wic: Copy rootfs dir if fstab needs updating

By default, wic updates the /etc/fstab in the rootfs to include details
of additional partitions described in the selected wks file. If this
modification is performed in place, other tasks which create an image
file from the rootfs directory (e.g. do_image_tar and do_image_ext4)
will pick up the modified fstab file which would not be appropriate for
those images as they do not include the additional partitions described
in the wks file. wic does undo modifications to the fstab file once it
has finished creating the filesystem image, however this leaves open a
race condition if one of the other tasks reads the contents of the fstab
file from the rootfs directory between the point where wic modifies the
fstab file and the point where wic restores the files original content.

This could be solved by adding a lockfile for tasks which use the rootfs
directory to ensure that no other such task is reading the rootfs
directory while do_image_wic is running. This would serialize several
do_image_* tasks and result in slower builds, especially for large
images. Another drawback of this solution is that it is hard to
selectively optimise - adding lockfiles to do_image_* tasks would result
in these tasks always being serialized even if no fstab modification
will take place.

An alternative solution is to copy the rootfs directory when fstab needs
to be modified. The code to do this in wic already exists as it is
needed when including or excluding content in the rootfs. This still
results in an impact on build times but the copy uses hardlinks if
possible (so little data is actually copied) and we can make selective
optimisations to improve things. The rootfs copy will only take place if
fstab modification is required (or if it was already needed to include
or exclude rootfs content). We can also follow up with further
optimisations after this commit. So this second solution is chosen.

Fixes [Yocto #13994]

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/scripts/lib/wic/partition.py b/scripts/lib/wic/partition.py
index 286c7867cb..f59eceb23d 100644
--- a/scripts/lib/wic/partition.py
+++ b/scripts/lib/wic/partition.py
@@ -54,6 +54,7 @@ class Partition():
         self.uuid = args.uuid
         self.fsuuid = args.fsuuid
         self.type = args.type
+        self.updated_fstab_path = None
 
         self.lineno = lineno
         self.source_file = ""
@@ -118,11 +119,13 @@ class Partition():
         return self.fixed_size if self.fixed_size else self.size
 
     def prepare(self, creator, cr_workdir, oe_builddir, rootfs_dir,
-                bootimg_dir, kernel_dir, native_sysroot):
+                bootimg_dir, kernel_dir, native_sysroot, updated_fstab_path):
         """
         Prepare content for individual partitions, depending on
         partition command parameters.
         """
+        self.updated_fstab_path = updated_fstab_path
+
         if not self.source:
             if not self.size and not self.fixed_size:
                 raise WicError("The %s partition has a size of zero. Please "