Clean up

src/backup.sh

@@ -16,27 +16,7 @@ source /src/borg-common.sh
 archive_prefix=$(borg_literal_prefix archive "$ARCHIVE_PREFIX")
 archive_glob=$(borg_literal_prefix glob "$ARCHIVE_PREFIX")
 
-# Only a missing repository (modern exit code 13) triggers initialization.
-info_status=0
-borg_wait info || info_status=$?
-case "$info_status" in
-    0) ;;
-    13)
-        echo "Repository does not exist. Initializing Borg..."
-        init_status=0
-        borg_wait init --encryption=repokey || init_status=$?
-        case "$init_status" in
-            0) ;;
-            # Another client may have initialized the repository in between.
-            10) borg_wait info ;;
-            *) exit "$init_status" ;;
-        esac
-        ;;
-    *)
-        echo "Cannot access repository (Borg exit status $info_status). Skipping backup." >&2
-        exit "$info_status"
-        ;;
-esac
+borg_ensure_repository
 
 cleanup() {
     local status=$?
@@ -54,7 +34,16 @@ fi
 
 /src/snapshot.sh create /btrfs-root /snapshot/btrfs-root
 
-cd "/snapshot/btrfs-root${BACKUP_RELATIVE_PATH:-}"
+case ${BACKUP_RELATIVE_PATH:-} in
+    ''|/*) ;;
+    *) echo 'BACKUP_RELATIVE_PATH must start with /.' >&2; exit 1 ;;
+esac
+IFS= read -r -d '' backup_directory < <(realpath -ez -- "/snapshot/btrfs-root${BACKUP_RELATIVE_PATH:-}")
+if [[ $backup_directory != /snapshot/btrfs-root && $backup_directory != /snapshot/btrfs-root/* ]]; then
+    echo 'BACKUP_RELATIVE_PATH must resolve to a directory within the snapshot.' >&2
+    exit 1
+fi
+cd -- "$backup_directory"
 
 borg_wait create --stats \
     --list \

src/borg-common.sh

@@ -2,6 +2,26 @@
 
 export BORG_RSH='ssh -oBatchMode=yes'
 
+borg_ensure_repository() {
+    local status=0
+    borg_wait info || status=$?
+    (( status != 0 )) || return 0
+    if (( status == 13 )) || (( status == 15 )); then
+        echo 'Repository is missing or confirmed empty. Initializing Borg...'
+        status=0
+        borg_wait init --encryption=repokey || status=$?
+        # Another client may have initialized it after the initial check.
+        if (( status == 10 )); then
+            borg_wait info
+        else
+            return "$status"
+        fi
+    else
+        echo "Cannot access repository (Borg exit status $status). Skipping backup." >&2
+        return "$status"
+    fi
+}
+
 # Borg expands placeholders in both names and archive filters. Only the filter
 # needs glob escaping; user-supplied prefixes are literal in both places.
 borg_literal_prefix() {