[#142] Fix multipart-objects download

Signed-off-by: Nikita Zinkevich <n.zinkevich@yadro.com>

cmd/http-gw/app.go

@@ -19,11 +19,10 @@ import (
 
 	v2container "git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/container"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/cache"
-	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/frostfs"
-	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/frostfs/services"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/handler"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/handler/middleware"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/service/frostfs"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/templates"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/metrics"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/resolver"
@@ -453,7 +452,7 @@ func (a *app) setHealthStatus() {
 }
 
 func (a *app) Serve() {
-	handler := handler.New(a.AppParams(), a.settings, tree.NewTree(services.NewPoolWrapper(a.treePool)))
+	handler := handler.New(a.AppParams(), a.settings, tree.NewTree(frostfs.NewPoolWrapper(a.treePool)))
 
 	// Configure router.
 	a.configureRouter(handler)

internal/data/object.go

@@ -0,0 +1,41 @@
+package data
+
+import (
+	"time"
+
+	cid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/container/id"
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+)
+
+type (
+	ObjectInfo struct {
+		ID  oid.ID
+		CID cid.ID
+
+		Bucket  string
+		Name    string
+		Size    uint64
+		Headers map[string]string
+	}
+
+	// PartInfo is upload information about part.
+	PartInfo struct {
+		Key      string    `json:"key"`
+		UploadID string    `json:"uploadId"`
+		Number   int       `json:"number"`
+		OID      oid.ID    `json:"oid"`
+		Size     uint64    `json:"size"`
+		ETag     string    `json:"etag"`
+		MD5      string    `json:"md5"`
+		Created  time.Time `json:"created"`
+	}
+)
+
+// Address returns object address.
+func (o *ObjectInfo) Address() oid.Address {
+	var addr oid.Address
+	addr.SetContainer(o.CID)
+	addr.SetObject(o.ID)
+
+	return addr
+}

internal/handler/frostfs_mock.go

@@ -229,6 +229,10 @@ func (t *TestFrostFS) SearchObjects(_ context.Context, prm PrmObjectSearch) (Res
 	return &resObjectSearchMock{res: res}, nil
 }
 
+func (t *TestFrostFS) InitMultiObjectReader(context.Context, PrmInitMultiObjectReader) (io.Reader, error) {
+	return nil, nil
+}
+
 func isMatched(attributes []object.Attribute, filter object.SearchFilter) bool {
 	for _, attr := range attributes {
 		if attr.Key() == filter.Header() {

internal/handler/handler.go

@@ -119,6 +119,16 @@ type PrmObjectSearch struct {
 	Filters object.SearchFilters
 }
 
+type PrmInitMultiObjectReader struct {
+	// payload range
+	Off, Ln uint64
+
+	ObjInfo *data.ObjectInfo
+	BktInfo *data.BucketInfo
+	Log     *zap.Logger
+	Bearer  *bearer.Token
+}
+
 type ResObjectSearch interface {
 	Read(buf []oid.ID) (int, error)
 	Iterate(f func(oid.ID) bool) error
@@ -140,6 +150,8 @@ type FrostFS interface {
 	RangeObject(context.Context, PrmObjectRange) (io.ReadCloser, error)
 	CreateObject(context.Context, PrmObjectCreate) (oid.ID, error)
 	SearchObjects(context.Context, PrmObjectSearch) (ResObjectSearch, error)
+	InitMultiObjectReader(ctx context.Context, p PrmInitMultiObjectReader) (io.Reader, error)
+
 	utils.EpochInfoFetcher
 }
 
@@ -179,7 +191,7 @@ func New(params *AppParams, config Config, tree *tree.Tree) *Handler {
 
 // byAddress is a wrapper for function (e.g. request.headObject, request.receiveFile) that
 // prepares request and object address to it.
-func (h *Handler) byAddress(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address)) {
+func (h *Handler) byAddress(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address, *data.BucketInfo)) {
 	var (
 		idCnr, _ = c.UserValue("cid").(string)
 		idObj, _ = c.UserValue("oid").(string)
@@ -205,12 +217,12 @@ func (h *Handler) byAddress(c *fasthttp.RequestCtx, f func(context.Context, requ
 	addr.SetContainer(bktInfo.CID)
 	addr.SetObject(*objID)
 
