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 <[email protected]>

server/storage/mvcc/key_index.go

@@ -116,6 +116,29 @@ func (ki *keyIndex) restore(lg *zap.Logger, created, modified Revision, ver int6
 	keysGauge.Inc()
 }
 
+// restoreTombstone is only used to build index when there is no previous revision.
+// Since there is no historical information, CreateRevision always is empty.
+func (ki *keyIndex) restoreTombstone(lg *zap.Logger, main, sub int64) {
+	if len(ki.generations) != 0 {
+		lg.Panic(
+			"'restore' got an unexpected non-empty generations",
+			zap.Int("generations-size", len(ki.generations)),
+		)
+	}
+
+	rev := Revision{Main: main, Sub: sub}
+
+	ki.modified = rev
+	ki.generations = append(ki.generations,
+		generation{
+			ver:     1,
+			created: Revision{}, // unknown for first revision as tombstone
+			revs:    []Revision{rev},
+		},
+		generation{},
+	)
+}
+
 // 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.

server/storage/mvcc/key_index_test.go

@@ -20,10 +20,51 @@ 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"
 	// modified: 16

server/storage/mvcc/kv_test.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	dto "github.com/prometheus/client_model/go"
+	"github.com/stretchr/testify/assert"
 	"go.uber.org/zap/zaptest"
 
 	"go.etcd.io/etcd/api/v3/mvccpb"
@@ -657,6 +658,8 @@ 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)
@@ -674,10 +677,23 @@ 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, _ := betesting.NewDefaultTmpBackend(t)
-		s := NewStore(zaptest.NewLogger(t), b, &lease.FakeLessor{}, 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++ {
@@ -689,7 +705,7 @@ func TestKVRestore(t *testing.T) {
 		s.Close()
 
 		// ns should recover the previous state from backend.
-		ns := NewStore(zaptest.NewLogger(t), b, &lease.FakeLessor{}, 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)

server/storage/mvcc/kvstore.go

@@ -477,6 +477,11 @@ func restoreIntoIndex(lg *zap.Logger, idx index) (chan<- revKeyValue, <-chan int
 				ki.restore(lg, Revision{Main: rkv.kv.CreateRevision}, rev, rkv.kv.Version)
 				idx.Insert(ki)
 				kiCache[rkv.kstr] = ki
+			} else {
+				lg.Warn("historical revision(s) has been compacted", zap.Int64("main", rev.Main), zap.Int64("sub", rev.Sub))
+				ki.restoreTombstone(lg, rev.Main, rev.Sub)
+				idx.Insert(ki)
+				kiCache[rkv.kstr] = ki
 			}
 		}
 	}()

tests/e2e/watch_test.go

@@ -488,3 +488,65 @@ func testStartWatcherFromCompactedRevision(t *testing.T, performCompactOnTombsto
 		}
 	}
 }
+
+func TestRestoreTombstone(t *testing.T) {
+	e2e.BeforeTest(t)
+
+	ctx := context.Background()
+	compactBatchLimit := 5
+
+	cfg := e2e.DefaultConfig()
+	clus, err := e2e.NewEtcdProcessCluster(context.Background(),
+		t,
+		e2e.WithConfig(cfg),
+		e2e.WithClusterSize(1),
+		e2e.WithCompactionBatchLimit(compactBatchLimit),
+		e2e.WithGoFailEnabled(true),
+		e2e.WithWatchProcessNotifyInterval(100*time.Millisecond),
+	)
+	require.NoError(t, err)
+	defer clus.Close()
+
+	c1 := newClient(t, clus.EndpointsGRPC(), cfg.Client)
+	defer c1.Close()
+
+	keyPrefix := "/key-"
+	for idx := 1; idx <= compactBatchLimit; idx++ {
+		key := fmt.Sprintf("%s%d", keyPrefix, idx)
+		value := fmt.Sprintf("%d", idx)
+
+		t.Logf("PUT key=%s, val=%s", key, value)
+		_, err = c1.KV.Put(ctx, key, value)
+		require.NoError(t, err)
+	}
+
+	firstKey := keyPrefix + "1"
+	t.Logf("DELETE key=%s", firstKey)
+	deleteResp, err := c1.KV.Delete(ctx, firstKey)
+	require.NoError(t, err)
+
+	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.NoError(t, clus.Restart(ctx))
+
+	c2 := newClient(t, clus.EndpointsGRPC(), cfg.Client)
+	defer c2.Close()
+
+	watchChan := c2.Watch(ctx, firstKey, clientv3.WithRev(deleteResp.Header.Revision))
+	select {
+	case watchResp := <-watchChan:
+		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)
+	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")
+	}
+}