tree bb37a32b3613982842ca49e56de2bdbc9ce59df9
parent b25389e583ee33e984c960a179f5cc5108b1432e
author Andrew Jeffery <andrew@aj.id.au> 1630971681 +0930
committer Andrew Jeffery <andrew@aj.id.au> 1631108016 +0930

Add overlay script

`overlay` does the tedious work of setting up and tearing down the
environment required for overlay filesystems. Example use:

```
root@bmc:~# touch /usr/share/vpd/foo
touch: /usr/share/vpd/foo: Read-only file system
root@bmc:~# ./overlay add /usr/share/vpd
[53864.956641] overlayfs: upper fs does not support xattr, falling back to index=off and metacopy=off.
root@bmc:~# touch /usr/share/vpd/foo
root@bmc:~# ls -l /usr/share/vpd/foo
-rw-r--r--    1 root     root             0 Sep  6 23:55 /usr/share/vpd/foo
root@bmc:~# rm /usr/share/vpd/foo
root@bmc:~# ls -l /usr/share/vpd/foo
ls: /usr/share/vpd/foo: No such file or directory
root@bmc:~# ./overlay remove /usr/share/vpd
root@bmc:~# touch /usr/share/vpd/foo
touch: /usr/share/vpd/foo: Read-only file system
```

The `rm -rf` specifically removes the temporary directories specified
in the mount options:

```
root@bmc:~# ./overlay add /usr/share/vpd
root@bmc:~# sh -x ./overlay remove /usr/share/vpd
+ set -eu
+ '[' 2 -lt 2 ']'
+ case $1 in
+ o_lowerdir=/usr/share/vpd
++ fstab_fs_mntopt /usr/share/vpd
++ awk '"overlay" == $1 && "/usr/share/vpd" == $2 { printf("%s\n", $4)
}'
+
o_options=rw,relatime,lowerdir=/usr/share/vpd,upperdir=/tmp/tmp.v8vJ35,workdir=/tmp/tmp.6v1YJY
+ umount /usr/share/vpd
+ '[' -z
rw,relatime,lowerdir=/usr/share/vpd,upperdir=/tmp/tmp.v8vJ35,workdir=/tmp/tmp.6v1YJY
']'
+ fstab_option_iter
+ tr , '\n'
+ echo
rw,relatime,lowerdir=/usr/share/vpd,upperdir=/tmp/tmp.v8vJ35,workdir=/tmp/tmp.6v1YJY
+ read FS_MNTOPT
+ case $FS_MNTOPT in
+ read FS_MNTOPT
+ case $FS_MNTOPT in
+ read FS_MNTOPT
+ case $FS_MNTOPT in
+ read FS_MNTOPT
+ case $FS_MNTOPT in
++ echo upperdir=/tmp/tmp.v8vJ35
++ fstab_option_value
++ cut -d= -f2
+ rm -rf /tmp/tmp.v8vJ35
+ read FS_MNTOPT
+ case $FS_MNTOPT in
++ fstab_option_value
++ cut -d= -f2
++ echo workdir=/tmp/tmp.6v1YJY
+ rm -rf /tmp/tmp.6v1YJY
+ read FS_MNTOPT
root@bmc:~#
```

It also safely handles bad remove invocations:

```
root@bmc:~# sh -x ./overlay remove /asdf
+ set -eu
+ '[' 2 -lt 2 ']'
+ case $1 in
+ o_lowerdir=/asdf
++ fstab_fs_mntopt /asdf
++ awk '"overlay" == $1 && "/asdf" == $2 { printf("%s\n", $4) }'
+ o_options=
+ umount /asdf
umount: /asdf: no mount point specified.
root@bmc:~# sh -x ./overlay remove /
+ set -eu
+ '[' 2 -lt 2 ']'
+ case $1 in
+ o_lowerdir=/
++ fstab_fs_mntopt /
++ awk '"overlay" == $1 && "/" == $2 { printf("%s\n", $4) }'
+ o_options=
+ umount /
+ '[' -z '' ']'
+ echo / is not an overlay
/ is not an overlay
+ exit 1
root@bmc:~#
```

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I0118c645d9edf391d9670aacc889f31d1e1b2f5f