server: Cache compaction hash for HashByRev API

Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>

server/mvcc/hash.go

@@ -17,9 +17,16 @@ package mvcc
 import (
 	"hash"
 	"hash/crc32"
+	"sort"
+	"sync"
 
 	"go.etcd.io/etcd/server/v3/mvcc/backend"
 	"go.etcd.io/etcd/server/v3/mvcc/buckets"
+	"go.uber.org/zap"
+)
+
+const (
+	hashStorageMaxSize = 10
 )
 
 func unsafeHashByRev(tx backend.ReadTx, compactRevision, revision int64, keep map[revision]struct{}) (KeyValueHash, error) {
@@ -83,15 +90,22 @@ type HashStorage interface {
 
 	// HashByRev computes the hash of all MVCC revisions up to a given revision.
 	HashByRev(rev int64) (hash KeyValueHash, currentRev int64, err error)
+
+	// Store adds hash value in local cache, allowing it can be returned by HashByRev.
+	Store(valueHash KeyValueHash)
 }
 
 type hashStorage struct {
-	store *store
+	store  *store
+	hashMu sync.RWMutex
+	hashes []KeyValueHash
+	lg     *zap.Logger
 }
 
-func newHashStorage(s *store) HashStorage {
+func newHashStorage(lg *zap.Logger, s *store) *hashStorage {
 	return &hashStorage{
 		store: s,
+		lg:    lg,
 	}
 }
 
@@ -100,5 +114,35 @@ func (s *hashStorage) Hash() (hash uint32, revision int64, err error) {
 }
 
 func (s *hashStorage) HashByRev(rev int64) (KeyValueHash, int64, error) {
+	s.hashMu.RLock()
+	for _, h := range s.hashes {
+		if rev == h.Revision {
+			s.hashMu.RUnlock()
+
+			s.store.revMu.RLock()
+			currentRev := s.store.currentRev
+			s.store.revMu.RUnlock()
+			return h, currentRev, nil
+		}
+	}
+	s.hashMu.RUnlock()
+
 	return s.store.hashByRev(rev)
 }
+
+func (s *hashStorage) Store(hash KeyValueHash) {
+	s.lg.Info("storing new hash",
+		zap.Uint32("hash", hash.Hash),
+		zap.Int64("revision", hash.Revision),
+		zap.Int64("compact-revision", hash.CompactRevision),
+	)
+	s.hashMu.Lock()
+	defer s.hashMu.Unlock()
+	s.hashes = append(s.hashes, hash)
+	sort.Slice(s.hashes, func(i, j int) bool {
+		return s.hashes[i].Revision < s.hashes[j].Revision
+	})
+	if len(s.hashes) > hashStorageMaxSize {
+		s.hashes = s.hashes[len(s.hashes)-hashStorageMaxSize:]
+	}
+}

server/mvcc/hash_test.go

@@ -187,3 +187,53 @@ func pickKey(i int64) string {
 		panic("Can't count")
 	}
 }
+
+func TestHasherStore(t *testing.T) {
+	lg := zaptest.NewLogger(t)
+	s := newHashStorage(lg, newFakeStore())
+	var hashes []KeyValueHash
+	for i := 0; i < hashStorageMaxSize; i++ {
+		hash := KeyValueHash{Hash: uint32(i), Revision: int64(i) + 10, CompactRevision: int64(i) + 100}
+		hashes = append(hashes, hash)
+		s.Store(hash)
+	}
+
+	for _, want := range hashes {
+		got, _, err := s.HashByRev(want.Revision)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want.Hash != got.Hash {
+			t.Errorf("Expected stored hash to match, got: %d, expected: %d", want.Hash, got.Hash)
+		}
+		if want.Revision != got.Revision {
+			t.Errorf("Expected stored revision to match, got: %d, expected: %d", want.Revision, got.Revision)
+		}
+		if want.CompactRevision != got.CompactRevision {
+			t.Errorf("Expected stored compact revision to match, got: %d, expected: %d", want.CompactRevision, got.CompactRevision)
+		}
+	}
+}
+
+func TestHasherStoreFull(t *testing.T) {
+	lg := zaptest.NewLogger(t)
+	s := newHashStorage(lg, newFakeStore())
+	var minRevision int64 = 100
+	var maxRevision = minRevision + hashStorageMaxSize
+	for i := 0; i < hashStorageMaxSize; i++ {
+		s.Store(KeyValueHash{Revision: int64(i) + minRevision})
+	}
+
+	// Hash for old revision should be discarded as storage is already full
+	s.Store(KeyValueHash{Revision: minRevision - 1})
+	hash, _, err := s.HashByRev(minRevision - 1)
+	if err == nil {
+		t.Errorf("Expected an error as old revision should be discarded, got: %v", hash)
+	}
+	// Hash for new revision should be stored even when storage is full
+	s.Store(KeyValueHash{Revision: maxRevision + 1})
+	_, _, err = s.HashByRev(maxRevision + 1)
+	if err != nil {
+		t.Errorf("Didn't expect error for new revision, err: %v", err)
+	}
+}

server/mvcc/kvstore.go

@@ -112,7 +112,7 @@ func NewStore(lg *zap.Logger, b backend.Backend, le lease.Lessor, cfg StoreConfi
 
 		lg: lg,
 	}
-	s.hashes = newHashStorage(s)
+	s.hashes = newHashStorage(lg, s)
 	s.ReadView = &readView{s}
 	s.WriteView = &writeView{s}
 	if s.le != nil {
@@ -234,11 +234,13 @@ func (s *store) compact(trace *traceutil.Trace, rev, prevCompactRev int64) (<-ch
 			s.compactBarrier(ctx, ch)
 			return
 		}
-		if _, err := s.scheduleCompaction(rev, prevCompactRev); err != nil {
+		hash, err := s.scheduleCompaction(rev, prevCompactRev)
+		if err != nil {
 			s.lg.Warn("Failed compaction", zap.Error(err))
 			s.compactBarrier(context.TODO(), ch)
 			return
 		}
+		s.hashes.Store(hash)
 		close(ch)
 	}
 

server/mvcc/kvstore_test.go

@@ -851,6 +851,7 @@ func newFakeStore() *store {
 		lg:             zap.NewExample(),
 	}
 	s.ReadView, s.WriteView = &readView{s}, &writeView{s}
+	s.hashes = newHashStorage(zap.NewExample(), s)
 	return s
 }