lxd/storage/drivers: Revert to filepath traversal if subvolume list f…

…ails.

Signed-off-by: Mark Laing <mark.laing@canonical.com>

lxd/storage/drivers/driver_btrfs_utils.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -107,39 +108,67 @@ func (d *btrfs) hasSubvolumes(path string) (bool, error) {
 }
 
 func (d *btrfs) getSubvolumes(path string) ([]string, error) {
+	// Make sure the path has a trailing slash.
+	if !strings.HasSuffix(path, "/") {
+		path = path + "/"
+	}
+
 	poolMountPath := GetPoolMountPath(d.name)
 	if !strings.HasPrefix(path, poolMountPath+"/") {
 		return nil, fmt.Errorf("%q is outside pool mount path %q", path, poolMountPath)
 	}
 
-	path = strings.TrimPrefix(path, poolMountPath+"/")
-
-	// Make sure the path has a trailing slash.
-	if !strings.HasSuffix(path, "/") {
-		path = path + "/"
-	}
+	var result []string
 
+	// Attempt to run `btrfs subvolume list`. This may fail inside a nested container, where the container does not have
+	// sufficient permission to run the command.
 	var stdout bytes.Buffer
 	err := shared.RunCommandWithFds(d.state.ShutdownCtx, nil, &stdout, "btrfs", "subvolume", "list", poolMountPath)
-	if err != nil {
-		return nil, err
+	if err == nil {
+		path = strings.TrimPrefix(path, poolMountPath+"/")
+		scanner := bufio.NewScanner(&stdout)
+		for scanner.Scan() {
+			fields := strings.Fields(scanner.Text())
+
+			if len(fields) != 9 {
+				continue
+			}
+
+			if !strings.HasPrefix(fields[8], path) {
+				continue
+			}
+
+			result = append(result, strings.TrimPrefix(fields[8], path))
+		}
+
+		return result, nil
 	}
 
-	result := []string{}
+	// Revert to walking the directory tree looking for subvolumes.
+	err = filepath.Walk(path, func(fpath string, entry fs.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
 
-	scanner := bufio.NewScanner(&stdout)
-	for scanner.Scan() {
-		fields := strings.Fields(scanner.Text())
+		// Ignore the base path.
+		if strings.TrimRight(fpath, "/") == strings.TrimRight(path, "/") {
+			return nil
+		}
 
-		if len(fields) != 9 {
-			continue
+		// Subvolumes can only be directories.
+		if !entry.IsDir() {
+			return nil
 		}
 
-		if !strings.HasPrefix(fields[8], path) {
-			continue
+		// Check if directory is a subvolume.
+		if d.isSubvolume(fpath) {
+			result = append(result, strings.TrimPrefix(fpath, path))
 		}
 
-		result = append(result, strings.TrimPrefix(fields[8], path))
+		return nil
+	})
+	if err != nil {
+		return nil, err
 	}
 
 	return result, nil