feat: add optional sidecar files for metadata

This adds the option to store metadata for objects and buckets
within a specified directory:
bucket: <sidecar dir>/<bucket>/meta/<attribute>
object: <sidecar dir>/bucket/<object>/meta/<attribute>

Example invocation:
./versitygw -a myaccess -s mysecret posix --sidecar /tmp/sidecar /tmp/gw

The attributes are stored by name within the hidden directory.

backend/meta/sidecar.go

@@ -0,0 +1,125 @@
+package meta
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// SideCar is a metadata storer that uses sidecar files to store metadata.
+type SideCar struct {
+	dir string
+}
+
+const (
+	sidecarmeta = "meta"
+)
+
+// NewSideCar creates a new SideCar metadata storer.
+func NewSideCar(dir string) (SideCar, error) {
+	fi, err := os.Lstat(dir)
+	if err != nil {
+		return SideCar{}, fmt.Errorf("failed to stat directory: %v", err)
+	}
+	if !fi.IsDir() {
+		return SideCar{}, fmt.Errorf("not a directory")
+	}
+
+	return SideCar{dir: dir}, nil
+}
+
+// RetrieveAttribute retrieves the value of a specific attribute for an object or a bucket.
+func (s SideCar) RetrieveAttribute(_ *os.File, bucket, object, attribute string) ([]byte, error) {
+	metadir := filepath.Join(s.dir, bucket, object, sidecarmeta)
+	if object == "" {
+		metadir = filepath.Join(s.dir, bucket, sidecarmeta)
+	}
+	attr := filepath.Join(metadir, attribute)
+
+	value, err := os.ReadFile(attr)
+	if errors.Is(err, os.ErrNotExist) {
+		return nil, ErrNoSuchKey
+	}
+	if err != nil {
+		return nil, fmt.Errorf("failed to read attribute: %v", err)
+	}
+
+	return value, nil
+}
+
+// StoreAttribute stores the value of a specific attribute for an object or a bucket.
+func (s SideCar) StoreAttribute(_ *os.File, bucket, object, attribute string, value []byte) error {
+	metadir := filepath.Join(s.dir, bucket, object, sidecarmeta)
+	if object == "" {
+		metadir = filepath.Join(s.dir, bucket, sidecarmeta)
+	}
+	err := os.MkdirAll(metadir, 0777)
+	if err != nil {
+		return fmt.Errorf("failed to create metadata directory: %v", err)
+	}
+
+	attr := filepath.Join(metadir, attribute)
+	err = os.WriteFile(attr, value, 0666)
+	if err != nil {
+		return fmt.Errorf("failed to write attribute: %v", err)
+	}
+
+	return nil
+}
+
+// DeleteAttribute removes the value of a specific attribute for an object or a bucket.
+func (s SideCar) DeleteAttribute(bucket, object, attribute string) error {
+	metadir := filepath.Join(s.dir, bucket, object, sidecarmeta)
+	if object == "" {
+		metadir = filepath.Join(s.dir, bucket, sidecarmeta)
+	}
+	attr := filepath.Join(metadir, attribute)
+
+	err := os.Remove(attr)
+	if errors.Is(err, os.ErrNotExist) {
+		return ErrNoSuchKey
+	}
+	if err != nil {
+		return fmt.Errorf("failed to remove attribute: %v", err)
+	}
+
+	return nil
+}
+
+// ListAttributes lists all attributes for an object or a bucket.
+func (s SideCar) ListAttributes(bucket, object string) ([]string, error) {
+	metadir := filepath.Join(s.dir, bucket, object, sidecarmeta)
+	if object == "" {
+		metadir = filepath.Join(s.dir, bucket, sidecarmeta)
+	}
+
+	ents, err := os.ReadDir(metadir)
+	if errors.Is(err, os.ErrNotExist) {
+		return []string{}, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("failed to list attributes: %v", err)
+	}
+
+	var attrs []string
+	for _, ent := range ents {
+		attrs = append(attrs, ent.Name())
+	}
+
+	return attrs, nil
+}
+
+// DeleteAttributes removes all attributes for an object or a bucket.
+func (s SideCar) DeleteAttributes(bucket, object string) error {
+	metadir := filepath.Join(s.dir, bucket, object, sidecarmeta)
+	if object == "" {
+		metadir = filepath.Join(s.dir, bucket, sidecarmeta)
+	}
+
+	err := os.RemoveAll(metadir)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("failed to remove attributes: %v", err)
+	}
+	return nil
+}

