mvcc: don't allocate end revision while computing range

Use 'nil' since it's only reading a single key. Also preallocates
the result slice based on limit / number of revisions fetched.

Fixes #8208

mvcc/kvstore.go

@@ -445,16 +445,3 @@ func appendMarkTombstone(b []byte) []byte {
 func isTombstone(b []byte) bool {
 	return len(b) == markedRevBytesLen && b[markBytePosition] == markTombstone
 }
-
-// revBytesRange returns the range of revision bytes at
-// the given revision.
-func revBytesRange(rev revision) (start, end []byte) {
-	start = newRevBytes()
-	revToBytes(rev, start)
-
-	end = newRevBytes()
-	endRev := revision{main: rev.main, sub: rev.sub + 1}
-	revToBytes(endRev, end)
-
-	return start, end
-}

mvcc/kvstore_test.go

@@ -217,9 +217,10 @@ func TestStoreRange(t *testing.T) {
 			t.Errorf("#%d: rev = %d, want %d", i, ret.Rev, wrev)
 		}
 
-		wstart, wend := revBytesRange(tt.idxr.revs[0])
+		wstart := newRevBytes()
+		revToBytes(tt.idxr.revs[0], wstart)
 		wact := []testutil.Action{
-			{"range", []interface{}{keyBucketName, wstart, wend, int64(0)}},
+			{"range", []interface{}{keyBucketName, wstart, []byte(nil), int64(0)}},
 		}
 		if g := b.tx.Action(); !reflect.DeepEqual(g, wact) {
 			t.Errorf("#%d: tx action = %+v, want %+v", i, g, wact)

mvcc/kvstore_txn.go

@@ -128,22 +128,22 @@ func (tr *storeTxnRead) rangeKeys(key, end []byte, curRev int64, ro RangeOptions
 		return &RangeResult{KVs: nil, Count: len(revpairs), Rev: curRev}, nil
 	}
 
-	var kvs []mvccpb.KeyValue
-	for _, revpair := range revpairs {
-		start, end := revBytesRange(revpair)
-		_, vs := tr.tx.UnsafeRange(keyBucketName, start, end, 0)
+	limit := int(ro.Limit)
+	if limit <= 0 || limit > len(revpairs) {
+		limit = len(revpairs)
+	}
+
+	kvs := make([]mvccpb.KeyValue, limit)
+	revBytes := newRevBytes()
+	for i, revpair := range revpairs[:len(kvs)] {
+		revToBytes(revpair, revBytes)
+		_, vs := tr.tx.UnsafeRange(keyBucketName, revBytes, nil, 0)
 		if len(vs) != 1 {
 			plog.Fatalf("range cannot find rev (%d,%d)", revpair.main, revpair.sub)
 		}
-
-		var kv mvccpb.KeyValue
-		if err := kv.Unmarshal(vs[0]); err != nil {
+		if err := kvs[i].Unmarshal(vs[0]); err != nil {
 			plog.Fatalf("cannot unmarshal event: %v", err)
 		}
-		kvs = append(kvs, kv)
-		if ro.Limit > 0 && len(kvs) >= int(ro.Limit) {
-			break
-		}
 	}
 	return &RangeResult{KVs: kvs, Count: len(revpairs), Rev: curRev}, nil
 }