-	f(ctx, *h.newRequest(c, log), addr)
+	f(ctx, *h.newRequest(c, log), addr, bktInfo)
 }
 
 // byObjectName is a wrapper for function (e.g. request.headObject, request.receiveFile) that
 // prepares request and object address to it.
-func (h *Handler) byObjectName(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address)) {
+func (h *Handler) byObjectName(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address, *data.BucketInfo)) {
 	var (
 		bucketname = c.UserValue("cid").(string)
 		key        = c.UserValue("oid").(string)
@@ -261,11 +273,11 @@ func (h *Handler) byObjectName(c *fasthttp.RequestCtx, f func(context.Context, r
 	addr.SetContainer(bktInfo.CID)
 	addr.SetObject(foundOid.OID)
 
-	f(ctx, *h.newRequest(c, log), addr)
+	f(ctx, *h.newRequest(c, log), addr, bktInfo)
 }
 
 // byAttribute is a wrapper similar to byAddress.
-func (h *Handler) byAttribute(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address)) {
+func (h *Handler) byAttribute(c *fasthttp.RequestCtx, f func(context.Context, request, oid.Address, *data.BucketInfo)) {
 	scid, _ := c.UserValue("cid").(string)
 	key, _ := c.UserValue("attr_key").(string)
 	val, _ := c.UserValue("attr_val").(string)
@@ -322,7 +334,7 @@ func (h *Handler) byAttribute(c *fasthttp.RequestCtx, f func(context.Context, re
 	addrObj.SetContainer(bktInfo.CID)
 	addrObj.SetObject(buf[0])
 
-	f(ctx, *h.newRequest(c, log), addrObj)
+	f(ctx, *h.newRequest(c, log), addrObj, bktInfo)
 }
 
 // resolveContainer decode container id, if it's not a valid container id

internal/handler/head.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 	"time"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/data"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/utils"
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
@@ -24,7 +25,7 @@ const (
 	hdrContainerID = "X-Container-Id"
 )
 
-func (h *Handler) headObject(ctx context.Context, req request, objectAddress oid.Address) {
+func (h *Handler) headObject(ctx context.Context, req request, objectAddress oid.Address, _ *data.BucketInfo) {
 	var start = time.Now()
 
 	btoken := bearerToken(ctx)

internal/handler/multipart.go

@@ -1,13 +1,21 @@
 package handler
 
 import (
+	"errors"
 	"io"
+	"strconv"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/data"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/handler/multipart"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
 	"go.uber.org/zap"
 )
 
+const (
+	frostFSSystemMetadataPrefix  = "S3-"
+	attributeMultipartObjectSize = frostFSSystemMetadataPrefix + "Multipart-Object-Size"
+)
+
 // MultipartFile provides standard ReadCloser interface and also allows one to
 // get file name, it's used for multipart uploads.
 type MultipartFile interface {
@@ -45,3 +53,45 @@ func fetchMultipartFile(l *zap.Logger, r io.Reader, boundary string) (MultipartF
 		return part, nil
 	}
 }
+
+// getPayload returns initial payload if object is not multipart else composes new reader with parts data.
+func (h *Handler) getPayload(p getPayloadParams) (io.ReadCloser, uint64, error) {
+	sizeValue, ok := p.attrs[attributeMultipartObjectSize]
+	if !ok {
+		return p.obj.Payload, p.obj.Header.PayloadSize(), nil
+	}
+	cid, ok := p.obj.Header.ContainerID()
+	if !ok {
+		return nil, 0, errors.New("no container id set")
+	}
+	oid, ok := p.obj.Header.ID()
+	if !ok {
+		return nil, 0, errors.New("no object id set")
+	}
+	size, err := strconv.ParseUint(sizeValue, 10, 64)
+	if err != nil {
+		return nil, 0, err
+	}
+	ctx := p.req.RequestCtx
+	params := PrmInitMultiObjectReader{
+		Off: 0,
+		Ln:  0,
+		ObjInfo: &data.ObjectInfo{
+			ID:      oid,
+			CID:     cid,
+			Bucket:  p.bktinfo.Name,
+			Name:    p.attrs["FilePath"],
+			Size:    size,
+			Headers: p.attrs,
+		},
+		BktInfo: p.bktinfo,
+		Log:     h.log,
+		Bearer:  bearerToken(ctx),
+	}
+	payload, err := h.frostfs.InitMultiObjectReader(ctx, params)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	return io.NopCloser(payload), size, nil
+}

