[v2] feat: lvm: Add lvm-based backup functionality

This commit implements the needs of #75[^1]: it allows for the
creation of atomic backups when the backup target is a file/dir whose
fs rests on LVM. This ensures the snapshot will be atomic.

By using a mount namespace, the LVM snapshot can be done in the same
directory -- so existing LVM-based applications of ansible_role_restic
can be migrated to this implementation without any discontinuity in
what appears to be backed up.

This combination of LVM's snapshotting layer and mount namespaces
comes with some caveats:

- you cannot backup / due to namespace issues
- subdirs with a separate fs won't be correctly detected
- not all filesystems are happy about LVM snapshots -- btrfs, e.g.
- LVM snapshots come with a performance penalty when active
- fstrim and LVM snapshots don't like each other whatsoever

[^1]: https://github.com/roles-ansible/ansible_role_restic/issues/75

--
Changes in v2:
- Use `findmnt -v` to find snapshot when cleaning up
- Check for _snap before `lvremove -y`

Signed-off-by: Martin Kennedy <hurricos@gmail.com>

templates/restic.service.j2

@@ -1,8 +1,15 @@
 [Unit]
 Description=Backup {{ item.name }} using restic
+{% if item.lvm is defined %}
+Conflicts=fstrim.service
+After=fstrim.timer
+{% endif %}
 
 [Service]
 Type=oneshot
+{% if item.lvm is defined %}
+PrivateMounts=on
+{% endif %}
 ExecStart={{ restic_script_dir }}/backup-{{ item.name }}.sh
 TimeoutStartSec=0
 Environment="CRON=true"

templates/restic_script_Linux.j2

@@ -62,6 +62,67 @@ export B2_ACCOUNT_KEY={{ restic_repos[item.repo].b2_account_key }}
 BACKUP_SOURCE={{ item.src }}
 {% endif %}
 
+{% if item.lvm is defined %}
+# Set up functions for LVM.
+
+function mount_opt_map {
+    mount_type="$1"
+    case "$mount_type" in
+        xfs)
+            echo "noatime,nouuid"
+            ;;
+        ext4)
+            echo "noatime"
+            ;;
+        *)
+            echo "noatime"
+    esac
+}
+
+function prepare_vol {
+    local path="$1"
+    [ -d "$path" ] || path="$(dirname "$path")"
+
+    # TODO: path cannot be /,
+    ## nor can it be where restic is
+    {
+        local source="$(findmnt -J -T ${path} | jq -r '.filesystems[0].source')"
+        local target="$(findmnt -J -T ${path} | jq -r '.filesystems[0].target')"
+        subdir=${path##$target}
+        echo "Creating snapshot ..."
+        lvcreate -y -L "${size:-10G}" -s -n "${source}_snap" "${source}"
+
+        local tmpdir="$(mktemp -d)"
+        local fs="$(lsblk -J --fs "$source" | jq -r '.blockdevices[0]|.fstype')"
+        echo "Identified fstype: $fs; using opts $(mount_opt_map "$fs") ..."
+        mount -t "$fs" \
+              -o "$(mount_opt_map "$fs")" \
+              --make-private \
+              -m \
+              "${source}_snap" "${tmpdir}"
+
+        mount -m --bind --make-private "${tmpdir}/${subdir}" "${path}"
+    }
+}
+
+function cleanup_vol {
+    local path="$1"
+    [ -d "$path" ] || path="$(dirname "$path")"
+
+    {
+        local source="$(findmnt -v -f -J -T ${path} | jq -r '.filesystems[0].source')"
+        echo "Cleaning up mount ..."
+        umount "${path}"
+
+        echo "Cleaning up snapshot ..."
+        if ! grep -q '_snap$' <<< $source; then
+            echo "Snapshot for ${path} could not be found (found: ${source}). Exiting!" && return 1;
+        fi
+        umount "${source}"
+        lvremove -y "${source}";
+    }
+}
+{% endif %}
 
 set -uxo pipefail
 {#
@@ -150,10 +211,14 @@ fi
     {% if item.exclude is defined %}{{ exclude(item.exclude) }}{% endif %} \
     $@ \
 {% else %}
+{
+  {% if item.lvm is defined %}prepare_vol $BACKUP_SOURCE &&{% endif %}
   {{ restic_install_path }}/restic backup $BACKUP_SOURCE $MODE_TAG \
     {{ tags(item.tags) }} \
     {% if item.exclude is defined %}{{ exclude(item.exclude) }}{% endif %} \
     $@ \
+    {% if item.lvm is defined %}&& cleanup_vol $BACKUP_SOURCE{% endif %};
+} \
 {% endif %} {{ backup_output_log }}
 if [[ $? -eq 0 ]]
 then
@@ -166,6 +231,9 @@ else
         {{ ' ' }}We tried to backup '{{ item.src }}'.
       {%- endif -%}
       {{ ' ' }}Please repair the restic-{{ item.name | replace(' ', '') }} job."
+{% if item.lvm is defined %}
+    cleanup_vol $BACKUP_SOURCE
+{% endif %}
 {% endif %}
 fi