mvcc: restore tombstone index if it's first revision

The tombstone could be the only one available revision in database.
It happens when all historical revisions have been deleted in previous
compactions. Since tombstone revision is still in database, we should
restore it as valid key index. Otherwise, we lost that event.

Signed-off-by: Wei Fu <fuweid89@gmail.com>
(cherry picked from commit d8b4192)
Signed-off-by: Wei Fu <fuweid89@gmail.com>

mvcc/key_index.go

@@ -127,6 +127,15 @@ func (ki *keyIndex) restore(lg *zap.Logger, created, modified revision, ver int6
 	keysGauge.Inc()
 }
 
+// restoreTombstone is used to restore a tombstone revision, which is the only
+// revision so far for a key. We don't know the creating revision (i.e. already
+// compacted) of the key, so set it empty.
+func (ki *keyIndex) restoreTombstone(lg *zap.Logger, main, sub int64) {
+	ki.restore(lg, revision{}, revision{main, sub}, 1)
+	ki.generations = append(ki.generations, generation{})
+	keysGauge.Dec()
+}
+
 // tombstone puts a revision, pointing to a tombstone, to the keyIndex.
 // It also creates a new empty generation in the keyIndex.
 // It returns ErrRevisionNotFound when tombstone on an empty generation.

mvcc/key_index_test.go

@@ -19,9 +19,51 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"go.uber.org/zap"
+	"go.uber.org/zap/zaptest"
 )
 
+func TestRestoreTombstone(t *testing.T) {
+	lg := zaptest.NewLogger(t)
+
+	// restore from tombstone
+	//
+	// key: "foo"
+	// modified: 16
+	// "created": 16
+	// generations:
+	//    {empty}
+	//    {{16, 0}(t)[0]}
+	//
+	ki := &keyIndex{key: []byte("foo")}
+	ki.restoreTombstone(lg, 16, 0)
+
+	// get should return not found
+	for retAt := 16; retAt <= 20; retAt++ {
+		_, _, _, err := ki.get(lg, int64(retAt))
+		require.ErrorIs(t, err, ErrRevisionNotFound)
+	}
+
+	// doCompact should keep that tombstone
+	availables := map[revision]struct{}{}
+	ki.doCompact(16, availables)
+	require.Len(t, availables, 1)
+	_, ok := availables[revision{main: 16}]
+	require.True(t, ok)
+
+	// should be able to put new revisions
+	ki.put(lg, 17, 0)
+	ki.put(lg, 18, 0)
+	revs := ki.since(lg, 16)
+	require.Equal(t, []revision{{16, 0}, {17, 0}, {18, 0}}, revs)
+
+	// compaction should remove restored tombstone
+	ki.compact(lg, 17, map[revision]struct{}{})
+	require.Len(t, ki.generations, 1)
+	require.Equal(t, []revision{{17, 0}, {18, 0}}, ki.generations[0].revs)
+}
+
 func TestKeyIndexGet(t *testing.T) {
 	// key: "foo"
 	// rev: 16

mvcc/kv_test.go

@@ -29,7 +29,9 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	dto "github.com/prometheus/client_model/go"
+	"github.com/stretchr/testify/assert"
 	"go.uber.org/zap"
+	"go.uber.org/zap/zaptest"
 )
 
 // Functional tests for features implemented in v3 store. It treats v3 store
@@ -617,6 +619,8 @@ func TestKVHash(t *testing.T) {
 }
 
 func TestKVRestore(t *testing.T) {
+	compactBatchLimit := 5
+
 	tests := []func(kv KV){
 		func(kv KV) {
 			kv.Put([]byte("foo"), []byte("bar0"), 1)
@@ -634,10 +638,23 @@ func TestKVRestore(t *testing.T) {
 			kv.Put([]byte("foo"), []byte("bar1"), 2)
 			kv.Compact(traceutil.TODO(), 1)
 		},
+		func(kv KV) { // after restore, foo1 key only has tombstone revision
+			kv.Put([]byte("foo1"), []byte("bar1"), 0)
+			kv.Put([]byte("foo2"), []byte("bar2"), 0)
+			kv.Put([]byte("foo3"), []byte("bar3"), 0)
+			kv.Put([]byte("foo4"), []byte("bar4"), 0)
+			kv.Put([]byte("foo5"), []byte("bar5"), 0)
+			_, delAtRev := kv.DeleteRange([]byte("foo1"), nil)
+			assert.Equal(t, int64(7), delAtRev)
+
+			// after compaction and restore, foo1 key only has tombstone revision
+			ch, _ := kv.Compact(traceutil.TODO(), delAtRev)
+			<-ch
+		},
 	}
 	for i, tt := range tests {
 		b, tmpPath := backend.NewDefaultTmpBackend()
-		s := NewStore(zap.NewExample(), b, &lease.FakeLessor{}, nil, StoreConfig{})
+		s := NewStore(zaptest.NewLogger(t), b, &lease.FakeLessor{}, StoreConfig{CompactionBatchLimit: compactBatchLimit})
 		tt(s)
 		var kvss [][]mvccpb.KeyValue
 		for k := int64(0); k < 10; k++ {
@@ -649,7 +666,7 @@ func TestKVRestore(t *testing.T) {
 		s.Close()
 
