storage: allow to seek backwards

pkg/core/storage/boltdb_store.go

@@ -113,7 +113,15 @@ func (s *BoltDBStore) Seek(rng SeekRange, f func(k, v []byte)) {
 	start := make([]byte, len(rng.Prefix)+len(rng.Start))
 	copy(start, rng.Prefix)
 	copy(start[len(rng.Prefix):], rng.Start)
-	prefix := util.BytesPrefix(rng.Prefix)
+	if rng.Backwards {
+		s.seekBackwards(rng.Prefix, start, f)
+	} else {
+		s.seek(rng.Prefix, start, f)
+	}
+}
+
+func (s *BoltDBStore) seek(key []byte, start []byte, f func(k, v []byte)) {
+	prefix := util.BytesPrefix(key)
 	prefix.Start = start
 	err := s.db.View(func(tx *bbolt.Tx) error {
 		c := tx.Bucket(Bucket).Cursor()
@@ -127,6 +135,26 @@ func (s *BoltDBStore) Seek(rng SeekRange, f func(k, v []byte)) {
 	}
 }
 
+func (s *BoltDBStore) seekBackwards(key []byte, start []byte, f func(k, v []byte)) {
+	err := s.db.View(func(tx *bbolt.Tx) error {
+		c := tx.Bucket(Bucket).Cursor()
+		// Move cursor to the first kv pair which is followed by the pair matching the specified prefix.
+		if len(start) == 0 {
+			lastKey, _ := c.Last()
+			start = lastKey
+		}
+		rng := util.BytesPrefix(start) // in fact, we only need limit based on start slice to iterate backwards starting from this limit
+		c.Seek(rng.Limit)
+		for k, v := c.Prev(); k != nil && bytes.HasPrefix(k, key); k, v = c.Prev() {
+			f(k, v)
+		}
+		return nil
+	})
+	if err != nil {
+		panic(err)
+	}
+}
+
 // Batch implements the Batch interface and returns a boltdb
 // compatible Batch.
 func (s *BoltDBStore) Batch() Batch {

pkg/core/storage/leveldb_store.go

@@ -89,7 +89,15 @@ func (s *LevelDBStore) Seek(rng SeekRange, f func(k, v []byte)) {
 	start := make([]byte, len(rng.Prefix)+len(rng.Start))
 	copy(start, rng.Prefix)
 	copy(start[len(rng.Prefix):], rng.Start)
-	prefix := util.BytesPrefix(rng.Prefix)
+	if rng.Backwards {
+		s.seekBackwards(rng.Prefix, start, f)
+	} else {
+		s.seek(rng.Prefix, start, f)
+	}
+}
+
+func (s *LevelDBStore) seek(key []byte, start []byte, f func(k, v []byte)) {
+	prefix := util.BytesPrefix(key)
 	prefix.Start = start
 	iter := s.db.NewIterator(prefix, nil)
 	for iter.Next() {
@@ -98,6 +106,17 @@ func (s *LevelDBStore) Seek(rng SeekRange, f func(k, v []byte)) {
 	iter.Release()
 }
 
+func (s *LevelDBStore) seekBackwards(key []byte, start []byte, f func(k, v []byte)) {
+	iRange := util.BytesPrefix(start)
+	iRange.Start = key
+
+	iter := s.db.NewIterator(iRange, nil)
+	for ok := iter.Last(); ok; ok = iter.Prev() {
+		f(iter.Key(), iter.Value())
+	}
+	iter.Release()
+}
+
 // Batch implements the Batch interface and returns a leveldb
 // compatible Batch.
 func (s *LevelDBStore) Batch() Batch {

