feat: add metadata storage abstraction layer

Closes #511. This adds an abstraction layer to the metadata
storage to allow for future non-xattr metadata storage
implementations.

cmd/versitygw/gateway_test.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 	"testing"
 
+	"github.com/versity/versitygw/backend/meta"
 	"github.com/versity/versitygw/backend/posix"
 	"github.com/versity/versitygw/tests/integration"
 )
@@ -56,7 +57,7 @@ func initPosix(ctx context.Context) {
 		log.Fatalf("make temp directory: %v", err)
 	}
 
-	be, err := posix.New(tempdir, posix.PosixOpts{})
+	be, err := posix.New(tempdir, meta.XattrMeta{}, posix.PosixOpts{})
 	if err != nil {
 		log.Fatalf("init posix: %v", err)
 	}

cmd/versitygw/posix.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 
 	"github.com/urfave/cli/v2"
+	"github.com/versity/versitygw/backend/meta"
 	"github.com/versity/versitygw/backend/posix"
 )
 
@@ -62,7 +63,13 @@ func runPosix(ctx *cli.Context) error {
 		return fmt.Errorf("no directory provided for operation")
 	}
 
-	be, err := posix.New(ctx.Args().Get(0), posix.PosixOpts{
+	gwroot := (ctx.Args().Get(0))
+	ok := meta.XattrMeta{}.Test(gwroot)
+	if !ok {
+		return fmt.Errorf("posix backend requires extended attributes support")
+	}
+
+	be, err := posix.New(gwroot, meta.XattrMeta{}, posix.PosixOpts{
 		ChownUID: chownuid,
 		ChownGID: chowngid,
 	})