Error when etcd3 watch finds delete event with nil prevKV

staging/src/k8s.io/apiserver/pkg/storage/etcd3/BUILD

@@ -10,6 +10,7 @@ go_test(
     name = "go_default_test",
     srcs = [
         "compact_test.go",
+        "event_test.go",
         "lease_manager_test.go",
         "store_test.go",
         "watcher_test.go",
@@ -36,7 +37,10 @@ go_test(
         "//vendor/github.com/coreos/etcd/clientv3:go_default_library",
         "//vendor/github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes:go_default_library",
         "//vendor/github.com/coreos/etcd/integration:go_default_library",
+        "//vendor/github.com/coreos/etcd/mvcc/mvccpb:go_default_library",
         "//vendor/github.com/coreos/pkg/capnslog:go_default_library",
+        "//vendor/github.com/stretchr/testify/assert:go_default_library",
+        "//vendor/github.com/stretchr/testify/require:go_default_library",
     ],
 )
 

staging/src/k8s.io/apiserver/pkg/storage/etcd3/event.go

@@ -17,6 +17,7 @@ limitations under the License.
 package etcd3
 
 import (
+	"fmt"
 	"github.com/coreos/etcd/clientv3"
 	"github.com/coreos/etcd/mvcc/mvccpb"
 )
@@ -42,7 +43,12 @@ func parseKV(kv *mvccpb.KeyValue) *event {
 	}
 }
 
-func parseEvent(e *clientv3.Event) *event {
+func parseEvent(e *clientv3.Event) (*event, error) {
+	if !e.IsCreate() && e.PrevKv == nil {
+		// If the previous value is nil, error. One example of how this is possible is if the previous value has been compacted already.
+		return nil, fmt.Errorf("etcd event received with PrevKv=nil (key=%q, modRevision=%d, type=%s)", string(e.Kv.Key), e.Kv.ModRevision, e.Type.String())
+
+	}
 	ret := &event{
 		key:       string(e.Kv.Key),
 		value:     e.Kv.Value,
@@ -53,5 +59,5 @@ func parseEvent(e *clientv3.Event) *event {
 	if e.PrevKv != nil {
 		ret.prevValue = e.PrevKv.Value
 	}
-	return ret
+	return ret, nil
 }

staging/src/k8s.io/apiserver/pkg/storage/etcd3/event_test.go

+/*
+Copyright 2019 The Kubernetes 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 etcd3
+
+import (
+	"github.com/coreos/etcd/clientv3"
+	"github.com/coreos/etcd/mvcc/mvccpb"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestParseEvent(t *testing.T) {
+	for _, tc := range []struct {
+		name          string
+		etcdEvent     *clientv3.Event
+		expectedEvent *event
+		expectedErr   string
+	}{
+		{
+			name: "successful create",
+			etcdEvent: &clientv3.Event{
+				Type:   clientv3.EventTypePut,
+				PrevKv: nil,
+				Kv: &mvccpb.KeyValue{
+					// key is the key in bytes. An empty key is not allowed.
+					Key:            []byte("key"),
+					ModRevision:    1,
+					CreateRevision: 1,
+					Value:          []byte("value"),
+				},
+			},
+			expectedEvent: &event{
+				key:       "key",
+				value:     []byte("value"),
+				prevValue: nil,
+				rev:       1,
+				isDeleted: false,
+				isCreated: true,
+			},
+			expectedErr: "",
+		},
+		{
+			name: "unsuccessful delete",
+			etcdEvent: &clientv3.Event{
+				Type:   mvccpb.DELETE,
+				PrevKv: nil,
+				Kv: &mvccpb.KeyValue{
+					Key:            []byte("key"),
+					CreateRevision: 1,
+					ModRevision:    2,
+					Value:          nil,
+				},
+			},
+			expectedErr: "etcd event received with PrevKv=nil",
+		},
+		{
+			name: "successful delete",
+			etcdEvent: &clientv3.Event{
+				Type: mvccpb.DELETE,
+				PrevKv: &mvccpb.KeyValue{
+					Key:            []byte("key"),
+					CreateRevision: 1,
+					ModRevision:    1,
+					Value:          []byte("value"),
+				},
+				Kv: &mvccpb.KeyValue{
+					Key:            []byte("key"),
+					CreateRevision: 1,
+					ModRevision:    2,
+					Value:          nil,
+				},
+			},
+			expectedEvent: &event{
+				key:       "key",
+				value:     nil,
+				prevValue: []byte("value"),
+				rev:       2,
+				isDeleted: true,
+				isCreated: false,
+			},
+			expectedErr: "",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			actualEvent, err := parseEvent(tc.etcdEvent)
+			if tc.expectedErr != "" {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), tc.expectedErr)
+			} else {
+				require.NoError(t, err)
+				assert.Equal(t, tc.expectedEvent, actualEvent)
+			}
+		})
+	}
+}

staging/src/k8s.io/apiserver/pkg/storage/etcd3/watcher.go

@@ -210,7 +210,13 @@ func (wc *watchChan) startWatching(watchClosedCh chan struct{}) {
 			return
 		}
 		for _, e := range wres.Events {
-			wc.sendEvent(parseEvent(e))
+			parsedEvent, err := parseEvent(e)
+			if err != nil {
+				klog.Errorf("watch chan error: %v", err)
+				wc.sendError(err)
+				return
+			}
+			wc.sendEvent(parsedEvent)
 		}
 	}
 	// When we come to this point, it's only possible that client side ends the watch.