ociindex: allow readonly access

If file lock can't be taken because of a permission or readonly
error then assume that the index is immutable and locks are not needed.

Writer side is unchanged as if lock would be skipped then the write
is assumed to fail anyway.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

Author: tonistiigi

client/ociindex/ociindex.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"os"
 	"path"
+	"syscall"
 
 	"github.com/gofrs/flock"
 	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
@@ -36,15 +37,18 @@ func (s StoreIndex) Read() (*ocispecs.Index, error) {
 	lock := flock.New(s.lockPath)
 	locked, err := lock.TryRLock()
 	if err != nil {
-		return nil, errors.Wrapf(err, "could not lock %s", s.lockPath)
-	}
-	if !locked {
-		return nil, errors.Errorf("could not lock %s", s.lockPath)
+		if !errors.Is(err, syscall.EPERM) && !errors.Is(err, syscall.EROFS) {
+			return nil, errors.Wrapf(err, "could not lock %s", s.lockPath)
+		}
+	} else {
+		if !locked {
+			return nil, errors.Errorf("could not lock %s", s.lockPath)
+		}
+		defer func() {
+			lock.Unlock()
+			os.RemoveAll(s.lockPath)
+		}()
 	}
-	defer func() {
-		lock.Unlock()
-		os.RemoveAll(s.lockPath)
-	}()
 
 	b, err := os.ReadFile(s.indexPath)
 	if err != nil {