Return not exist error in metadata store

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/metadata/store/metadata_store.go

@@ -17,12 +17,21 @@ limitations under the License.
 package store
 
 import (
-	"fmt"
+	"errors"
 	"sync"
 
 	"github.com/golang/glog"
 )
 
+var (
+	// ErrNotExist is the error returned when specified id does
+	// not exist.
+	ErrNotExist = errors.New("does not exist")
+	// ErrAlreadyExist is the error returned when specified id already
+	// exists.
+	ErrAlreadyExist = errors.New("already exists")
+)
+
 // All byte arrays are expected to be read-only. User MUST NOT modify byte
 // array element directly!!
 
@@ -46,11 +55,12 @@ type MetadataStore interface {
 	// * The id and data MUST be added in one transaction to the store.
 	Create(string, []byte) error
 	// Get the data by id.
-	// Note that Get MUST return nil without error if the id
-	// doesn't exist.
+	// Note that Get MUST return ErrNotExist if the id doesn't exist.
 	Get(string) ([]byte, error)
-	// Update the data by id. Note that the update MUST be applied in
-	// one transaction.
+	// Update the data by id.
+	// Note:
+	// * Update MUST return ErrNotExist is the id doesn't exist.
+	// * The update MUST be applied in one transaction.
 	Update(string, UpdateFunc) error
 	// List returns entire array of data from the store.
 	List() ([][]byte, error)
@@ -135,7 +145,7 @@ func (m *metadataStore) createMetadata(id string, meta *metadata) error {
 	m.Lock()
 	defer m.Unlock()
 	if _, found := m.metas[id]; found {
-		return fmt.Errorf("id %q already exists", id)
+		return ErrAlreadyExist
 	}
 	m.metas[id] = meta
 	return nil
@@ -172,7 +182,7 @@ func (m *metadataStore) getMetadata(id string) (*metadata, bool) {
 func (m *metadataStore) Get(id string) ([]byte, error) {
 	meta, found := m.getMetadata(id)
 	if !found {
-		return nil, nil
+		return nil, ErrNotExist
 	}
 	return meta.get(), nil
 }
@@ -181,7 +191,7 @@ func (m *metadataStore) Get(id string) ([]byte, error) {
 func (m *metadataStore) Update(id string, u UpdateFunc) error {
 	meta, found := m.getMetadata(id)
 	if !found {
-		return fmt.Errorf("id %q doesn't exist", id)
+		return ErrNotExist
 	}
 	return meta.update(u)
 }

pkg/metadata/store/metadata_store_test.go

@@ -97,9 +97,10 @@ func TestMetadataStore(t *testing.T) {
 	assert.Equal(testMeta[testIds[1]], meta)
 
 	t.Logf("update should take effect")
-	m.Update(testIds[1], func(in []byte) ([]byte, error) {
+	err = m.Update(testIds[1], func(in []byte) ([]byte, error) {
 		return []byte("updated-metadata-1"), nil
 	})
+	assert.NoError(err)
 	newMeta, err := m.Get(testIds[1])
 	assert.NoError(err)
 	assert.Equal([]byte("updated-metadata-1"), newMeta)
@@ -111,9 +112,15 @@ func TestMetadataStore(t *testing.T) {
 	assert.Len(metas, 1)
 	assert.Equal(testMeta[testIds[0]], metas[0])
 	meta, err = m.Get(testIds[1])
-	assert.NoError(err)
+	assert.Equal(ErrNotExist, err)
 	assert.Nil(meta)
 
+	t.Logf("update should return not exist error after metadata got deleted")
+	err = m.Update(testIds[1], func(in []byte) ([]byte, error) {
+		return in, nil
+	})
+	assert.Equal(ErrNotExist, err)
+
 	t.Logf("existing reference should not be affected by delete")
 	assert.Equal([]byte("updated-metadata-1"), newMeta)
 
@@ -181,9 +188,14 @@ func TestMultithreadAccess(t *testing.T) {
 			assert.NoError(err)
 
 			got, err = m.Get(id)
-			assert.NoError(err)
+			assert.Equal(ErrNotExist, err)
 			assert.Nil(got)
 
+			err = m.Update(id, func(in []byte) ([]byte, error) {
+				return in, nil
+			})
+			assert.Equal(ErrNotExist, err)
+
 			gotList, err = m.List()
 			assert.NoError(err)
 			assert.NotContains(gotList, expect)