Only detect subvolume nesting

src/snapshot.sh

@@ -1,106 +1,43 @@
 #!/bin/bash
 
-# Read mountinfo from stdin and fill the caller's associative array. Kernel
-# escapes protect whitespace and backslashes in the mount-point field.
-snapshot_mounted_subvolumes() {
-    local source=${1%/} mount remainder _ignored
-    local -n discovered=$2
-    while IFS=' ' read -r _ignored _ignored _ignored _ignored mount remainder; do
-        [[ $remainder == *' - btrfs '* ]] || continue
-        printf -v mount '%b' "$mount"
-        if [[ $mount == "$source/"* && $mount != "$source/" ]]; then
-            # The nameref fills the caller's array.
-            # shellcheck disable=SC2034
-            discovered["${mount#"$source/"}"]=1
-        fi
-    done
-}
-
 snapshot_create() (
-    local source target parent name directory child relative inode original
-    local -a pending
-    local -A mounts=()
-    unset GLOBIGNORE
-    shopt -s dotglob nullglob
+    local source target=$2 children mount remainder inode _ignored
     IFS= read -r -d '' source < <(realpath -ez -- "$1") || return 1
-    target=$2
     while [[ $target == */ && $target != / ]]; do target=${target%/}; done
-    name=${target##*/}
-    parent=${target%/*}
-    [[ $target == */* ]] || parent=.
-    IFS= read -r -d '' parent < <(realpath -ez -- "${parent:-/}") || return 1
-    target=${parent%/}/$name
     if [[ -e $target || -L $target ]]; then
         printf 'Snapshot destination already exists: %s\n' "$target" >&2
         return 1
     fi
-    snapshot_mounted_subvolumes "$source" mounts < /proc/self/mountinfo || return 1
-    btrfs subvolume snapshot "$source" "$target" || return 1
-    pending=("$target")
-    while (( ${#pending[@]} )); do
-        directory=${pending[-1]}
-        unset 'pending[-1]'
-        if [[ ! -r $directory || ! -x $directory ]]; then
-            printf 'Cannot traverse snapshot directory: %s\n' "$directory" >&2
+
+    children=$(btrfs subvolume list -o "$source") || return 1
+    if [[ -n $children ]]; then
+        printf 'Nested subvolumes are not supported under %s:\n%s\n' "$source" "$children" >&2
+        return 1
+    fi
+
+    # Mounted subvolumes may live elsewhere and are absent from the list above.
+    while IFS=' ' read -r _ignored _ignored _ignored _ignored mount remainder; do
+        [[ $remainder == *' - btrfs '* ]] || continue
+        printf -v mount '%b' "$mount"
+        [[ $mount == "${source%/}/"* && $mount != "$source" ]] || continue
+        inode=$(stat -c %i -- "$mount") || return 1
+        if [[ $inode == 256 && -d $mount ]]; then
+            printf 'Nested subvolumes are not supported: %s\n' "$mount" >&2
             return 1
         fi
-        for child in "$directory"/*; do
-            [[ -d $child && ! -L $child ]] || continue
-            relative=${child#"$target/"}
-            inode=$(stat -c %i -- "$child") || return 1
-            # Unsnapshotted children have inode 2. A mounted subvolume may
-            # instead hide an ordinary directory in its parent's snapshot.
-            if [[ $inode == 2 || ${mounts[$relative]:-} == 1 ]]; then
-                original=${source%/}/$relative
-                if [[ -L $original ]]; then
-                    printf 'Subvolume changed into a symlink: %s\n' "$original" >&2
-                    return 1
-                fi
-                # Never recurse into the scratch snapshot, even below a child.
-                if [[ $original -ef $target ]]; then
-                    rmdir -- "$child" || return 1
-                    continue
-                fi
-                # Refuse to discard data hidden beneath a mounted subvolume.
-                rmdir -- "$child" || return 1
-                btrfs subvolume snapshot "$original" "$child" || return 1
-            fi
-            pending+=("$child")
-        done
-    done
+    done < /proc/self/mountinfo || return 1
+
+    btrfs subvolume snapshot "$source" "$target"
 )
 
 snapshot_delete() (
-    local target=$1 directory child inode index
-    local -a pending subvolumes
-    unset GLOBIGNORE
-    shopt -s dotglob nullglob
+    local target=$1
     while [[ $target == */ && $target != / ]]; do target=${target%/}; done
     if [[ -L $target ]]; then
         printf 'Refusing to delete a symlink as a snapshot: %s\n' "$target" >&2
         return 1
     fi
-    btrfs subvolume show "$target" > /dev/null || return 1
-    pending=("$target")
-    subvolumes=("$target")
-    while (( ${#pending[@]} )); do
-        directory=${pending[-1]}
-        unset 'pending[-1]'
-        if [[ ! -r $directory || ! -x $directory ]]; then
-            printf 'Cannot traverse snapshot directory: %s\n' "$directory" >&2
-            return 1
-        fi
-        for child in "$directory"/*; do
-            [[ -d $child && ! -L $child ]] || continue
-            inode=$(stat -c %i -- "$child") || return 1
-            [[ $inode != 256 ]] || subvolumes+=("$child")
-            pending+=("$child")
-        done
-    done
-    # Reverse discovery order puts every child before its parent.
-    for ((index = ${#subvolumes[@]} - 1; index >= 0; index--)); do
-        btrfs subvolume delete "${subvolumes[index]}" || return 1
-    done
+    btrfs subvolume delete "$target"
 )
 
 if [[ ${BASH_SOURCE[0]} == "$0" ]]; then