server: optimizing memory overhead of copy operation in ConcurrentReadTxn

server/storage/backend/tx_buffer.go

@@ -17,6 +17,8 @@ package backend
 import (
 	"bytes"
 	"sort"
+
+	"go.etcd.io/etcd/client/pkg/v3/verify"
 )
 
 const bucketBufferInitialSize = 512
@@ -124,7 +126,7 @@ func (txr *txReadBuffer) unsafeCopy() txReadBuffer {
 		bufVersion: 0,
 	}
 	for bucketName, bucket := range txr.txBuffer.buckets {
-		txrCopy.txBuffer.buckets[bucketName] = bucket.Copy()
+		txrCopy.txBuffer.buckets[bucketName] = bucket.CopyUsed()
 	}
 	return txrCopy
 }
@@ -221,11 +223,13 @@ func (bb *bucketBuffer) Less(i, j int) bool {
 }
 func (bb *bucketBuffer) Swap(i, j int) { bb.buf[i], bb.buf[j] = bb.buf[j], bb.buf[i] }
 
-func (bb *bucketBuffer) Copy() *bucketBuffer {
+func (bb *bucketBuffer) CopyUsed() *bucketBuffer {
+	verify.Assert(bb.used <= len(bb.buf),
+		"used (%d) should never be bigger than the length of buf (%d)", bb.used, len(bb.buf))
 	bbCopy := bucketBuffer{
-		buf:  make([]kv, len(bb.buf)),
+		buf:  make([]kv, bb.used),
 		used: bb.used,
 	}
-	copy(bbCopy.buf, bb.buf)
+	copy(bbCopy.buf, bb.buf[:bb.used])
 	return &bbCopy
 }

server/storage/backend/tx_buffer_test.go

@@ -0,0 +1,92 @@
+// Copyright 2023 The etcd Authors
+//
+// 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 backend
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_bucketBuffer_CopyUsed_After_Add(t *testing.T) {
+	bb := &bucketBuffer{buf: make([]kv, 10), used: 0}
+	for i := 0; i < 20; i++ {
+		k := fmt.Sprintf("key%d", i)
+		v := fmt.Sprintf("val%d", i)
+		bb.add([]byte(k), []byte(v))
+		bbCopy := bb.CopyUsed()
+		assert.Equal(t, bb.used, bbCopy.used)
+		assert.Equal(t, bbCopy.used, len(bbCopy.buf))
+		assert.GreaterOrEqual(t, len(bb.buf), len(bbCopy.buf))
+	}
+}
+
+func Test_bucketBuffer_CopyUsed(t *testing.T) {
+	tests := []struct {
+		name       string
+		bufLen     int
+		used       int
+		wantPanic  bool
+		wantUsed   int
+		wantBufLen int
+	}{
+		{
+			name:       "used is 0",
+			bufLen:     10,
+			used:       0,
+			wantPanic:  false,
+			wantUsed:   0,
+			wantBufLen: 0,
+		},
+		{
+			name:       "used is greater than 0 and less than len(buf)",
+			bufLen:     10,
+			used:       5,
+			wantPanic:  false,
+			wantUsed:   5,
+			wantBufLen: 5,
+		},
+		{
+			name:       "used is equal to len(buf)",
+			bufLen:     10,
+			used:       10,
+			wantPanic:  false,
+			wantUsed:   10,
+			wantBufLen: 10,
+		},
+		{
+			name:      "used is greater than len(buf)",
+			bufLen:    10,
+			used:      11,
+			wantPanic: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			bb := &bucketBuffer{buf: make([]kv, tt.bufLen), used: tt.used}
+			if tt.wantPanic {
+				assert.Panicsf(t, func() {
+					bb.CopyUsed()
+				}, "expected panic when used (%d) and the length of buf (%d)", tt.used, tt.bufLen)
+			} else {
+				bbCopy := bb.CopyUsed()
+				assert.Equal(t, tt.wantUsed, bbCopy.used)
+				assert.Equal(t, tt.wantBufLen, len(bbCopy.buf))
+			}
+		})
+	}
+}