[Buildroot] [PATCH] fs: allow extra arguments to common tarball extraction

[Carlos Santos]

Since commit 118534fe54b (fs: use a common tarball as base for the other
filesystems) Buildroot creates a .tar filesystem image and re-extracts
it in a private directory to create each format-specific image. Add an
option to pass extra arguments to tar when that commom root image is
extracted.

This option is useful when the root filesystem is volatile (e.g. initrd)
or read-only but a read-write subtree is still necessary for persistent
data modified by programs as they run.

For example, one can pass "--exclude='./var/lib/*'" to exclude that path
from the rootfs image and use a post-fakeroot script to make a separate
filesystem image for /var/lib.

Signed-off-by: Carlos Santos <casantos at datacom.com.br>
---
Additional explanation

This change solves a real-life problem of my current project. Before
buildroot 2018.05 I had to use a trickier approach:

1. Add a post-fakeroot script to
   1.1. Create a filesystem image (ext4) from $(TARGET_DIR)/var/lib.
   1.2. Move $(TARGET_DIR)/var/lib to a backup path under $(BUILD_DIR).
   1.3. Create an empty $(TARGET_DIR)/var/lib.
2. Add a post-image script to
   2.1. Copy the rootfs and /var/lib images to the disk image.
   2.2. Remove the empty $(TARGET_DIR)/var/lib.
   2.3. Restore $(TARGET_DIR)/var/lib from the backup path.

That solution is fragile: if something goes wrong during the execution
of the post-image script it can lead to a cripled $(TARGET_DIR) with an
empty var/lib subdir. Rebuilding a package without restoring the backup
can easily cause a kaboom and force me to run a lengthy clean build.

I added some protection mechanisms to stop the build if a stale /var/lib
backup is found but quite frankly the whole thing stinks.

The solution with custom extraction arguments, OTOH, is bullet-proof
because it does not change $(TARGET_DIR), so it is always safe to run
dirty builds.
---
 docs/manual/customize-rootfs.txt | 10 ++++++++++
 fs/Config.in                     | 13 +++++++++++++
 fs/common.mk                     |  3 ++-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/docs/manual/customize-rootfs.txt b/docs/manual/customize-rootfs.txt
index bb6d8da6bf..6dd9365723 100644
--- a/docs/manual/customize-rootfs.txt
+++ b/docs/manual/customize-rootfs.txt
@@ -121,6 +121,16 @@ This method is not recommended because it duplicates the entire
   skeleton, which prevents taking advantage of the fixes or improvements
   brought to the default skeleton in later Buildroot releases.
 
+Rootfs tarball extraction arguments (+BR2_ROOTFS_COMMON_UNTAR_ARGS+)::
++
+Buildroot creates a .tar filesystem image and re-extracts it in a private
+ directory to create each format-specific image. Use this option to pass
+ extra arguments to tar when the commom root image is extracted.
++
+For example, you can pass "--exclude=\'./var/lib/*\'" to exclude that path from
+ the rootfs image and use a post-fakeroot script (see below) to create a
+ separate filesystem image for '/var/lib'.
+
 Post-fakeroot scripts (+BR2_ROOTFS_POST_FAKEROOT_SCRIPT+)::
 +
 When aggregating the final images, some parts of the process requires
diff --git a/fs/Config.in b/fs/Config.in
index c25b01c3de..74487b1172 100644
--- a/fs/Config.in
+++ b/fs/Config.in
@@ -1,5 +1,18 @@
 menu "Filesystem images"
 
+config BR2_ROOTFS_COMMON_UNTAR_ARGS
+	string "Rootfs tarball extraction arguments"
+	help
+	  Buildroot creates a .tar filesystem image and re-extracts it
+	  in a private directory to create each format-specific image.
+	  Use this option to pass extra arguments to tar when the commom
+	  root image is extracted.
+
+	  For example, you can pass "--exclude='./var/lib/*'" to exclude
+	  that path from the rootfs image and use a post-fakeroot script
+	  (see BR2_ROOTFS_POST_FAKEROOT_SCRIPT) to create a separate
+	  filesystem image for /var/lib.
+
 source "fs/axfs/Config.in"
 source "fs/cloop/Config.in"
 source "fs/cpio/Config.in"
diff --git a/fs/common.mk b/fs/common.mk
index abf35418cb..33ad1ab2a9 100644
--- a/fs/common.mk
+++ b/fs/common.mk
@@ -50,9 +50,10 @@ define ROOTFS_COMMON_TAR_CMD
 endef
 
 # Command to extract the common tarball into the per-rootfs target directory
+ROOTFS_COMMON_UNTAR_ARGS = $(call qstrip,$(BR2_ROOTFS_COMMON_UNTAR_ARGS))
 define ROOTFS_COMMON_UNTAR_CMD
 	mkdir -p $(TARGET_DIR)
-	tar xf $(ROOTFS_COMMON_TAR) -C $(TARGET_DIR)
+	tar xf $(ROOTFS_COMMON_TAR) -C $(TARGET_DIR) $(ROOTFS_COMMON_UNTAR_ARGS)
 endef
 
 .PHONY: rootfs-common
-- 
2.17.0