Working MultipartUpload that needs minor tweaks.

api/api.go

@@ -71,6 +71,11 @@ func (h *Handler) handleGetObject(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if uploadID := r.URL.Query().Get("uploadId"); uploadID != "" {
+		h.handleListMultipartParts(w, r, bucket, key, uploadID)
+		return
+	}
+
 	stream, manifest, err := h.svc.GetObject(bucket, key)
 	if err != nil {
 		writeMappedS3Error(w, r, err)
@@ -94,6 +99,7 @@ func (h *Handler) handlePostObject(w http.ResponseWriter, r *http.Request) {
 		writeS3Error(w, r, s3ErrInvalidObjectKey, r.URL.Path)
 		return
 	}
+	defer r.Body.Close()
 
 	if _, ok := r.URL.Query()["uploads"]; ok {
 		upload, err := h.svc.CreateMultipartUpload(bucket, key)
@@ -119,6 +125,39 @@ func (h *Handler) handlePostObject(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if uploadID := r.URL.Query().Get("uploadId"); uploadID != "" {
+		var req models.CompleteMultipartUploadRequest
+		if err := xml.NewDecoder(r.Body).Decode(&req); err != nil {
+			writeS3Error(w, r, s3ErrMalformedXML, r.URL.Path)
+			return
+		}
+
+		manifest, err := h.svc.CompleteMultipartUpload(bucket, key, uploadID, req.Parts)
+		if err != nil {
+			writeMappedS3Error(w, r, err)
+			return
+		}
+
+		response := models.CompleteMultipartUploadResult{
+			Xmlns:    "http://s3.amazonaws.com/doc/2006-03-01/",
+			Bucket:   bucket,
+			Key:      key,
+			ETag:     `"` + manifest.ETag + `"`,
+			Location: r.URL.Path,
+		}
+		payload, err := xml.MarshalIndent(response, "", "    ")
+		if err != nil {
+			writeMappedS3Error(w, r, err)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(xml.Header))
+		_, _ = w.Write(payload)
+		return
+	}
+
 	writeS3Error(w, r, s3ErrNotImplemented, r.URL.Path)
 }
 
@@ -129,9 +168,31 @@ func (h *Handler) handlePutObject(w http.ResponseWriter, r *http.Request) {
 		writeS3Error(w, r, s3ErrInvalidObjectKey, r.URL.Path)
 		return
 	}
-	if r.URL.Query().Get("uploads") != "" {
-		if r.URL.Query().Get("partNumber") != "" {
+	defer r.Body.Close()
+
+	uploadID := r.URL.Query().Get("uploadId")
+	partNumberRaw := r.URL.Query().Get("partNumber")
+	if uploadID != "" || partNumberRaw != "" {
+		if uploadID == "" || partNumberRaw == "" {
+			writeS3Error(w, r, s3ErrInvalidPart, r.URL.Path)
+			return
 		}
+
+		partNumber, err := strconv.Atoi(partNumberRaw)
+		if err != nil {
+			writeS3Error(w, r, s3ErrInvalidPart, r.URL.Path)
+			return
+		}
+
+		etag, err := h.svc.UploadPart(bucket, key, uploadID, partNumber, r.Body)
+		if err != nil {
+			writeMappedS3Error(w, r, err)
+			return
+		}
+		w.Header().Set("ETag", `"`+etag+`"`)
+		w.Header().Set("Content-Length", "0")
+		w.WriteHeader(http.StatusOK)
+		return
 	}
 
 	contentType := r.Header.Get("Content-Type")
@@ -140,7 +201,6 @@ func (h *Handler) handlePutObject(w http.ResponseWriter, r *http.Request) {
 	}
 
 	manifest, err := h.svc.PutObject(bucket, key, contentType, r.Body)
-	defer r.Body.Close()
 
 	if err != nil {
 		writeMappedS3Error(w, r, err)
@@ -153,6 +213,41 @@ func (h *Handler) handlePutObject(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusOK)
 }
 
