Re: [PATCH] expand-on-first-boot: wait for udev to create symlink

[Henning Schild <henning.schild@siemens.com>]

Am Thu, 8 Dec 2022 17:55:42 +0100
schrieb "T. Schaffner" <tobias.schaffner@siemens.com>:

> From: Tobias Schaffner <tobias.schaffner@siemens.com>
> 
> systemd-growfs depends on a symlink to the partition of the filesystem
> that should be resized. This symlink is created by udev in
> /dev/block/.
> 
> If this symlink is not yet created for example because systemd-udev is
> not up yet systemd-growfs will fail.
> 
> We could use Require and After to depend on the systemd-udev service
> but this could again create a race condition if udev is up but not
> fast enough after the partx -u.
> 
> Resolve the symlinks in /dev/block/ periodically until the symlink
> appears before running systemd-growfs.
> ---
>  .../files/expand-last-partition.sh               | 16
> +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git
> a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> index 57055cc..7ebb3e5 100755 ---
> a/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> +++
> b/meta/recipes-support/expand-on-first-boot/files/expand-last-partition.sh
> @@ -38,7 +38,8 @@ if [ "$DISK_SIZE" -lt "$MINIMAL_SIZE" ]; then exit
> 0 fi -LAST_PART="$(sfdisk -d "${BOOT_DEV}" 2>/dev/null | tail -1 |
> cut -d ' ' -f 1)" +LAST_PART_NAME="$(lsblk -l -o NAME "${BOOT_DEV}" |
> tail -1)" +LAST_PART="/dev/${LAST_PART_NAME}"
>  
>  # Transform the partition table as follows:
>  #
> @@ -79,6 +80,19 @@ if [ ! -d "${MOUNT_POINT}" ]; then
>  	exit 1
>  fi
>  
> +START_TIME="$(date +%s)"
> +
> +# Wait for udev to create the symlink to the partition in

Newer systemd versions will not need that anymore. Maybe write down
which version that would be and starting from which debian distro we
can drop that. Who knows when that udev stuff changes and the symlinks
will never appear ... We should only wait for them in systemd versions
that use them.

If we already have a version out there ... like bookworm? We can
already implement it without the wait.

> /dev/block/ as +# systemd-growfs depends on it
> +while ! readlink /dev/block/* | grep -q "${LAST_PART_NAME}"; do

I would make that much stricter to not be tricked by partial matches 

sda4 vs sda42

readlink -f /dev/block/* | grep -e -q "^${LAST_PART}$"

> +    sleep 0.1

I wonder if there is anything we could do here. Maybe "udevadm trigger"
and depend on udev after all. Because the 5 is a nasty guess ...

> +    CURRENT_TIME="$(date +%s)"
> +    if [ $(( CURRENT_TIME - START_TIME )) -gt "5" ]; then

we can simply i++ > 50, no need to call date

so maybe

err=1
for i in $(seq 0 50); do
	if readlink -f /dev/block/* | grep -e -q "^${LAST_PART}$"; then
		err=0
		break
	fi
	sleep 0.1
done

if $err ...
> +        echo "Could not find symlink to last part in /dev/block/."
> +        exit 1
> +    fi
> +done
> +
>  mount "${LAST_PART}" "${MOUNT_POINT}"
>  /lib/systemd/systemd-growfs "${MOUNT_POINT}"

we could also loop over calling this until it goes "0" or we reach a
retry counter, that way we magically handle new systemd versions that
do not need symlinks and do not implement anything with symlinks.

Henning

>  umount "${MOUNT_POINT}"