lexi: fix backwards iteration with prefix

dex/lexi/db_test.go

@@ -70,6 +70,10 @@ func valueIndex(k, v encoding.BinaryMarshaler) ([]byte, error) {
 	return v.(*tValue).idx, nil
 }
 
+func valueKey(k, v encoding.BinaryMarshaler) ([]byte, error) {
+	return v.(*tValue).k, nil
+}
+
 func TestIndex(t *testing.T) {
 	db, shutdown := newTestDB(t)
 	defer shutdown()
@@ -84,11 +88,26 @@ func TestIndex(t *testing.T) {
 		t.Fatalf("Error adding index: %v", err)
 	}
 
+	keyIdx, err := tbl.AddIndex("K", valueKey)
+	if err != nil {
+		t.Fatalf("Error adding index: %v", err)
+	}
+
+	// Put 100 values in.
 	const nVs = 100
 	vs := make([]*tValue, nVs)
 	for i := 0; i < nVs; i++ {
+		// Random value, but with a flag at the end.
 		k := append(encode.RandomBytes(5), byte(i))
-		v := &tValue{k: []byte{byte(i)}, v: encode.RandomBytes(10), idx: []byte{byte(i)}}
+		// The index is keyed on i, with a prefix of 0, until 40, after which
+		// the prefix is 1.
+		indexKey := []byte{byte(i)}
+		prefix := []byte{0}
+		if i >= 40 {
+			prefix = []byte{1}
+		}
+		indexKey = append(prefix, indexKey...)
+		v := &tValue{k: indexKey, v: encode.RandomBytes(10), idx: []byte{byte(i)}}
 		vs[i] = v
 		if err := tbl.Set(B(k), v); err != nil {
 			t.Fatalf("Error setting table entry: %v", err)
@@ -128,6 +147,37 @@ func TestIndex(t *testing.T) {
 		t.Fatalf("Expected to iterate back to zero but only got to %d", i)
 	}
 
+	// Iterate forwards with prefix.
+	keyIdx.Iterate([]byte{0}, func(it *Iter) error {
+		v := vs[i]
+		it.V(func(vB []byte) error {
+			if !bytes.Equal(vB, v.v) {
+				t.Fatalf("Wrong bytes for forward iteration index %d", i)
+			}
+			return nil
+		})
+		i++
+		return nil
+	})
+	if i != 40 {
+		t.Fatalf("Expected to iterate 40 items but only did %d", i)
+	}
+
+	// Iterate backwards with prefix.
+	keyIdx.Iterate([]byte{0}, func(it *Iter) error {
+		i--
+		v := vs[i]
+		return it.V(func(vB []byte) error {
+			if !bytes.Equal(vB, v.v) {
+				t.Fatalf("Wrong bytes for reverse iteration index %d", i)
+			}
+			return nil
+		})
+	}, WithReverse())
+	if i != 0 {
+		t.Fatalf("Expected to iterate back to zero but only got to %d", i)
+	}
+
 	// Iterate forward and delete the first half.
 	i = 0
 	if err := idx.Iterate(nil, func(it *Iter) error {

dex/lexi/index.go

@@ -4,6 +4,7 @@
 package lexi
 
 import (
+	"bytes"
 	"encoding"
 	"encoding/binary"
 	"errors"
@@ -265,6 +266,26 @@ func iteratePrefix(txn *badger.Txn, prefix, seek []byte, f func(iter *badger.Ite
 	return nil
 }
 
+// https://github.com/dgraph-io/badger/issues/436#issuecomment-1073008604
+func seekLast(it *badger.Iterator, prefix []byte) {
+	tweaked := make([]byte, len(prefix))
+	copy(tweaked, prefix)
+	n := len(prefix)
+	for n > 0 {
+		if tweaked[n-1] == 0xff {
+			n -= 1
+		} else {
+			tweaked[n-1] += 1
+			break
+		}
+	}
+	tweaked = tweaked[0:n]
+	it.Seek(tweaked)
+	if it.Valid() && bytes.Equal(tweaked, it.Item().Key()) {
+		it.Next()
+	}
+}
+
 func reverseIteratePrefix(txn *badger.Txn, prefix, seek []byte, f func(iter *badger.Iterator) error, iterOpts ...badgerIterationOption) error {
 	opts := badger.DefaultIteratorOptions
 	opts.Prefix = prefix
@@ -276,15 +297,12 @@ func reverseIteratePrefix(txn *badger.Txn, prefix, seek []byte, f func(iter *bad
 	defer iter.Close()
 
 	if len(seek) == 0 {
-		var p keyPrefix
-		copy(p[:], prefix)
-		nextPrefix := incrementPrefix(p)
-		seek = nextPrefix[:]
+		seekLast(iter, prefix)
 	} else {
-		seek = append(seek, lastDBID[:]...)
+		iter.Seek(append(seek, lastDBID[:]...))
 	}
 
-	for iter.Seek(seek); iter.ValidForPrefix(prefix); iter.Next() {
+	for ; iter.ValidForPrefix(prefix); iter.Next() {
 		if err := f(iter); err != nil {
 			if errors.Is(err, ErrEndIteration) {
 				return nil