+func (h *Handler) handleListMultipartParts(w http.ResponseWriter, r *http.Request, bucket, key, uploadID string) {
+	parts, err := h.svc.ListMultipartParts(bucket, key, uploadID)
+	if err != nil {
+		writeMappedS3Error(w, r, err)
+		return
+	}
+
+	response := models.ListPartsResult{
+		Xmlns:    "http://s3.amazonaws.com/doc/2006-03-01/",
+		Bucket:   bucket,
+		Key:      key,
+		UploadID: uploadID,
+		Parts:    make([]models.PartItem, 0, len(parts)),
+	}
+	for _, part := range parts {
+		response.Parts = append(response.Parts, models.PartItem{
+			PartNumber:   part.PartNumber,
+			LastModified: time.Unix(part.CreatedAt, 0).UTC().Format("2006-01-02T15:04:05.000Z"),
+			ETag:         `"` + part.ETag + `"`,
+			Size:         part.Size,
+		})
+	}
+
+	payload, err := xml.MarshalIndent(response, "", "    ")
+	if err != nil {
+		writeMappedS3Error(w, r, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte(xml.Header))
+	_, _ = w.Write(payload)
+}
+
 func (h *Handler) handleHeadObject(w http.ResponseWriter, r *http.Request) {
 	bucket := chi.URLParam(r, "bucket")
 	key := chi.URLParam(r, "*")
@@ -198,7 +293,15 @@ func (h *Handler) handleDeleteObject(w http.ResponseWriter, r *http.Request) {
 		writeS3Error(w, r, s3ErrInvalidObjectKey, r.URL.Path)
 		return
 	}
-
+	if uploadId := r.URL.Query().Get("uploadId"); uploadId != "" {
+		err := h.svc.AbortMultipartUpload(bucket, key, uploadId)
+		if err != nil {
+			writeMappedS3Error(w, r, err)
+			return
+		}
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
 	err := h.svc.DeleteObject(bucket, key)
 	if err != nil {
 		if errors.Is(err, metadata.ErrObjectNotFound) {

api/s3_errors.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fs/metadata"
 	"fs/models"
+	"fs/service"
 	"net/http"
 )
 
@@ -25,6 +26,21 @@ var (
 		Code:    "NotImplemented",
 		Message: "A header you provided implies functionality that is not implemented.",
 	}
+	s3ErrInvalidPart = s3APIError{
+		Status:  http.StatusBadRequest,
+		Code:    "InvalidPart",
+		Message: "One or more of the specified parts could not be found.",
+	}
+	s3ErrInvalidPartOrder = s3APIError{
+		Status:  http.StatusBadRequest,
+		Code:    "InvalidPartOrder",
+		Message: "The list of parts was not in ascending order.",
+	}
+	s3ErrMalformedXML = s3APIError{
+		Status:  http.StatusBadRequest,
+		Code:    "MalformedXML",
+		Message: "The XML you provided was not well-formed or did not validate against our published schema.",
+	}
 	s3ErrInternal = s3APIError{
 		Status:  http.StatusInternalServerError,
 		Code:    "InternalError",
@@ -64,6 +80,24 @@ func mapToS3Error(err error) s3APIError {
 			Code:    "NoSuchKey",
 			Message: "The specified key does not exist.",
 		}
+	case errors.Is(err, metadata.ErrMultipartNotFound):
+		return s3APIError{
+			Status:  http.StatusNotFound,
+			Code:    "NoSuchUpload",
+			Message: "The specified multipart upload does not exist.",
+		}
+	case errors.Is(err, metadata.ErrMultipartNotPending):
+		return s3APIError{
+			Status:  http.StatusBadRequest,
+			Code:    "InvalidRequest",
+			Message: "The multipart upload is not in a valid state for this operation.",
+		}
+	case errors.Is(err, service.ErrInvalidPart):
+		return s3ErrInvalidPart
+	case errors.Is(err, service.ErrInvalidPartOrder):
+		return s3ErrInvalidPartOrder
+	case errors.Is(err, service.ErrInvalidCompleteRequest):
+		return s3ErrMalformedXML
 	default:
 		return s3ErrInternal
 	}

metadata/metadata.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"fs/models"
 	"regexp"
+	"sort"
 	"strings"
 	"time"
 