pkg/core/storage/memcached_store.go

@@ -115,7 +115,8 @@ func (s *MemCachedStore) SeekAsync(ctx context.Context, rng SeekRange, cutPrefix
 // seek is internal representations of Seek* capable of seeking for the given key
 // and supporting early stop using provided context. `cutPrefix` denotes whether provided
 // key needs to be cut off the resulting keys. `rng` specifies prefix items must match
-// and point to start seeking from.
+// and point to start seeking from. Backwards seeking from some point is supported
+// with corresponding `rng` field set.
 func (s *MemCachedStore) seek(ctx context.Context, rng SeekRange, cutPrefix bool, f func(k, v []byte)) {
 	// Create memory store `mem` and `del` snapshot not to hold the lock.
 	var memRes []KeyValueExists
@@ -126,6 +127,11 @@ func (s *MemCachedStore) seek(ctx context.Context, rng SeekRange, cutPrefix bool
 	isKeyOK := func(key string) bool {
 		return strings.HasPrefix(key, sPrefix) && (lStart == 0 || strings.Compare(key[lPrefix:], sStart) >= 0)
 	}
+	if rng.Backwards {
+		isKeyOK = func(key string) bool {
+			return strings.HasPrefix(key, sPrefix) && (lStart == 0 || strings.Compare(key[lPrefix:], sStart) <= 0)
+		}
+	}
 	s.mut.RLock()
 	for k, v := range s.MemoryStore.mem {
 		if isKeyOK(k) {
@@ -149,9 +155,14 @@ func (s *MemCachedStore) seek(ctx context.Context, rng SeekRange, cutPrefix bool
 	}
 	ps := s.ps
 	s.mut.RUnlock()
+
+	less := func(k1, k2 []byte) bool {
+		res := bytes.Compare(k1, k2)
+		return res != 0 && rng.Backwards == (res > 0)
+	}
 	// Sort memRes items for further comparison with ps items.
 	sort.Slice(memRes, func(i, j int) bool {
-		return bytes.Compare(memRes[i].Key, memRes[j].Key) < 0
+		return less(memRes[i].Key, memRes[j].Key)
 	})
 
 	var (
@@ -181,7 +192,7 @@ func (s *MemCachedStore) seek(ctx context.Context, rng SeekRange, cutPrefix bool
 				done = true
 				break loop
 			default:
-				var isMem = haveMem && (bytes.Compare(kvMem.Key, kvPs.Key) < 0)
+				var isMem = haveMem && less(kvMem.Key, kvPs.Key)
 				if isMem {
 					if kvMem.Exists {
 						if cutPrefix {

pkg/core/storage/memory_store.go

@@ -128,7 +128,8 @@ func (s *MemoryStore) SeekAll(key []byte, f func(k, v []byte)) {
 }
 
 // seek is an internal unlocked implementation of Seek. `start` denotes whether
-// seeking starting from the provided prefix should be performed.
+// seeking starting from the provided prefix should be performed. Backwards
+// seeking from some point is supported with corresponding SeekRange field set.
 func (s *MemoryStore) seek(rng SeekRange, f func(k, v []byte)) {
 	sPrefix := string(rng.Prefix)
 	lPrefix := len(sPrefix)
@@ -139,6 +140,16 @@ func (s *MemoryStore) seek(rng SeekRange, f func(k, v []byte)) {
 	isKeyOK := func(key string) bool {
 		return strings.HasPrefix(key, sPrefix) && (lStart == 0 || strings.Compare(key[lPrefix:], sStart) >= 0)
 	}
+	if rng.Backwards {
+		isKeyOK = func(key string) bool {
+			return strings.HasPrefix(key, sPrefix) && (lStart == 0 || strings.Compare(key[lPrefix:], sStart) <= 0)
+		}
+	}
+	less := func(k1, k2 []byte) bool {
+		res := bytes.Compare(k1, k2)
+		return res != 0 && rng.Backwards == (res > 0)
+	}
+
 	for k, v := range s.mem {
 		if isKeyOK(k) {
 			memList = append(memList, KeyValue{
@@ -148,7 +159,7 @@ func (s *MemoryStore) seek(rng SeekRange, f func(k, v []byte)) {
 		}
 	}
 	sort.Slice(memList, func(i, j int) bool {
-		return bytes.Compare(memList[i].Key, memList[j].Key) < 0
+		return less(memList[i].Key, memList[j].Key)
 	})
 	for _, kv := range memList {
 		f(kv.Key, kv.Value)

