Script that safely updates with Pacman without crashes when using btrfs and snapper

.sh

#!/bin/bash
# This bash script bypasses the crashing problem when using BTRFS and Snapper with Pacman. Useful as long as the Pacman
# space computation bug is not fixed.
# This script expects only a single mountpoint to be updated, and only the root config of snapper to be used.
# `bc` (basic calculator) needs to be installed which did NOT come by default with my Garuda Plasma installation.
if [ $(id -u) != 0 ]; then
	exec sudo -s "$0" "$@";
fi

# does not work, it will not show the dialog for unknown reason, same with send-notify
true || {
	notificationCommand='
	if (( $exitCode )); then
		kdialog --warningyesno "System Update: Snapshot cleanup aborted"'\''!'\'' "Still require $(bc -q <<< "$(( requiredSpace - availableSpace )) / 1024^2" ) MiB." --yes-label "pacman -Scc" --no-label "cancel"
		case $? in
			0 ) pacman -Scc; exec sudo -s "'$0'" "'$@'" ;;
			1 ) ;;
			* ) ;;
		esac
	else
		kdialog --passivepopup "System Update ready"'\''!'\''
	fi
	'
	trap "$notificationCommand" EXIT   # notify user when script finished to take action for pacman
}

# debugCommand=echo; # if you want to test this script without applying changes

updatedMountpoint="/";
snapperconfig="root";
syncronizationPace=$(( 3 ));  # each synchronization takes long, this tells the script how many snapshots to delete at once before syncronization of BTRFS space
maxSnapshotPercentToRemove=$(( 50 ));   # set here, how many oldest snapshots from the entire `snapper list` may be deleted at most by this script
minSnapshotsPreserved=$(( 12 ));   # do not delete more snapshots if the number is less equal to this limit
maxSnapshotPercentToRemove=$(( $maxSnapshotPercentToRemove >= 100 ? 100 : $maxSnapshotPercentToRemove ));

computeSpaceExpression() {
	echo "$*" | sed -E -e 's/GiB/*1024^3/g' -e 's/GB?/*1000^3/g' -e 's/MiB/*1024^2/g' -e 's/MB?/*1000^2/g' -e 's/KiB/*1024/g' -e 's/KB?/*1000/g' | bc -q;
}

if [ -n "${debugCommand+used}" ]; then
	requiredSpaceThreshold='0';
else
	requiredSpaceThreshold="$(computeSpaceExpression "200MiB")";   # safety gap margin in Bytes; minimum additional required space that must be available
fi

# min x, minimum x, at least x, require x, required x, > x, >= x  is accepted as argument
if minArgument=$(echo "${*}" | pcre2grep -i -o1 '(?<=^|\s)(?:min(?:imum)|at least|required?|>=?) (\S+)' ) \
	&& minArgument=$(computeSpaceExpression "$minArgument") \
	&& minArgument=${minArgument%.*} \
	&& (( $minArgument > ${requiredSpaceThreshold} ));
then
	requiredSpaceThreshold=$minArgument;
fi

computeAvailableSpace() {
	if [ -n "${debugCommand+used}" ]; then echo $(( $RANDOM + ${requiredSpaceThreshold} )); return 0; fi

	# Using grep and cut on program output is fragile in general but there is no easy usable alternative in shell languages.
	computeSpaceExpression "$(btrfs filesystem df "$updatedMountpoint" | grep 'Data, single:' | cut -d' ' -f3- | sed -E 's/total=(.*?),.*? used=(.*?)/\1-\2/')";
}
computeRequiredSpace() {
	if [ -n "${debugCommand+used}" ]; then echo $(( $RANDOM + ${requiredSpaceThreshold} )); return 0; fi

	# alternative to pacman -Qu: checkupdates (slow!)
	computeSpaceExpression "$(pacman -Qu | cut -d' ' -f1 | xargs pacman -Si | grep 'Installed Size' | cut -d':' -f2 | tr '\n' '+' | tr ',' '.') 0";
}

isMoreThanSnapshotLimit() (( $(wc -w <<< "$*") >= ${minSnapshotsPreserved} ))

pacman -Sy > /dev/null; # should be automatically called when the system is updated

