fix: return method not allowed for read only fs for fallback tempfile

We had put the error handling in for the read only filesystems
when O_TMPFILE is supported, but missed the CreateTemp() fallback
case. This fixes this case to also return the method not allowed
error.

This also adds the error handling for the scoutfs case as well.

Fixes #1195

backend/posix/with_otmpfile.go

@@ -70,11 +70,17 @@ func (p *Posix) openTmpFile(dir, bucket, obj string, size int64, acct auth.Accou
 		// O_TMPFILE not supported, try fallback
 		err = backend.MkdirAll(dir, uid, gid, doChown, p.newDirPerm)
 		if err != nil {
+			if errors.Is(err, syscall.EROFS) {
+				return nil, s3err.GetAPIError(s3err.ErrMethodNotAllowed)
+			}
 			return nil, fmt.Errorf("make temp dir: %w", err)
 		}
 		f, err := os.CreateTemp(dir,
 			fmt.Sprintf("%x.", sha256.Sum256([]byte(obj))))
 		if err != nil {
+			if errors.Is(err, syscall.EROFS) {
+				return nil, s3err.GetAPIError(s3err.ErrMethodNotAllowed)
+			}
 			return nil, err
 		}
 		tmp := &tmpfile{

backend/scoutfs/scoutfs_compat.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"syscall"
 
 	"golang.org/x/sys/unix"
 
@@ -31,6 +32,7 @@ import (
 	"github.com/versity/versitygw/backend"
 	"github.com/versity/versitygw/backend/meta"
 	"github.com/versity/versitygw/backend/posix"
+	"github.com/versity/versitygw/s3err"
 )
 
 func New(rootdir string, opts ScoutfsOpts) (*ScoutFS, error) {
@@ -90,6 +92,9 @@ func (s *ScoutFS) openTmpFile(dir, bucket, obj string, size int64, acct auth.Acc
 	// file descriptor into the namespace.
 	fd, err := unix.Open(dir, unix.O_RDWR|unix.O_TMPFILE|unix.O_CLOEXEC, defaultFilePerm)
 	if err != nil {
+		if errors.Is(err, syscall.EROFS) {
+			return nil, s3err.GetAPIError(s3err.ErrMethodNotAllowed)
+		}
 		return nil, err
 	}