@@ -20,6 +21,7 @@ type MetadataHandler struct {
 
 var systemIndex = []byte("__SYSTEM_BUCKETS__")
 var multipartUploadIndex = []byte("__MULTIPART_UPLOADS__")
+var multipartUploadPartsIndex = []byte("__MULTIPART_UPLOAD_PARTS__")
 
 var validBucketName = regexp.MustCompile(`^[a-z0-9.-]{3,63}$`)
 
@@ -29,6 +31,8 @@ var (
 	ErrBucketNotFound      = errors.New("bucket not found")
 	ErrBucketNotEmpty      = errors.New("bucket not empty")
 	ErrObjectNotFound      = errors.New("object not found")
+	ErrMultipartNotFound   = errors.New("multipart upload not found")
+	ErrMultipartNotPending = errors.New("multipart upload is not pending")
 )
 
 func NewMetadataHandler(dbPath string) (*MetadataHandler, error) {
@@ -54,6 +58,14 @@ func NewMetadataHandler(dbPath string) (*MetadataHandler, error) {
 		_ = db.Close()
 		return nil, err
 	}
+	err = h.db.Update(func(tx *bbolt.Tx) error {
+		_, err := tx.CreateBucketIfNotExists(multipartUploadPartsIndex)
+		return err
+	})
+	if err != nil {
+		_ = db.Close()
+		return nil, err
+	}
 
 	return h, nil
 }
@@ -325,3 +337,219 @@ func (h *MetadataHandler) CreateMultipartUpload(bucket, key string) (*models.Mul
 
