refactor: single-pass hashing during upload via TeeReader

Previous approach read the file twice (once for SHA-256, once for upload),
doubling disk I/O. Under concurrent multi-target uploads this becomes a
bottleneck.

New design — hashingReader wraps io.TeeReader + sha256.Hash:
  file.Read() → TeeReader → sha256.Write() (hash) + provider (upload)
Single read pass yields both byte count and SHA-256 simultaneously.

Each upload goroutine independently opens the file and computes its own
hash. The first successful target writes checksum to the record via
sync.Once. Zero extra disk I/O, zero extra memory copies, fully
concurrent-safe.

server/internal/backup/helpers.go

@@ -1,10 +1,7 @@
 package backup
 
 import (
-	"crypto/sha256"
-	"encoding/hex"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -24,20 +21,6 @@ func createTempArtifact(baseDir, taskName string, extension string) (string, str
 	return tempDir, filepath.Join(tempDir, fileName), nil
 }
 
-// SHA256File 计算文件的 SHA-256 哈希值，返回十六进制字符串
-func SHA256File(path string) (string, error) {
-	file, err := os.Open(path)
-	if err != nil {
-		return "", fmt.Errorf("open file for checksum: %w", err)
-	}
-	defer file.Close()
-	hash := sha256.New()
-	if _, err := io.Copy(hash, file); err != nil {
-		return "", fmt.Errorf("compute checksum: %w", err)
-	}
-	return hex.EncodeToString(hash.Sum(nil)), nil
-}
-
 func sanitizeFileName(value string) string {
 	builder := strings.Builder{}
 	for _, char := range strings.TrimSpace(value) {

server/internal/service/backup_execution_service.go

@@ -2,8 +2,11 @@ package service
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"hash"
 	"io"
 	"os"
 	"path/filepath"
@@ -326,17 +329,6 @@ func (s *BackupExecutionService) executeTask(ctx context.Context, task *model.Ba
 	fileName = filepath.Base(finalPath)
 	storagePath = backup.BuildStorageKey(task.Type, startedAt, fileName)
 
-	// 计算文件 SHA-256 哈希（上传前）
-	logger.Infof("计算备份文件校验和...")
-	localChecksum, checksumErr := backup.SHA256File(finalPath)
-	if checksumErr != nil {
-		errMessage = checksumErr.Error()
-		logger.Errorf("计算文件校验和失败：%v", checksumErr)
-		return
-	}
-	checksum = localChecksum
-	logger.Infof("文件校验和: SHA-256=%s, 大小=%d bytes", checksum, fileSize)
-
 	// 收集所有存储目标
 	targetIDs := collectTargetIDs(task)
 	if len(targetIDs) == 0 {
@@ -347,6 +339,7 @@ func (s *BackupExecutionService) executeTask(ctx context.Context, task *model.Ba
 
 	// 并行上传到所有目标
 	uploadResults = make([]StorageUploadResultItem, len(targetIDs))
+	var checksumOnce sync.Once
 	var wg sync.WaitGroup
 	for i, tid := range targetIDs {
 		wg.Add(1)
@@ -371,22 +364,25 @@ func (s *BackupExecutionService) executeTask(ctx context.Context, task *model.Ba
 			}
 			defer artifact.Close()
 			logger.Infof("开始上传备份到存储目标：%s", targetName)
-			// 用 CountingReader 包装，上传过程中统计实际传输字节数（零额外开销）
+			// hashingReader: 上传过程中同步计算字节数 + SHA-256，单次读取零额外 I/O
-			counter := &countingReader{reader: artifact}
+			hr := newHashingReader(artifact)
-			if uploadErr := provider.Upload(ctx, storagePath, counter, fileSize, map[string]string{"taskId": fmt.Sprintf("%d", task.ID), "recordId": fmt.Sprintf("%d", recordID)}); uploadErr != nil {
+			if uploadErr := provider.Upload(ctx, storagePath, hr, fileSize, map[string]string{"taskId": fmt.Sprintf("%d", task.ID), "recordId": fmt.Sprintf("%d", recordID)}); uploadErr != nil {
 				uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: uploadErr.Error()}
 				logger.Warnf("存储目标 %s 上传失败：%v", targetName, uploadErr)
 				return
 			}
-			// 完整性校验：对比实际传输字节数与本地文件大小
+			// 完整性校验：对比实际传输字节数
-			if counter.n != fileSize {
+			if hr.n != fileSize {
-				uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: fmt.Sprintf("完整性校验失败: 预期 %d bytes, 实际传输 %d bytes", fileSize, counter.n)}
+				uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "failed", Error: fmt.Sprintf("完整性校验失败: 预期 %d bytes, 实际传输 %d bytes", fileSize, hr.n)}
-				logger.Errorf("存储目标 %s 完整性校验失败：预期 %d bytes, 实际传输 %d bytes", targetName, fileSize, counter.n)
+				logger.Errorf("存储目标 %s 完整性校验失败：预期 %d bytes, 实际传输 %d bytes", targetName, fileSize, hr.n)
 				_ = provider.Delete(ctx, storagePath)
 				return
 			}
+			// 取第一个成功目标的哈希写入 record（所有目标读同一文件，哈希一定相同）
+			targetChecksum := hr.Sum()
+			checksumOnce.Do(func() { checksum = targetChecksum })
 			uploadResults[index] = StorageUploadResultItem{StorageTargetID: targetID, StorageTargetName: targetName, Status: "success", StoragePath: storagePath, FileSize: fileSize}
-			logger.Infof("存储目标 %s 上传成功 (%d bytes, SHA-256=%s)", targetName, fileSize, localChecksum)
+			logger.Infof("存储目标 %s 上传成功 (%d bytes, SHA-256=%s)", targetName, fileSize, targetChecksum)
 			// 每个成功目标独立执行保留策略
 			if s.retention != nil {
 				cleanupResult, cleanupErr := s.retention.Cleanup(ctx, task, provider)
@@ -601,14 +597,27 @@ func buildStorageProviderFromRepos(ctx context.Context, storageTargetID uint, st
 	return provider, target, nil
 }
 
-// countingReader 包装 io.Reader，统计实际读取字节数
+// hashingReader 在上传过程中同步计算字节数和 SHA-256，零额外 I/O
-type countingReader struct {
+type hashingReader struct {
 	reader io.Reader
+	hash   hash.Hash
 	n      int64
 }
 
-func (r *countingReader) Read(p []byte) (int, error) {
+func newHashingReader(reader io.Reader) *hashingReader {
+	h := sha256.New()
+	return &hashingReader{
+		reader: io.TeeReader(reader, h),
+		hash:   h,
+	}
+}
+
+func (r *hashingReader) Read(p []byte) (int, error) {
 	n, err := r.reader.Read(p)
 	r.n += int64(n)
 	return n, err
 }
+
+func (r *hashingReader) Sum() string {
+	return hex.EncodeToString(r.hash.Sum(nil))
+}