internal/handler/reader.go

@@ -5,10 +5,10 @@ import (
 	"context"
 	"io"
 	"net/http"
-	"path"
 	"strconv"
 	"time"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/data"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/response"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/utils"
@@ -47,20 +47,24 @@ func readContentType(maxSize uint64, rInit func(uint64) (io.Reader, error)) (str
 	return http.DetectContentType(buf), buf, err // to not lose io.EOF
 }
 
-func (h *Handler) receiveFile(ctx context.Context, req request, objectAddress oid.Address) {
+type getPayloadParams struct {
+	obj     *Object
+	req     request
+	bktinfo *data.BucketInfo
+	attrs   map[string]string
+}
+
+func (h *Handler) receiveFile(ctx context.Context, req request, objAddress oid.Address, bktInfo *data.BucketInfo) {
 	var (
-		err      error
-		dis      = "inline"
-		start    = time.Now()
-		filename string
-		filepath string
+		shouldDownload = req.QueryArgs().GetBool("download")
+		start          = time.Now()
 	)
 
 	prm := PrmObjectGet{
 		PrmAuth: PrmAuth{
 			BearerToken: bearerToken(ctx),
 		},
-		Address: objectAddress,
+		Address: objAddress,
 	}
 
 	rObj, err := h.frostfs.GetObject(ctx, prm)
@@ -71,53 +75,40 @@ func (h *Handler) receiveFile(ctx context.Context, req request, objectAddress oi
 
 	// we can't close reader in this function, so how to do it?
 
-	if req.Request.URI().QueryArgs().GetBool("download") {
-		dis = "attachment"
+	attrs := makeAttributesMap(rObj.Header.Attributes())
+	req.setAttributes(attrs)
+
+	req.setIDs(rObj.Header)
+	req.setDisposition(shouldDownload, attrs)
+
+	if err = req.setTimestamp(attrs[object.AttributeTimestamp]); err != nil {
+		req.log.Error(logs.CouldntParseCreationDate,
+			zap.String("val", attrs[object.AttributeTimestamp]),
+			zap.Error(err))
+		response.Error(req.RequestCtx, "failed to convert timestamp: "+err.Error(), fasthttp.StatusInternalServerError)
 	}
 
-	payloadSize := rObj.Header.PayloadSize()
+	payloadParams := getPayloadParams{
+		obj:     rObj,
+		req:     req,
+		bktinfo: bktInfo,
+		attrs:   attrs,
+	}
 
+	payload, payloadSize, err := h.getPayload(payloadParams)
+	if err != nil {
+		req.handleFrostFSErr(err, start)
+		return
+	}
 	req.Response.Header.Set(fasthttp.HeaderContentLength, strconv.FormatUint(payloadSize, 10))
-	var contentType string
-	for _, attr := range rObj.Header.Attributes() {
-		key := attr.Key()
-		val := attr.Value()
-		if !isValidToken(key) || !isValidValue(val) {
-			continue
-		}
-
-		key = utils.BackwardTransformIfSystem(key)
-
-		req.Response.Header.Set(utils.UserAttributeHeaderPrefix+key, val)
-		switch key {
-		case object.AttributeFileName:
-			filename = val
-		case object.AttributeTimestamp:
-			value, err := strconv.ParseInt(val, 10, 64)
-			if err != nil {
-				req.log.Info(logs.CouldntParseCreationDate,
-					zap.String("key", key),
-					zap.String("val", val),
-					zap.Error(err))
-				continue
-			}
-			req.Response.Header.Set(fasthttp.HeaderLastModified,
-				time.Unix(value, 0).UTC().Format(http.TimeFormat))
-		case object.AttributeContentType:
-			contentType = val
-		case object.AttributeFilePath:
-			filepath = val
-		}
-	}
-
-	idsToResponse(&req.Response, &rObj.Header)
 
