add key dedupe when a write buffer writeback to an empty read buffer bucket.

Signed-off-by: Siyuan Zhang <sizhang@google.com>

server/storage/backend/tx_buffer.go

@@ -97,6 +97,9 @@ func (txw *txWriteBuffer) writeback(txr *txReadBuffer) {
 		rb, ok := txr.buckets[k]
 		if !ok {
 			delete(txw.buckets, k)
+			if seq, ok := txw.bucket2seq[k]; ok && !seq {
+				wb.dedupe()
+			}
 			txr.buckets[k] = wb
 			continue
 		}
@@ -218,10 +221,15 @@ func (bb *bucketBuffer) merge(bbsrc *bucketBuffer) {
 	if bytes.Compare(bb.buf[(bb.used-bbsrc.used)-1].key, bbsrc.buf[0].key) < 0 {
 		return
 	}
+	bb.dedupe()
+}
 
+// dedupe removes duplicates, using only newest update
+func (bb *bucketBuffer) dedupe() {
+	if bb.used <= 1 {
+		return
+	}
 	sort.Stable(bb)
-
-	// remove duplicates, using only newest update
 	widx := 0
 	for ridx := 1; ridx < bb.used; ridx++ {
 		if !bytes.Equal(bb.buf[ridx].key, bb.buf[widx].key) {

server/storage/backend/tx_buffer_test.go

@@ -90,3 +90,53 @@ func Test_bucketBuffer_CopyUsed(t *testing.T) {
 		})
 	}
 }
+
+func TestDedupe(t *testing.T) {
+	tests := []struct {
+		name                                   string
+		keys, vals, expectedKeys, expectedVals []string
+	}{
+		{
+			name:         "empty",
+			keys:         []string{},
+			vals:         []string{},
+			expectedKeys: []string{},
+			expectedVals: []string{},
+		},
+		{
+			name:         "single kv",
+			keys:         []string{"key1"},
+			vals:         []string{"val1"},
+			expectedKeys: []string{"key1"},
+			expectedVals: []string{"val1"},
+		},
+		{
+			name:         "duplicate key",
+			keys:         []string{"key1", "key1"},
+			vals:         []string{"val1", "val2"},
+			expectedKeys: []string{"key1"},
+			expectedVals: []string{"val2"},
+		},
+		{
+			name:         "unordered keys",
+			keys:         []string{"key3", "key1", "key4", "key2", "key1", "key4"},
+			vals:         []string{"val1", "val5", "val3", "val4", "val2", "val6"},
+			expectedKeys: []string{"key1", "key2", "key3", "key4"},
+			expectedVals: []string{"val2", "val4", "val1", "val6"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			bb := &bucketBuffer{buf: make([]kv, 10), used: 0}
+			for i := 0; i < len(tt.keys); i++ {
+				bb.add([]byte(tt.keys[i]), []byte(tt.vals[i]))
+			}
+			bb.dedupe()
+			assert.Equal(t, bb.used, len(tt.expectedKeys))
+			for i := 0; i < bb.used; i++ {
+				assert.Equal(t, bb.buf[i].key, []byte(tt.expectedKeys[i]))
+				assert.Equal(t, bb.buf[i].val, []byte(tt.expectedVals[i]))
+			}
+		})
+	}
+}