 	return upload, nil
 }
+
+func getMultipartUploadBucket(tx *bbolt.Tx) (*bbolt.Bucket, error) {
+	multipartUploadBucket := tx.Bucket(multipartUploadIndex)
+	if multipartUploadBucket == nil {
+		return nil, errors.New("multipart upload index not found")
+	}
+	return multipartUploadBucket, nil
+}
+
+func getMultipartPartsBucket(tx *bbolt.Tx) (*bbolt.Bucket, error) {
+	multipartPartsBucket := tx.Bucket(multipartUploadPartsIndex)
+	if multipartPartsBucket == nil {
+		return nil, errors.New("multipart upload parts index not found")
+	}
+	return multipartPartsBucket, nil
+}
+
+func getMultipartUploadFromBucket(multipartUploadBucket *bbolt.Bucket, uploadID string) (*models.MultipartUpload, error) {
+	payload := multipartUploadBucket.Get([]byte(uploadID))
+	if payload == nil {
+		return nil, fmt.Errorf("%w: %s", ErrMultipartNotFound, uploadID)
+	}
+	upload := models.MultipartUpload{}
+	if err := json.Unmarshal(payload, &upload); err != nil {
+		return nil, err
+	}
+	return &upload, nil
+}
+
+func getMultipartUploadFromTx(tx *bbolt.Tx, uploadID string) (*models.MultipartUpload, *bbolt.Bucket, error) {
+	multipartUploadBucket, err := getMultipartUploadBucket(tx)
+	if err != nil {
+		return nil, nil, err
+	}
+	upload, err := getMultipartUploadFromBucket(multipartUploadBucket, uploadID)
+	if err != nil {
+		return nil, nil, err
+	}
+	return upload, multipartUploadBucket, nil
+}
+
+func putMultipartUpload(multipartUploadBucket *bbolt.Bucket, uploadID string, upload *models.MultipartUpload) error {
+	payload, err := json.Marshal(upload)
+	if err != nil {
+		return err
+	}
+	return multipartUploadBucket.Put([]byte(uploadID), payload)
+}
+
+func deleteMultipartPartsByUploadID(tx *bbolt.Tx, uploadID string) error {
+	multipartPartsBucket, err := getMultipartPartsBucket(tx)
+	if err != nil {
+		return err
+	}
+
+	prefix := uploadID + ":"
+	cursor := multipartPartsBucket.Cursor()
+	keysToDelete := make([][]byte, 0)
+	for k, _ := cursor.Seek([]byte(prefix)); k != nil && strings.HasPrefix(string(k), prefix); k, _ = cursor.Next() {
+		keyCopy := make([]byte, len(k))
+		copy(keyCopy, k)
+		keysToDelete = append(keysToDelete, keyCopy)
+	}
+	for _, key := range keysToDelete {
+		if err := multipartPartsBucket.Delete(key); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (h *MetadataHandler) GetMultipartUpload(uploadID string) (*models.MultipartUpload, error) {
+	var upload *models.MultipartUpload
+	err := h.db.View(func(tx *bbolt.Tx) error {
+		var err error
+		upload, _, err = getMultipartUploadFromTx(tx, uploadID)
+		if err != nil {
+			return err
+		}
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return upload, nil
+}
+func (h *MetadataHandler) PutMultipartPart(uploadID string, part models.UploadedPart) error {
+	if part.PartNumber < 1 || part.PartNumber > 10000 {
+		return fmt.Errorf("invalid part number: %d", part.PartNumber)
+	}
+
+	err := h.db.Update(func(tx *bbolt.Tx) error {
+		upload, _, err := getMultipartUploadFromTx(tx, uploadID)
+		if err != nil {
+			return err
+		}
+		if upload.State != "pending" {
+			return fmt.Errorf("%w: %s", ErrMultipartNotPending, uploadID)
+		}
+
+		multipartPartsBucket, err := getMultipartPartsBucket(tx)
+		if err != nil {
+			return err
+		}
+
+		key := fmt.Sprintf("%s:%05d", uploadID, part.PartNumber)
+		payload, err := json.Marshal(part)
+		if err != nil {
+			return err
+		}
+		return multipartPartsBucket.Put([]byte(key), payload)
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}
+func (h *MetadataHandler) ListMultipartParts(uploadID string) ([]models.UploadedPart, error) {
+	parts := make([]models.UploadedPart, 0)
+
+	err := h.db.View(func(tx *bbolt.Tx) error {
+		if _, _, err := getMultipartUploadFromTx(tx, uploadID); err != nil {
+			return err
+		}
+
+		multipartPartsBucket, err := getMultipartPartsBucket(tx)
+		if err != nil {
+			return err
+		}
+		prefix := uploadID + ":"
+		cursor := multipartPartsBucket.Cursor()
+		for k, v := cursor.Seek([]byte(prefix)); k != nil && strings.HasPrefix(string(k), prefix); k, v = cursor.Next() {
+			part := models.UploadedPart{}
+			if err := json.Unmarshal(v, &part); err != nil {
+				return err
+			}
+			parts = append(parts, part)
+		}
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	sort.Slice(parts, func(i, j int) bool {
+		return parts[i].PartNumber < parts[j].PartNumber
+	})
+	return parts, nil
+}
+func (h *MetadataHandler) CompleteMultipartUpload(uploadID string, final *models.ObjectManifest) error {
+	if final == nil {
+		return errors.New("final object manifest is required")
+	}
+
+	err := h.db.Update(func(tx *bbolt.Tx) error {
+		upload, multipartUploadBucket, err := getMultipartUploadFromTx(tx, uploadID)
+		if err != nil {
+			return err
+		}
+		if upload.State != "pending" {
+			return fmt.Errorf("%w: %s", ErrMultipartNotPending, uploadID)
+		}
+
+		metadataBucket := tx.Bucket([]byte(upload.Bucket))
+		if metadataBucket == nil {
+			return fmt.Errorf("%w: %s", ErrBucketNotFound, upload.Bucket)
+		}
+		final.Bucket = upload.Bucket
+		final.Key = upload.Key
+		finalPayload, err := json.Marshal(final)
+		if err != nil {
+			return err
+		}
+		if err := metadataBucket.Put([]byte(upload.Key), finalPayload); err != nil {
+			return err
+		}
+
+		upload.State = "completed"
+		if err := putMultipartUpload(multipartUploadBucket, uploadID, upload); err != nil {
+			return err
+		}
+
+		if err := deleteMultipartPartsByUploadID(tx, uploadID); err != nil {
+			return err
+		}
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}
+func (h *MetadataHandler) AbortMultipartUpload(uploadID string) error {
+	err := h.db.Update(func(tx *bbolt.Tx) error {
+		upload, multipartUploadBucket, err := getMultipartUploadFromTx(tx, uploadID)
+		if err != nil {
+			return err
+		}
+		if upload.State == "completed" {
+			return fmt.Errorf("%w: %s", ErrMultipartNotPending, uploadID)
+		}
+		upload.State = "aborted"
+		if err := putMultipartUpload(multipartUploadBucket, uploadID, upload); err != nil {
+			return err
+		}
+
+		if err := deleteMultipartPartsByUploadID(tx, uploadID); err != nil {
+			return err
+		}
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}

