fix row merging bug

trying to be clever, we reused the memory allocated for the left
operand when doing partial merges
this had been tested to be safe, in general.  however, the
implementation was then written such that we always reused
globally defined operands, this meant that we mutated
the operands which were intended to always represent
+1/-1
this then cascades quickly to making increment/decrement
values much larger/smaller than they should be
related to #197

index/upside_down/row_merge.go

@@ -61,8 +61,9 @@ func (m *upsideDownMerge) FullMerge(key, existingValue []byte, operands [][]byte
 func (m *upsideDownMerge) PartialMerge(key, leftOperand, rightOperand []byte) ([]byte, bool) {
 	left := int64(binary.LittleEndian.Uint64(leftOperand))
 	right := int64(binary.LittleEndian.Uint64(rightOperand))
-	binary.LittleEndian.PutUint64(leftOperand, uint64(left+right))
-	return leftOperand, true
+	rv := make([]byte, 8)
+	binary.LittleEndian.PutUint64(rv, uint64(left+right))
+	return rv, true
 }
 
 func (m *upsideDownMerge) Name() string {

index/upside_down/row_merge_test.go

@@ -0,0 +1,52 @@
+//  Copyright (c) 2014 Couchbase, Inc.
+//  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+//  except in compliance with the License. You may obtain a copy of the License at
+//    http://www.apache.org/licenses/LICENSE-2.0
+//  Unless required by applicable law or agreed to in writing, software distributed under the
+//  License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+//  either express or implied. See the License for the specific language governing permissions
+//  and limitations under the License.
+
+package upside_down
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+)
+
+func TestPartialMerge(t *testing.T) {
+
+	tests := []struct {
+		in  [][]byte
+		out uint64
+	}{
+		{
+			in:  [][]byte{dictionaryTermIncr, dictionaryTermIncr, dictionaryTermIncr, dictionaryTermIncr, dictionaryTermIncr},
+			out: 5,
+		},
+	}
+
+	mo := &upsideDownMerge{}
+	for _, test := range tests {
+		curr := test.in[0]
+		for _, next := range test.in[1:] {
+			var ok bool
+			curr, ok = mo.PartialMerge([]byte("key"), curr, next)
+			if !ok {
+				t.Errorf("expected partial merge ok")
+			}
+		}
+		actual := decodeCount(curr)
+		if actual != test.out {
+			t.Errorf("expected %d, got %d", test.out, actual)
+		}
+	}
+
+}
+
+func decodeCount(in []byte) uint64 {
+	buf := bytes.NewBuffer(in)
+	count, _ := binary.ReadUvarint(buf)
+	return count
+}