[RFC] fs: allow passing the image file name to inner-rootfs

Suppose that you want to create a filesystem image containing only the
/var/lib subtree. That would something like this on a br2-external:

fs/rootfs-var-lib/Config.in:

    menuconfig BR2_TARGET_ROOTFS_VAR_LIB
    bool "/var/lib"

    if BR2_TARGET_ROOTFS_VAR_LIB
    source "$BR2_EXTERNAL_MFS_PATH/fs/rootfs-var-lib/ext4/Config.in"
    endif

fs/rootfs-var-lib/rootfs-var-lib.mk:
    ifeq ($(BR2_TARGET_ROOTFS_VAR_LIB),y)
    rootfs-var-lib = $(call inner-rootfs,var-lib-$(pkgname),VAR_LIB_$(call UPPERCASE,$(pkgname)))
    include $(sort $(wildcard $(BR2_EXTERNAL_MFS_PATH)/fs/rootfs-var-lib/*/*.mk))
    endif

fs/rootfs-var-lib/ext4/Config.in:
    config BR2_TARGET_ROOTFS_VAR_LIB_EXT4
        bool "ext4 /var/lib filesystem"
        select BR2_PACKAGE_HOST_E2FSPROGS
        help
          Build an ext4 /var/lib filesystem

    if BR2_TARGET_ROOTFS_VAR_LIB_EXT4
        [...]
    endif # BR2_TARGET_ROOTFS_VAR_LIB_EXT4

fs/rootfs-var-lib/ext4/ext4.mk:
    # Build the ext4 /var/lib filesystem image
    [...]
    ROOTFS_VAR_LIB_EXT4_OPTS = \
        -d $(TARGET_DIR)/var/lib \
                        ^^^^^^^^---- /var/lib, only
    [...]
    define ROOTFS_VAR_LIB_EXT4_CMD
        rm -f $@
        $(HOST_DIR)/sbin/mkfs.ext4 $(ROOTFS_VAR_LIB_EXT4_OPTS) $@ "$(ROOTFS_VAR_LIB_EXT4_SIZE)"
    endef
    $(eval $(rootfs-var-lib))

Selecting BR2_TARGET_ROOTFS_VAR_LIB_EXT4 would produce the filesystem
image $(BINARIES_DIR)/rootfs.var-lib-ext4 coresponging to the contents
of $(TARGET_DIR)/var/lib. This is correct but the file name is a bit
awkward. It would be better to call it "rootfs-var-lib.ext4", instead.

This can be achieved by passing a third argument to inner-rootfs with
the desired file name:

    rootfs-var-lib = $(call inner-rootfs,var-lib-$(pkgname),VAR_LIB_$(call UPPERCASE,$(pkgname)),rootfs-var-lib.$(pkgname))

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
---
 fs/common.mk | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Carlos, All,

On 2018-06-21 23:31 -0300, Carlos Santos spake thusly:
> Suppose that you want to create a filesystem image containing only the
> /var/lib subtree. That would something like this on a br2-external:
[--SNIP--]
> This can be achieved by passing a third argument to inner-rootfs with
> the desired file name:
>     rootfs-var-lib = $(call inner-rootfs,var-lib-$(pkgname),VAR_LIB_$(call UPPERCASE,$(pkgname)),rootfs-var-lib.$(pkgname))

We've discussed this patch during the developers days, and we came to
the conclusion that this should be better done by allowing filesystems
to provide the name of their output file as a variable, something like:

    ROOTFS_FOOBAR_IMAGE_NAME = my-own-filesystem.data

Then the infrastructure would do:

    ROOTFS_$(2)_IMAGE_NAME ?= rootfs.$(1)

    $$(BINARIES_DIR)/$$(ROOTFS_$(2)_IMAGE_NAME): ROOTFS=$(2)
    $$(BINARIES_DIR)/$$(ROOTFS_$(2)_IMAGE_NAME): FAKEROOT_SCRIPT=$$(ROOTFS_$(2)_DIR)/fakeroot
    [...and so on...]

Using a variable is more inline with how we handle this kind of
customisations in Buildrooot, and allows one to continue calling the
'rootfs' macro instad of the 'rootfs-inner' one (which, as its name
implies, is an inner macro, aka purely for internal use, not public).

Besides, the commit log was seen as too decorelated from the actual
change. It is not needed to explain the 'sub-tree-only' case, and just
state that some filesystems may want to tweak their output name, rather
than the current scheme.

Regards,
Yann E. MORIN.

> Signed-off-by: Carlos Santos <casantos@datacom.com.br>
> ---
>  fs/common.mk | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/common.mk b/fs/common.mk
> index abf35418cb..2380c1dd3f 100644
> --- a/fs/common.mk
> +++ b/fs/common.mk
> @@ -139,10 +139,10 @@ ROOTFS_$(2)_COMPRESS_EXT = .xz
>  ROOTFS_$(2)_COMPRESS_CMD = xz -9 -C crc32 -c
>  endif
>  
> -$$(BINARIES_DIR)/rootfs.$(1): ROOTFS=$(2)
> -$$(BINARIES_DIR)/rootfs.$(1): FAKEROOT_SCRIPT=$$(ROOTFS_$(2)_DIR)/fakeroot
> -$$(BINARIES_DIR)/rootfs.$(1): $$(ROOTFS_$(2)_DEPENDENCIES)
> -	@$$(call MESSAGE,"Generating root filesystem image rootfs.$(1)")
> +$$(BINARIES_DIR)/$(3): ROOTFS=$(2)
> +$$(BINARIES_DIR)/$(3): FAKEROOT_SCRIPT=$$(ROOTFS_$(2)_DIR)/fakeroot
> +$$(BINARIES_DIR)/$(3): $$(ROOTFS_$(2)_DEPENDENCIES)
> +	@$$(call MESSAGE,"Generating root filesystem image $(3)")
>  	rm -rf $$(ROOTFS_$(2)_DIR)
>  	mkdir -p $$(ROOTFS_$(2)_DIR)
>  	echo '#!/bin/sh' > $$(FAKEROOT_SCRIPT)
> @@ -163,7 +163,7 @@ endif
>  rootfs-$(1)-show-depends:
>  	@echo $$(ROOTFS_$(2)_DEPENDENCIES)
>  
> -rootfs-$(1): $$(BINARIES_DIR)/rootfs.$(1)
> +rootfs-$(1): $$(BINARIES_DIR)/$(3)
>  
>  .PHONY: rootfs-$(1) rootfs-$(1)-show-depends
>  
> @@ -181,6 +181,6 @@ endif
>  endef
>  
>  # $(pkgname) also works well to return the filesystem name
> -rootfs = $(call inner-rootfs,$(pkgname),$(call UPPERCASE,$(pkgname)))
> +rootfs = $(call inner-rootfs,$(pkgname),$(call UPPERCASE,$(pkgname)),rootfs.$(pkgname))
>  
>  include $(sort $(wildcard fs/*/*.mk))
> -- 
> 2.17.1
> 
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Superseded by https://patchwork.ozlabs.org/patch/988446/