+	contentType := attrs[object.AttributeContentType]
 	if len(contentType) == 0 {
 		// determine the Content-Type from the payload head
 		var payloadHead []byte
 
 		contentType, payloadHead, err = readContentType(payloadSize, func(uint64) (io.Reader, error) {
-			return rObj.Payload, nil
+			return payload, nil
 		})
 		if err != nil && err != io.EOF {
 			req.log.Error(logs.CouldNotDetectContentTypeFromPayload, zap.Error(err))
@@ -129,20 +120,26 @@ func (h *Handler) receiveFile(ctx context.Context, req request, objectAddress oi
 		var headReader io.Reader = bytes.NewReader(payloadHead)
 
 		if err != io.EOF { // otherwise, we've already read full payload
-			headReader = io.MultiReader(headReader, rObj.Payload)
+			headReader = io.MultiReader(headReader, payload)
 		}
 
 		// note: we could do with io.Reader, but SetBodyStream below closes body stream
 		// if it implements io.Closer and that's useful for us.
-		rObj.Payload = readCloser{headReader, rObj.Payload}
+		payload = readCloser{headReader, payload}
 	}
 	req.SetContentType(contentType)
-
-	if filename == "" {
-		filename = filepath
-	}
-
-	req.Response.Header.Set(fasthttp.HeaderContentDisposition, dis+"; filename="+path.Base(filename))
-
-	req.Response.SetBodyStream(rObj.Payload, int(payloadSize))
+	req.Response.SetBodyStream(payload, int(payloadSize))
+}
+
+func makeAttributesMap(attrs []object.Attribute) map[string]string {
+	attributes := make(map[string]string)
+	for _, attr := range attrs {
+		if !isValidToken(attr.Key()) || !isValidValue(attr.Value()) {
+			continue
+		}
+		key := utils.BackwardTransformIfSystem(attr.Key())
+
+		attributes[key] = attr.Value()
+	}
+	return attributes
 }

internal/handler/utils.go

@@ -3,6 +3,9 @@ package handler
 import (
 	"context"
 	"errors"
+	"net/http"
+	"path"
+	"strconv"
 	"strings"
 	"time"
 
@@ -10,8 +13,10 @@ import (
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/response"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/tokens"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/tree"
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/utils"
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/bearer"
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client"
+	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
 	"github.com/valyala/fasthttp"
 	"go.uber.org/zap"
 )
@@ -33,6 +38,51 @@ func (r *request) handleFrostFSErr(err error, start time.Time) {
 	response.Error(r.RequestCtx, msg, statusCode)
 }
 
+func (r *request) setAttributes(attrs map[string]string) {
+	for key, val := range attrs {
+		r.Response.Header.Set(utils.UserAttributeHeaderPrefix+key, val)
+	}
+}
+
+func (r *request) setIDs(obj object.Object) {
+	objID, _ := obj.ID()
+	cnrID, _ := obj.ContainerID()
+	r.Response.Header.Set(hdrObjectID, objID.String())
+	r.Response.Header.Set(hdrOwnerID, obj.OwnerID().String())
+	r.Response.Header.Set(hdrContainerID, cnrID.String())
+}
+
+func (r *request) setDisposition(shouldDownload bool, attrs map[string]string) {
+	const (
+		inlineDisposition     = "inline"
+		attachmentDisposition = "attachment"
+	)
+
+	dis := inlineDisposition
+	if shouldDownload {
+		dis = attachmentDisposition
+	}
+
+	filename := attrs[object.AttributeFileName]
+	filepath := attrs[object.AttributeFilePath]
+	if filename == "" {
+		filename = filepath
+	}
+
+	r.Response.Header.Set(fasthttp.HeaderContentDisposition, dis+"; filename="+path.Base(filename))
+}
+
+func (r *request) setTimestamp(timestamp string) error {
+	value, err := strconv.ParseInt(timestamp, 10, 64)
+	if err != nil {
+		return err
+	}
+	r.Response.Header.Set(fasthttp.HeaderLastModified,
+		time.Unix(value, 0).UTC().Format(http.TimeFormat))
+
+	return nil
+}
+
 func bearerToken(ctx context.Context) *bearer.Token {
 	if tkn, err := tokens.LoadBearerToken(ctx); err == nil {
 		return tkn