pkg/core/storage/store.go

@@ -59,6 +59,10 @@ type SeekRange struct {
 	// Empty Prefix and empty Start can be combined, which means seeking
 	// through all keys in the DB.
 	Start []byte
+	// Backwards denotes whether Seek direction should be reversed, i.e.
+	// whether seeking should be performed in a descending way.
+	// Backwards can be safely combined with Prefix and Start.
+	Backwards bool
 }
 
 // ErrKeyNotFound is an error returned by Store implementations

pkg/core/storage/storeandbatch_test.go

@@ -81,17 +81,27 @@ func testStoreSeek(t *testing.T, s Store) {
 		require.NoError(t, s.Put(v.Key, v.Value))
 	}
 
-	check := func(t *testing.T, goodprefix, start []byte, goodkvs []KeyValue) {
+	check := func(t *testing.T, goodprefix, start []byte, goodkvs []KeyValue, backwards bool) {
-		// Seek result expected to be sorted in an ascending way.
+		// Seek result expected to be sorted in an ascending (for forwards seeking) or descending (for backwards seeking) way.
-		sort.Slice(goodkvs, func(i, j int) bool {
+		cmpFunc := func(i, j int) bool {
 			return bytes.Compare(goodkvs[i].Key, goodkvs[j].Key) < 0
-		})
+		}
+		if backwards {
+			cmpFunc = func(i, j int) bool {
+				return bytes.Compare(goodkvs[i].Key, goodkvs[j].Key) > 0
+			}
+		}
+		sort.Slice(goodkvs, cmpFunc)
 
-		actual := make([]KeyValue, 0, len(goodkvs))
+		rng := SeekRange{
-		s.Seek(SeekRange{
 			Prefix: goodprefix,
 			Start:  start,
-		}, func(k, v []byte) {
+		}
+		if backwards {
+			rng.Backwards = true
+		}
+		actual := make([]KeyValue, 0, len(goodkvs))
+		s.Seek(rng, func(k, v []byte) {
 			actual = append(actual, KeyValue{
 				Key:   slice.Copy(k),
 				Value: slice.Copy(v),
@@ -101,6 +111,7 @@ func testStoreSeek(t *testing.T, s Store) {
 	}
 
 	t.Run("non-empty prefix, empty start", func(t *testing.T) {
+		t.Run("forwards", func(t *testing.T) {
 			t.Run("good", func(t *testing.T) {
 				// Given this prefix...
 				goodprefix := []byte("2")
@@ -112,16 +123,36 @@ func testStoreSeek(t *testing.T, s Store) {
 					kvs[3], // key = "21"
 					kvs[4], // key = "22"
 				}
-			check(t, goodprefix, start, goodkvs)
+				check(t, goodprefix, start, goodkvs, false)
 			})
 			t.Run("no matching items", func(t *testing.T) {
 				goodprefix := []byte("0")
 				start := []byte{}
-			check(t, goodprefix, start, []KeyValue{})
+				check(t, goodprefix, start, []KeyValue{}, false)
+			})
+		})
+
+		t.Run("backwards", func(t *testing.T) {
+			t.Run("good", func(t *testing.T) {
+				goodprefix := []byte("2")
+				start := []byte{}
+				goodkvs := []KeyValue{
+					kvs[4], // key = "22"
+					kvs[3], // key = "21"
+					kvs[2], // key = "20"
+				}
+				check(t, goodprefix, start, goodkvs, true)
+			})
+			t.Run("no matching items", func(t *testing.T) {
+				goodprefix := []byte("0")
+				start := []byte{}
+				check(t, goodprefix, start, []KeyValue{}, true)
+			})
 		})
 	})
 