backend/posix/posix.go

@@ -107,9 +107,14 @@ type PosixOpts struct {
 	BucketLinks   bool
 	VersioningDir string
 	NewDirPerm    fs.FileMode
+	SideCarDir    string
 }
 
 func New(rootdir string, meta meta.MetadataStorer, opts PosixOpts) (*Posix, error) {
+	if opts.SideCarDir != "" && strings.HasPrefix(opts.SideCarDir, rootdir) {
+		return nil, fmt.Errorf("sidecar directory cannot be inside the gateway root directory")
+	}
+
 	err := os.Chdir(rootdir)
 	if err != nil {
 		return nil, fmt.Errorf("chdir %v: %w", rootdir, err)
@@ -159,7 +164,13 @@ func New(rootdir string, meta meta.MetadataStorer, opts PosixOpts) (*Posix, erro
 		}
 	}
 
-	fmt.Printf("Bucket versioning enabled with directory: %v\n", verioningdirAbs)
+	if verioningdirAbs != "" {
+		fmt.Printf("Bucket versioning enabled with directory: %v\n", verioningdirAbs)
+	}
+
+	if opts.SideCarDir != "" {
+		fmt.Println("Using sidecar directory for metadata: ", opts.SideCarDir)
+	}
 
 	return &Posix{
 		meta:          meta,
@@ -219,6 +230,11 @@ func (p *Posix) ListBuckets(_ context.Context, input s3response.ListBucketsInput
 
 	var buckets []s3response.ListAllMyBucketsEntry
 	for _, entry := range entries {
+		if !entry.IsDir() {
+			// buckets must be a directory
+			continue
+		}
+
 		fi, err := entry.Info()
 		if err != nil {
 			// skip entries returning errors

cmd/versitygw/posix.go

@@ -29,6 +29,7 @@ var (
 	bucketlinks        bool
 	versioningDir      string
 	dirPerms           uint
+	sidecar            string
 )
 
 func posixCommand() *cli.Command {
@@ -79,6 +80,12 @@ will be translated into the file /mnt/fs/gwroot/mybucket/a/b/c/myobject`,
 				DefaultText: "0755",
 				Value:       0755,
 			},
+			&cli.StringFlag{
+				Name:        "sidecar",
+				Usage:       "use provided sidecar directory to store metadata",
+				EnvVars:     []string{"VGW_META_SIDECAR"},
+				Destination: &sidecar,
+			},
 		},
 	}
 }
@@ -89,24 +96,39 @@ func runPosix(ctx *cli.Context) error {
 	}
 
 	gwroot := (ctx.Args().Get(0))
-	err := meta.XattrMeta{}.Test(gwroot)
-	if err != nil {
-		return fmt.Errorf("posix xattr check: %v", err)
-	}
 
 	if dirPerms > math.MaxUint32 {
 		return fmt.Errorf("invalid directory permissions: %d", dirPerms)
 	}
 
-	be, err := posix.New(gwroot, meta.XattrMeta{}, posix.PosixOpts{
+	opts := posix.PosixOpts{
 		ChownUID:      chownuid,
 		ChownGID:      chowngid,
 		BucketLinks:   bucketlinks,
 		VersioningDir: versioningDir,
 		NewDirPerm:    fs.FileMode(dirPerms),
-	})
+	}
+
+	var ms meta.MetadataStorer
+	switch {
+	case sidecar != "":
+		sc, err := meta.NewSideCar(sidecar)
+		if err != nil {
+			return fmt.Errorf("failed to init sidecar metadata: %w", err)
+		}
+		ms = sc
+		opts.SideCarDir = sidecar
+	default:
+		ms = meta.XattrMeta{}
+		err := meta.XattrMeta{}.Test(gwroot)
+		if err != nil {
+			return fmt.Errorf("xattr check failed: %w", err)
+		}
+	}
+
+	be, err := posix.New(gwroot, ms, opts)
 	if err != nil {
-		return fmt.Errorf("init posix: %v", err)
+		return fmt.Errorf("failed to init posix backend: %w", err)
 	}
 
 	return runGateway(ctx.Context, be)