPENDENCIES += $(BR2_XZCAT_HOST_DEPENDENCY)
> > > +endif  
> > 
> > So, all packages will now depend on this, even those that are not
> > compressed with xz?
> 
> Yes, if there is no xzcat available on the system. This is exactly what
> the current behavior is with DEPENDENCIES_HOST_PREREQ. My series does
> not intend to solve this problem, which is a separate one.

I somehow interpreted your patches in my head as "only build them when
needed". ;-)

> I do believe however that by moving those dependencies (host-xz and
> host-lzip) as proper package dependencies, it will be easier to have
> follow-up patches that add the dependency only if lzip or xz are
> needed. But again, my series does not intend to fix that, it's a
> separate problem.

OK, agreed.

> > Currently, filesystems that want to compress with xz all depend on
> > host-xz unconditioanlly. Do we want to change that as well, so they only
> > depend on it if needed?
> 
> I don't understand this part, because filesystems do not depend on
> host-xz unconditionally. They only depend on it if xz compression of
> the filesystem image is requested. From fs/common.mk:
> 
> ifeq ($$(BR2_TARGET_ROOTFS_$(2)_XZ),y)
> ROOTFS_$(2)_DEPENDENCIES += host-xz
> ROOTFS_$(2)_COMPRESS_EXT = .xz
> ROOTFS_$(2)_COMPRESS_CMD = xz -9 -C crc32 -c
> endif

Well, first, let's assume that when the host has a suitable xzcat, it
also has the xz command as well; I think it's not too far-fetched an
assumption.

Then, we have BR2_XZCAT_HOST_DEPENDENCY empty, because we do have a
suitable xzcat, and thus a suitable xz. Yet, when the user requires
their filesystem to be xz-compressed, we still forcefully depend on
host-xz, even though we do have a suitable xz command.

What I mean is that, probably, we will want to make it:

    ROOTFS_$(2)_DEPENDENCIES += $$(BR2_XZCAT_HOST_DEPENDENCY)

Regards,
Yann E. MORIN.

> Best regards,
> 
> Thomas Petazzoni
> -- 
> Thomas Petazzoni, CTO, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com

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