 	t.Run("non-empty prefix, non-empty start", func(t *testing.T) {
+		t.Run("forwards", func(t *testing.T) {
 			t.Run("good", func(t *testing.T) {
 				goodprefix := []byte("2")
 				start := []byte("1") // start will be upended to goodprefix to start seek from
@@ -129,16 +160,34 @@ func testStoreSeek(t *testing.T, s Store) {
 					kvs[3], // key = "21"
 					kvs[4], // key = "22"
 				}
-			check(t, goodprefix, start, goodkvs)
+				check(t, goodprefix, start, goodkvs, false)
 			})
 			t.Run("no matching items", func(t *testing.T) {
 				goodprefix := []byte("2")
-			start := []byte("3") // start will be upended to goodprefix to start seek from
+				start := []byte("3") // start is more than all keys prefixed by '2'.
-			check(t, goodprefix, start, []KeyValue{})
+				check(t, goodprefix, start, []KeyValue{}, false)
+			})
+		})
+		t.Run("backwards", func(t *testing.T) {
+			t.Run("good", func(t *testing.T) {
+				goodprefix := []byte("2")
+				start := []byte("1") // start will be upended to goodprefix to start seek from
+				goodkvs := []KeyValue{
+					kvs[3], // key = "21"
+					kvs[2], // key = "20"
+				}
+				check(t, goodprefix, start, goodkvs, true)
+			})
+			t.Run("no matching items", func(t *testing.T) {
+				goodprefix := []byte("2")
+				start := []byte(".") // start is less than all keys prefixed by '2'.
+				check(t, goodprefix, start, []KeyValue{}, true)
+			})
 		})
 	})
 
 	t.Run("empty prefix, non-empty start", func(t *testing.T) {
+		t.Run("forwards", func(t *testing.T) {
 			t.Run("good", func(t *testing.T) {
 				goodprefix := []byte{}
 				start := []byte("21")
@@ -148,12 +197,31 @@ func testStoreSeek(t *testing.T, s Store) {
 					kvs[5], // key = "30"
 					kvs[6], // key = "31"
 				}
-			check(t, goodprefix, start, goodkvs)
+				check(t, goodprefix, start, goodkvs, false)
 			})
 			t.Run("no matching items", func(t *testing.T) {
 				goodprefix := []byte{}
-			start := []byte("32")
+				start := []byte("32") // start is more than all keys.
-			check(t, goodprefix, start, []KeyValue{})
+				check(t, goodprefix, start, []KeyValue{}, false)
+			})
+		})
+		t.Run("backwards", func(t *testing.T) {
+			t.Run("good", func(t *testing.T) {
+				goodprefix := []byte{}
+				start := []byte("21")
+				goodkvs := []KeyValue{
+					kvs[3], // key = "21"
+					kvs[2], // key = "20"
+					kvs[1], // key = "11"
+					kvs[0], // key = "10"
+				}
+				check(t, goodprefix, start, goodkvs, true)
+			})
+			t.Run("no matching items", func(t *testing.T) {
+				goodprefix := []byte{}
+				start := []byte("0") // start is less than all keys.
+				check(t, goodprefix, start, []KeyValue{}, true)
+			})
 		})
 	})
 
@@ -162,7 +230,12 @@ func testStoreSeek(t *testing.T, s Store) {
 		start := []byte{}
 		goodkvs := make([]KeyValue, len(kvs))
 		copy(goodkvs, kvs)
-		check(t, goodprefix, start, goodkvs)
+		t.Run("forwards", func(t *testing.T) {
+			check(t, goodprefix, start, goodkvs, false)
+		})
+		t.Run("backwards", func(t *testing.T) {
+			check(t, goodprefix, start, goodkvs, true)
+		})
 	})
 
 	require.NoError(t, s.Close())