Detecting whether v2store is "empty" (metadata only).

server/etcdserver/api/membership/store.go

@@ -31,7 +31,7 @@ const (
 	attributesSuffix     = "attributes"
 	raftAttributesSuffix = "raftAttributes"
 
-	// the prefix for stroing membership related information in store provided by store pkg.
+	// the prefix for storing membership related information in store provided by store pkg.
 	storePrefix = "/0"
 )
 

server/etcdserver/api/membership/storev2.go

@@ -0,0 +1,36 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package membership
+
+import (
+	"go.etcd.io/etcd/server/v3/etcdserver/api/v2store"
+)
+
+// IsMetaStoreOnly verifies if the given `store` contains only
+// a meta-information (members, version) that can be recovered from the
+// backend (storev3) as well as opposed to user-data.
+func IsMetaStoreOnly(store v2store.Store) (bool, error) {
+	event, err := store.Get("/", true, false)
+	if err != nil {
+		return false, err
+	}
+	for _, n := range event.Node.Nodes {
+		if n.Key != storePrefix && n.Nodes.Len() > 0 {
+			return false, nil
+		}
+	}
+
+	return true, nil
+}

server/etcdserver/api/membership/storev2_test.go

@@ -0,0 +1,54 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package membership
+
+import (
+	"testing"
+
+	"github.com/coreos/go-semver/semver"
+	"github.com/stretchr/testify/assert"
+	"go.etcd.io/etcd/server/v3/etcdserver/api/v2store"
+	"go.uber.org/zap/zaptest"
+)
+
+func TestIsMetaStoreOnly(t *testing.T) {
+	lg := zaptest.NewLogger(t)
+	s := v2store.New("/0", "/1")
+
+	metaOnly, err := IsMetaStoreOnly(s)
+	assert.NoError(t, err)
+	assert.True(t, metaOnly, "Just created v2store should be meta-only")
+
+	mustSaveClusterVersionToStore(lg, s, semver.New("3.5.17"))
+	metaOnly, err = IsMetaStoreOnly(s)
+	assert.NoError(t, err)
+	assert.True(t, metaOnly, "Just created v2store should be meta-only")
+
+	mustSaveMemberToStore(lg, s, &Member{ID: 0x00abcd})
+	metaOnly, err = IsMetaStoreOnly(s)
+	assert.NoError(t, err)
+	assert.True(t, metaOnly, "Just created v2store should be meta-only")
+
+	_, err = s.Create("/1/foo", false, "v1", false, v2store.TTLOptionSet{ExpireTime: v2store.Permanent})
+	assert.NoError(t, err)
+	metaOnly, err = IsMetaStoreOnly(s)
+	assert.NoError(t, err)
+	assert.False(t, metaOnly, "Just created v2store should be meta-only")
+
+	_, err = s.Delete("/1/foo", false, false)
+	assert.NoError(t, err)
+	assert.NoError(t, err)
+	assert.False(t, metaOnly, "Just created v2store should be meta-only")
+}