 		// ns should recover the the previous state from backend.
-		ns := NewStore(zap.NewExample(), b, &lease.FakeLessor{}, nil, StoreConfig{})
+		ns := NewStore(zaptest.NewLogger(t), b, &lease.FakeLessor{}, StoreConfig{CompactionBatchLimit: compactBatchLimit})
 
 		if keysRestore := readGaugeInt(keysGauge); keysBefore != keysRestore {
 			t.Errorf("#%d: got %d key count, expected %d", i, keysRestore, keysBefore)

mvcc/kvstore.go

@@ -571,8 +571,12 @@ func restoreIntoIndex(lg *zap.Logger, idx index) (chan<- revKeyValue, <-chan int
 					continue
 				}
 				ki.put(lg, rev.main, rev.sub)
-			} else if !isTombstone(rkv.key) {
-				ki.restore(lg, revision{rkv.kv.CreateRevision, 0}, rev, rkv.kv.Version)
+			} else {
+				if isTombstone(rkv.key) {
+					ki.restoreTombstone(lg, rev.main, rev.sub)
+				} else {
+					ki.restore(lg, revision{rkv.kv.CreateRevision, 0}, rev, rkv.kv.Version)
+				}
 				idx.Insert(ki)
 				kiCache[rkv.kstr] = ki
 			}

tests/e2e/watch_test.go

@@ -476,3 +476,81 @@ func testStartWatcherFromCompactedRevision(t *testing.T, performCompactOnTombsto
 		}
 	}
 }
+
+// TestResumeCompactionOnTombstone verifies whether a deletion event is preserved
+// when etcd restarts and resumes compaction on a key that only has a tombstone revision.
+func TestResumeCompactionOnTombstone(t *testing.T) {
+	defer testutil.AfterTest(t)
+
+	ctx := context.Background()
+	compactBatchLimit := 5
+
+	clus, cerr := newEtcdProcessCluster(t, &etcdProcessClusterConfig{
+		clusterSize:                1,
+		goFailEnabled:              true,
+		clientTLS:                  clientTLS,
+		isClientAutoTLS:            true,
+		CompactionBatchLimit:       compactBatchLimit,
+		WatchProcessNotifyInterval: 100 * time.Millisecond,
+	})
+	require.NoError(t, cerr)
+	defer clus.Close()
+
+	c1 := newClient(t, clus.EndpointsGRPC(), clientTLS, true)
+	defer c1.Close()
+
+	keyPrefix := "/key-"
+	for i := 0; i < compactBatchLimit; i++ {
+		key := fmt.Sprintf("%s%d", keyPrefix, i)
+		value := fmt.Sprintf("%d", i)
+
+		t.Logf("PUT key=%s, val=%s", key, value)
+		_, err := c1.KV.Put(ctx, key, value)
+		require.NoError(t, err)
+	}
+
+	firstKey := keyPrefix + "0"
+	t.Logf("DELETE key=%s", firstKey)
+	deleteResp, err := c1.KV.Delete(ctx, firstKey)
+	require.NoError(t, err)
+
+	var deleteEvent *clientv3.Event
+	select {
+	case watchResp := <-c1.Watch(ctx, firstKey, clientv3.WithRev(deleteResp.Header.Revision)):
+		require.Len(t, watchResp.Events, 1)
+
+		require.Equal(t, mvccpb.DELETE, watchResp.Events[0].Type)
+		deletedKey := string(watchResp.Events[0].Kv.Key)
+		require.Equal(t, firstKey, deletedKey)
+
+		deleteEvent = watchResp.Events[0]
+	case <-time.After(100 * time.Millisecond):
+		t.Fatal("timed out getting watch response")
+	}
+
+	require.NoError(t, clus.procs[0].Failpoints().SetupHTTP(ctx, "compactBeforeSetFinishedCompact", `panic`))
+
+	t.Logf("COMPACT rev=%d", deleteResp.Header.Revision)
+	_, err = c1.KV.Compact(ctx, deleteResp.Header.Revision, clientv3.WithCompactPhysical())
+	require.Error(t, err)
+
+	require.Error(t, clus.procs[0].Stop())
+	// NOTE: The proc panics and exit code is 2. It's impossible to restart
+	// that etcd proc because last exit code is 2 and Restart() refuses to
+	// start new one. Using IsRunning() function is to cleanup status.
+	require.False(t, clus.procs[0].IsRunning())
+	require.NoError(t, clus.Restart())
+
+	c2 := newClient(t, clus.EndpointsGRPC(), clientTLS, true)
+	defer c2.Close()
+
+	watchChan := c2.Watch(ctx, firstKey, clientv3.WithRev(deleteResp.Header.Revision))
+	select {
+	case watchResp := <-watchChan:
+		require.Equal(t, []*clientv3.Event{deleteEvent}, watchResp.Events)
+	case <-time.After(100 * time.Millisecond):
+		// we care only about the first response, but have an
+		// escape hatch in case the watch response is delayed.
+		t.Fatal("timed out getting watch response")
+	}
+}