internal/service/frostfs/frostfs.go

@@ -33,9 +33,9 @@ func NewFrostFS(p *pool.Pool) *FrostFS {
 }
 
 // Container implements frostfs.FrostFS interface method.
-func (x *FrostFS) Container(ctx context.Context, layerPrm handler.PrmContainer) (*container.Container, error) {
+func (x *FrostFS) Container(ctx context.Context, containerPrm handler.PrmContainer) (*container.Container, error) {
 	prm := pool.PrmContainerGet{
-		ContainerID: layerPrm.ContainerID,
+		ContainerID: containerPrm.ContainerID,
 	}
 
 	res, err := x.pool.GetContainer(ctx, prm)

internal/service/frostfs/multi_object_reader.go

@@ -0,0 +1,234 @@
+package frostfs
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/data"
+	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/handler"
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+	"go.uber.org/zap"
+)
+
+type GetFrostFSParams struct {
+	// payload range
+	Off, Ln uint64
+
+	Oid     oid.ID
+	BktInfo *data.BucketInfo
+}
+
+type PartObj struct {
+	OID  oid.ID
+	Size uint64
+}
+
+type readerInitiator interface {
+	InitFrostFSObjectPayloadReader(ctx context.Context, p GetFrostFSParams) (io.Reader, error)
+}
+
+// MultiObjectReader implements io.Reader of payloads of the object list stored in the FrostFS network.
+type MultiObjectReader struct {
+	ctx context.Context
+	log *zap.Logger
+
+	layer readerInitiator
+
+	startPartOffset uint64
+	endPartLength   uint64
+
+	prm GetFrostFSParams
+
+	curIndex  int
+	curReader io.Reader
+
+	parts []PartObj
+}
+
+type MultiObjectReaderConfig struct {
+	Initiator readerInitiator
+	Log       *zap.Logger
+
+	// the offset of complete object and total size to read
+	Off, Ln uint64
+
+	BktInfo *data.BucketInfo
+	Parts   []PartObj
+}
+
+var (
+	errOffsetIsOutOfRange = errors.New("offset is out of payload range")
+	errLengthIsOutOfRange = errors.New("length is out of payload range")
+	errEmptyPartsList     = errors.New("empty parts list")
+	errorZeroRangeLength  = errors.New("zero range length")
+)
+
+func (x *FrostFS) InitMultiObjectReader(ctx context.Context, p handler.PrmInitMultiObjectReader) (io.Reader, error) {
+	combinedObj, err := x.GetObject(ctx, handler.PrmObjectGet{
+		PrmAuth: handler.PrmAuth{BearerToken: p.Bearer},
+		Address: p.ObjInfo.Address(),
+	})
+	if err != nil {
+		return nil, fmt.Errorf("get combined object '%s': %w", p.ObjInfo.ID.EncodeToString(), err)
+	}
+
+	var parts []*data.PartInfo
+	if err = json.NewDecoder(combinedObj.Payload).Decode(&parts); err != nil {
+		return nil, fmt.Errorf("unmarshal combined object parts: %w", err)
+	}
+
+	objParts := make([]PartObj, len(parts))
+	for i, part := range parts {
+		objParts[i] = PartObj{
+			OID:  part.OID,
+			Size: part.Size,
+		}
+	}
+
+	return NewMultiObjectReader(ctx, MultiObjectReaderConfig{
+		Initiator: x,
+		Off:       p.Off,
+		Ln:        p.Ln,
+		Parts:     objParts,
+		BktInfo:   p.BktInfo,
+		Log:       p.Log,
+	})
+}
+
+func NewMultiObjectReader(ctx context.Context, cfg MultiObjectReaderConfig) (*MultiObjectReader, error) {
+	if len(cfg.Parts) == 0 {
+		return nil, errEmptyPartsList
+	}
+
+	r := &MultiObjectReader{
+		ctx:   ctx,
+		layer: cfg.Initiator,
+		prm: GetFrostFSParams{
+			BktInfo: cfg.BktInfo,
+		},
+		parts: cfg.Parts,
+		log:   cfg.Log,
+	}
+
+	if cfg.Off+cfg.Ln == 0 {
+		return r, nil
+	}
+
+	if cfg.Off > 0 && cfg.Ln == 0 {
+		return nil, errorZeroRangeLength
+	}
+
