[arch-projects] [devtools] [PATCH] makechrootpkg: Be recursive when deleting btrfs subvolumes.

[Dave Reisner]

On Fri, Feb 10, 2017 at 03:46:08AM -0500, lukeshu at lukeshu.com wrote:
> From: Luke Shumaker <lukeshu at sbcglobal.net>
> 
> Motivation:
> 
>   When installing the necessaryssary dependencies in the chroot, the
>   ALPM hooks run; and if 'systemd' is a dependency, then one of the
>   hooks is to run systemd-tmpfiles.  There are several tmpfiles.d(5)
>   commands that instruct it to create btrfs subvolumes if on btrfs
>   (the `v`, `q`, and `Q` commands).
> 
>   This causes a problem when we go to delete the chroot.  The command
>   `btrfs subvolume delete` won't recursively delete subvolumes; if a
>   child subvolume was created, it will fail with the fairly unhelpful
>   error message "directory not empty".
> 
> Solution:
> 
>   Because the subvolume that gets mounted isn't necessarily the
>   toplevel subvolume, and `btrfs subvolume list` gives us paths
>   relative to the toplevel; we need to figure out how our path relates
>   to the toplevel.  Figure out the mountpoint (which turns out to be
>   slightly tricky; see below), and call `btrfs subvolume list -a` on
>   it to get the list of subvolumes that are visible to us (and quite
>   possibly some that aren't; the logic for determining which ones it
>   shows is... absurd).  This gives us a list of subvolumes with
>   numeric IDs, and paths relative to the toplevel (actually it gives
>   us more than that, and we use a hopefully-correct `sed` expression
>   to trim it down; the format certainly isn't human-friendly, but it's
>   not machine-friendly either.)  So then we look at that list of pairs
>   and find the one that matches the ID of the subvolume we're trying
>   to delete (which is easy to get with `btrfs subvolume show`); once
>   we've found the path of our subvolume, we can use that to filter and
>   trim the complete list of paths.  From there the remainder of the
>   solution is obvious.
> 
>   Now, back to "figure out the mountpoint"; the normal `stat -c %m`
>   doesn't work.  It gives the mounted path of the subvolume closest to
>   the path we give it, not the actual mountpoint.  Now, it turns out
>   that `df` can figure out the correct mountpoint (though I haven't
>   investigated how it knows when stat doesn't; but I suspect it parses
>   `/proc/mounts`).  So we are reduced to parsing `df`'s output.
> ---

If you need to write a commit message this long just to remove what's
currently a single nested subvolume, then I agree with Jan that we need
to rethink our chroot management, or btrfs needs to improve its tooling
(I tend to think the latter is true independent of this patch).

That said, this patch does seem to work as of this writing. I have
concerns that it's a little fragile and might break in the future.

>  makechrootpkg.in | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 57 insertions(+), 2 deletions(-)
> 
> diff --git a/makechrootpkg.in b/makechrootpkg.in
> index 5c4b530..01e9e96 100644
> --- a/makechrootpkg.in
> +++ b/makechrootpkg.in
> @@ -80,6 +80,61 @@ load_vars() {
>  	return 0
>  }
>  
> +# Usage: btrfs_subvolume_id $SUBVOLUME
> +btrfs_subvolume_id() (
> +	set -o pipefail
> +	LC_ALL=C btrfs subvolume show "$1" | sed -n 's/^\tSubvolume ID:\s*//p'

This looks like you're parsing human readable output to get the subvol
ID. Is this really the only way to get this information?

> +)
> +
> +# Usage: btrfs_subvolume_list_all $FILEPATH
> +#
> +# Given $FILEPATH somewhere on a mounted btrfs filesystem, print the
> +# ID and full path of every subvolume on the filesystem, one per line
> +# in the format "$ID $PATH".
> +btrfs_subvolume_list_all() (
> +	set -o pipefail
> +	local mountpoint
> +	mountpoint="$(df --output=target "$1" | sed 1d)" || return $?

"return $?" is a long winded way of just "return". The status is
implied.

> +	LC_ALL=C btrfs subvolume list -a "$mountpoint" | sed -r 's|^ID ([0-9]+) .* path (<FS_TREE>/)?(\S*).*|\1 \3|'

You might want to validate that you actually get an integer back here,
rather than just assuming the calls succeed. You cannot rely on sed to
provide a useful exit status.

> +)
> +
> +# Usage: btrfs_subvolume_list $SUBVOLUME
> +#
> +# Assuming that $SUBVOLUME is a btrfs subvolume, list all child
> +# subvolumes; from most deeply nested to most shallowly nested.
> +#
> +# This is intended to be a sane version of `btrfs subvolume list`.
> +btrfs_subvolume_list() {
> +	local subvolume=$1
> +
> +	local id all path subpath
> +	id="$(btrfs_subvolume_id "$subvolume")" || return $?
> +	all="$(btrfs_subvolume_list_all "$subvolume")" || return $?
> +	path="$(sed -n "s/^$id //p" <<<"$all")"

Rather than injecting unvalidated data into a sed program, I'd suggest
using awk:

path=$(awk -v id="$1" '$1 == id { sub($1 FS, ""); print }' <<<"$all")

> +	while read -r id subpath; do
> +		if [[ "$subpath" = "$path"/* ]]; then
> +			printf '%s\n' "${subpath#"${path}/"}"
> +		fi
> +	done <<<"$all" | LC_ALL=C sort --reverse
> +}
> +
> +# Usage: btrfs_subvolume_delete $SUBVOLUME
> +#
> +# Assuming that $SUBVOLUME is a btrfs subvolume, delete it and all
> +# subvolumes below it.
> +#
> +# This is intended to be a recursive version of
> +# `btrfs subvolume delete`.
> +btrfs_subvolume_delete() {
> +	local dir="$1"
> +	local subvolumes subvolume
> +	subvolumes=($(btrfs_subvolume_list "$dir")) || return $?

This is a broken way to read lines into an array, because you're not
reading lines at all -- you're reading words.

> +	for subvolume in "${subvolumes[@]}"; do
> +		btrfs subvolume delete "$dir/$subvolume" || return $?
> +	done
> +	btrfs subvolume delete "$dir"
> +}
> +
>  create_chroot() {
>  	# Lock the chroot we want to use. We'll keep this lock until we exit.
>  	lock 9 "$copydir.lock" "Locking chroot copy [%s]" "$copy"
> @@ -92,7 +147,7 @@ create_chroot() {
>  		stat_busy "Creating clean working copy [%s]" "$copy"
>  		if [[ "$chroottype" == btrfs ]] && ! mountpoint -q "$copydir"; then
>  			if [[ -d $copydir ]]; then
> -				btrfs subvolume delete "$copydir" >/dev/null ||
> +				btrfs_subvolume_delete "$copydir" >/dev/null ||
>  					die "Unable to delete subvolume %s" "$copydir"
>  			fi
>  			btrfs subvolume snapshot "$chrootdir/root" "$copydir" >/dev/null ||
> @@ -114,7 +169,7 @@ create_chroot() {
>  clean_temporary() {
>  	stat_busy "Removing temporary copy [%s]" "$copy"
>  	if [[ "$chroottype" == btrfs ]] && ! mountpoint -q "$copydir"; then
> -		btrfs subvolume delete "$copydir" >/dev/null ||
> +		btrfs_subvolume_delete "$copydir" >/dev/null ||
>  			die "Unable to delete subvolume %s" "$copydir"
>  	else
>  		# avoid change of filesystem in case of an umount failure
> -- 
> 2.11.1