models/models.go

@@ -74,3 +74,45 @@ type InitiateMultipartUploadResult struct {
 	Key      string   `xml:"Key"`
 	UploadID string   `xml:"UploadId"`
 }
+type UploadedPart struct {
+	PartNumber int      `json:"part_number" xml:"PartNumber"`
+	ETag       string   `json:"etag" xml:"ETag"`
+	Size       int64    `json:"size" xml:"Size"`
+	Chunks     []string `json:"chunks"`
+	CreatedAt  int64    `json:"created_at"`
+}
+
+type CompletedPart struct {
+	PartNumber int    `xml:"PartNumber"`
+	ETag       string `xml:"ETag"`
+}
+
+type CompleteMultipartUploadRequest struct {
+	XMLName xml.Name        `xml:"CompleteMultipartUpload"`
+	Parts   []CompletedPart `xml:"Part"`
+}
+
+type CompleteMultipartUploadResult struct {
+	XMLName  xml.Name `xml:"CompleteMultipartUploadResult"`
+	Xmlns    string   `xml:"xmlns,attr"`
+	Bucket   string   `xml:"Bucket"`
+	Key      string   `xml:"Key"`
+	ETag     string   `xml:"ETag"`
+	Location string   `xml:"Location,omitempty"`
+}
+
+type ListPartsResult struct {
+	XMLName  xml.Name   `xml:"ListPartsResult"`
+	Xmlns    string     `xml:"xmlns,attr"`
+	Bucket   string     `xml:"Bucket"`
+	Key      string     `xml:"Key"`
+	UploadID string     `xml:"UploadId"`
+	Parts    []PartItem `xml:"Part"`
+}
+
+type PartItem struct {
+	PartNumber   int    `xml:"PartNumber"`
+	LastModified string `xml:"LastModified"`
+	ETag         string `xml:"ETag"`
+	Size         int64  `xml:"Size"`
+}

service/service.go

@@ -1,11 +1,15 @@
 package service
 
 import (
+	"crypto/md5"
+	"encoding/hex"
+	"errors"
 	"fmt"
 	"fs/metadata"
 	"fs/models"
 	"fs/storage"
 	"io"
+	"strings"
 	"time"
 )
 
@@ -13,6 +17,12 @@ type ObjectService struct {
 	metadataHandler *metadata.MetadataHandler
 }
 
+var (
+	ErrInvalidPart            = errors.New("invalid multipart part")
+	ErrInvalidPartOrder       = errors.New("invalid multipart part order")
+	ErrInvalidCompleteRequest = errors.New("invalid complete multipart request")
+)
+
 func NewObjectService(metadataHandler *metadata.MetadataHandler) *ObjectService {
 	return &ObjectService{metadataHandler: metadataHandler}
 }
@@ -102,6 +112,137 @@ func (s *ObjectService) CreateMultipartUpload(bucket, key string) (*models.Multi
 	return s.metadataHandler.CreateMultipartUpload(bucket, key)
 }
 