${debugCommand} btrfs subvolume sync "$updatedMountpoint";  # force update of BTRFS storage info

availableSpace=$(computeAvailableSpace);
availableSpace=${availableSpace%.*};  # availableSpace converted to int

requiredSpace=$(computeRequiredSpace);
requiredSpace=$((${requiredSpace%.*} + $requiredSpaceThreshold));

if (( $availableSpace >= $requiredSpace )); then
	echo "enough space available ${availableSpace} = $(bc -q <<<"${availableSpace} / 1024^2") MiB > required space ${requiredSpace} = $(bc -q <<<"${requiredSpace} / 1024^2") MiB";
	# all set, go to exit
else
	snapshotNumbers=$(snapper list | grep '^ \?[[:digit:]]' | cut -d' ' -f1 | tr '\n' ' ');

	toRemove=$(( $(wc -w <<< "$snapshotNumbers") * $maxSnapshotPercentToRemove / 100 ));
	toRemove=$(( ($toRemove <= 0 && $maxSnapshotPercentToRemove > 0) ? 1 : $toRemove ));  # as long as maxSnapshotPercentToRemove is set, do remove at least one snapshot

	# remove groups of contiguous snapshots in steps until sufficient memory is available
	while (( $availableSpace < $requiredSpace )) && (( --toRemove >= 0 )) && isMoreThanSnapshotLimit "$snapshotNumbers";
	do
		echo "removing snapshot ${snapshotNumbers%% *}";
		${debugCommand} snapper -c "$snapperconfig" delete ${snapshotNumbers%% *};
		snapshotNumbers=${snapshotNumbers#* };
		if (( ++removedCount % $syncronizationPace != 0 )) && (( toRemove >= 0 )) && isMoreThanSnapshotLimit "$snapshotNumbers"; then
			continue
		fi

		${debugCommand} btrfs subvolume sync "$updatedMountpoint";
		availableSpace=$(computeAvailableSpace);
		availableSpace=${availableSpace%.*};
	done

	if (( $availableSpace < $requiredSpace )); then
		echo -e "Not enough space ${availableSpace} for system update ${requiredSpace}"'!!'
		if (( $maxSnapshotPercentToRemove > 0 )) && isMoreThanSnapshotLimit "$snapshotNumbers"; then
			echo "Run this script again to remove more snapshots";
		elif (( isPaccacheCleanupAllowed )); then
			echo "Still require $(bc -q <<< "$(( requiredSpace - availableSpace )) / 1024^2" ) MiB." 2>&1
			# kdialog --warningyesno "System Update: Snapshot cleanup aborted"'!' "Still require $(bc -q <<< "$(( requiredSpace - availableSpace )) / 1024^2" ) MiB." --yes-label "pacman -Scc" --no-label "cancel"  # not working
			# case $? in
			# 	0 ) pacman -Scc; exec sudo -s "'$0'" "'$@'" ;;
			# 	1 ) ;;
			# 	* ) ;;
			# esac
		fi
		exit ${exitCode:=1};
	fi

	echo -e "Enough space ${availableSpace} = $(bc -q <<<"${availableSpace} / 1024^2") MiB for system update ${requiredSpace} = $(bc -q <<<"${requiredSpace} / 1024^2") MiB available"'!'
fi

echo -e "Manually check the total size as depicted by Pacman"'!'"\n--------------------";
# kdialog --passivepopup "System Update ready"'!'  # does not show anything and stops execution
exit ${exitCode:=0};

Update, I reduced the default batch size of snapshots to be removed in one step. This number depends on how large snapshots are which depends on how often snapshots are made.

I also added a setting "minSnapshotsPreserved" which won't remove snapshots when there are equal or less snapshots available. This number also depends on how big the snapshots are.

I tried to add some notification when the script finished but it seems, kdialog or notify-send do not work here while it works in other smaller scripts.