-func (s *ObjectService) PutMultipartObject(bucket, key, uploadId string, input io.Reader) (*models.MultipartUpload, error) {
-	return nil, nil
+func (s *ObjectService) UploadPart(bucket, key, uploadId string, partNumber int, input io.Reader) (string, error) {
+	if partNumber < 1 || partNumber > 10000 {
+		return "", ErrInvalidPart
+	}
+
+	upload, err := s.metadataHandler.GetMultipartUpload(uploadId)
+	if err != nil {
+		return "", err
+	}
+	if upload.Bucket != bucket || upload.Key != key {
+		return "", metadata.ErrMultipartNotFound
+	}
+
+	var uploadedPart models.UploadedPart
+	chunkIds, totalSize, etag, err := storage.IngestStream(input)
+	if err != nil {
+		return "", err
+	}
+	uploadedPart = models.UploadedPart{
+		PartNumber: partNumber,
+		ETag:       etag,
+		Size:       totalSize,
+		Chunks:     chunkIds,
+		CreatedAt:  time.Now().Unix(),
+	}
+	err = s.metadataHandler.PutMultipartPart(uploadId, uploadedPart)
+	if err != nil {
+		return "", err
+	}
+	return etag, nil
+}
+
+func (s *ObjectService) ListMultipartParts(bucket, key, uploadID string) ([]models.UploadedPart, error) {
+	upload, err := s.metadataHandler.GetMultipartUpload(uploadID)
+	if err != nil {
+		return nil, err
+	}
+	if upload.Bucket != bucket || upload.Key != key {
+		return nil, metadata.ErrMultipartNotFound
+	}
+	return s.metadataHandler.ListMultipartParts(uploadID)
+}
+
+func (s *ObjectService) CompleteMultipartUpload(bucket, key, uploadID string, completed []models.CompletedPart) (*models.ObjectManifest, error) {
+	if len(completed) == 0 {
+		return nil, ErrInvalidCompleteRequest
+	}
+
+	upload, err := s.metadataHandler.GetMultipartUpload(uploadID)
+	if err != nil {
+		return nil, err
+	}
+	if upload.Bucket != bucket || upload.Key != key {
+		return nil, metadata.ErrMultipartNotFound
+	}
+
+	storedParts, err := s.metadataHandler.ListMultipartParts(uploadID)
+	if err != nil {
+		return nil, err
+	}
+	partsByNumber := make(map[int]models.UploadedPart, len(storedParts))
+	for _, part := range storedParts {
+		partsByNumber[part.PartNumber] = part
+	}
+
+	lastPartNumber := 0
+	orderedParts := make([]models.UploadedPart, 0, len(completed))
+	chunks := make([]string, 0)
+	var totalSize int64
+
+	for _, part := range completed {
+		if part.PartNumber <= lastPartNumber {
+			return nil, ErrInvalidPartOrder
+		}
+		lastPartNumber = part.PartNumber
+
+		storedPart, ok := partsByNumber[part.PartNumber]
+		if !ok {
+			return nil, ErrInvalidPart
+		}
+		if normalizeETag(part.ETag) != normalizeETag(storedPart.ETag) {
+			return nil, ErrInvalidPart
+		}
+
+		orderedParts = append(orderedParts, storedPart)
+		chunks = append(chunks, storedPart.Chunks...)
+		totalSize += storedPart.Size
+	}
+
+	finalETag := buildMultipartETag(orderedParts)
+	manifest := &models.ObjectManifest{
+		Bucket:      bucket,
+		Key:         key,
+		Size:        totalSize,
+		ContentType: "application/octet-stream",
+		ETag:        finalETag,
+		Chunks:      chunks,
+		CreatedAt:   time.Now().Unix(),
+	}
+
+	if err := s.metadataHandler.CompleteMultipartUpload(uploadID, manifest); err != nil {
+		return nil, err
+	}
+
+	return manifest, nil
+}
+
+func (s *ObjectService) AbortMultipartUpload(bucket, key, uploadID string) error {
+	upload, err := s.metadataHandler.GetMultipartUpload(uploadID)
+	if err != nil {
+		return err
+	}
+	if upload.Bucket != bucket || upload.Key != key {
+		return metadata.ErrMultipartNotFound
+	}
+	return s.metadataHandler.AbortMultipartUpload(uploadID)
+}
+
+func normalizeETag(etag string) string {
+	return strings.Trim(etag, "\"")
+}
+
+func buildMultipartETag(parts []models.UploadedPart) string {
+	hasher := md5.New()
+	for _, part := range parts {
+		etagBytes, err := hex.DecodeString(normalizeETag(part.ETag))
+		if err == nil {
+			_, _ = hasher.Write(etagBytes)
+			continue
+		}
+		_, _ = hasher.Write([]byte(normalizeETag(part.ETag)))
+	}
+	return fmt.Sprintf("%x-%d", hasher.Sum(nil), len(parts))
 }