+	startPartIndex, startPartOffset := findStartPart(cfg)
+	if startPartIndex == -1 {
+		return nil, errOffsetIsOutOfRange
+	}
+	r.startPartOffset = startPartOffset
+
+	endPartIndex, endPartLength := findEndPart(cfg)
+	if endPartIndex == -1 {
+		return nil, errLengthIsOutOfRange
+	}
+	r.endPartLength = endPartLength
+
+	r.parts = cfg.Parts[startPartIndex : endPartIndex+1]
+
+	return r, nil
+}
+
+func findStartPart(cfg MultiObjectReaderConfig) (index int, offset uint64) {
+	position := cfg.Off
+	for i, part := range cfg.Parts {
+		// Strict inequality when searching for start position to avoid reading zero length part.
+		if position < part.Size {
+			return i, position
+		}
+		position -= part.Size
+	}
+
+	return -1, 0
+}
+
+func findEndPart(cfg MultiObjectReaderConfig) (index int, length uint64) {
+	position := cfg.Off + cfg.Ln
+	for i, part := range cfg.Parts {
+		// Non-strict inequality when searching for end position to avoid out of payload range error.
+		if position <= part.Size {
+			return i, position
+		}
+		position -= part.Size
+	}
+
+	return -1, 0
+}
+
+func (x *MultiObjectReader) Read(p []byte) (n int, err error) {
+	if x.curReader != nil {
+		n, err = x.curReader.Read(p)
+		if !errors.Is(err, io.EOF) {
+			return n, err
+		}
+		x.curIndex++
+	}
+
+	if x.curIndex == len(x.parts) {
+		return n, io.EOF
+	}
+
+	x.prm.Oid = x.parts[x.curIndex].OID
+
+	if x.curIndex == 0 {
+		x.prm.Off = x.startPartOffset
+		x.prm.Ln = x.parts[x.curIndex].Size - x.startPartOffset
+	}
+
+	if x.curIndex == len(x.parts)-1 {
+		x.prm.Ln = x.endPartLength - x.prm.Off
+	}
+
+	x.curReader, err = x.layer.InitFrostFSObjectPayloadReader(x.ctx, x.prm)
+	if err != nil {
+		return n, fmt.Errorf("init payload reader for the next part: %w", err)
+	}
+
+	x.prm.Off = 0
+	x.prm.Ln = 0
+
+	next, err := x.Read(p[n:])
+
+	return n + next, err
+}
+
+// InitFrostFSObjectPayloadReader initializes payload reader of the FrostFS object.
+// Zero range corresponds to full payload (panics if only offset is set).
+func (x *FrostFS) InitFrostFSObjectPayloadReader(ctx context.Context, p GetFrostFSParams) (io.Reader, error) {
+	var prmAuth handler.PrmAuth
+
+	var addr oid.Address
+	addr.SetContainer(p.BktInfo.CID)
+	addr.SetObject(p.Oid)
+
+	if p.Off+p.Ln != 0 {
+		prm := handler.PrmObjectRange{
+			PrmAuth:      prmAuth,
+			PayloadRange: [2]uint64{p.Off, p.Ln},
+			Address:      addr,
+		}
+
+		return x.RangeObject(ctx, prm)
+	}
+
+	prm := handler.PrmObjectGet{
+		PrmAuth: prmAuth,
+		Address: addr,
+	}
+
+	res, err := x.GetObject(ctx, prm)
+	if err != nil {
+		return nil, err
+	}
+
+	return res.Payload, nil
+}

internal/service/frostfs/multi_object_reader_test.go

@@ -0,0 +1,137 @@
+package frostfs
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"testing"
+
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+	oidtest "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id/test"
+	"github.com/stretchr/testify/require"
+)
+
+type readerInitiatorMock struct {
+	parts map[oid.ID][]byte
+}
+
+func (r *readerInitiatorMock) InitFrostFSObjectPayloadReader(_ context.Context, p GetFrostFSParams) (io.Reader, error) {
+	partPayload, ok := r.parts[p.Oid]
+	if !ok {
+		return nil, errors.New("part not found")
+	}
+
+	if p.Off+p.Ln == 0 {
+		return bytes.NewReader(partPayload), nil
+	}
+
+	if p.Off > uint64(len(partPayload)-1) {
+		return nil, fmt.Errorf("invalid offset: %d/%d", p.Off, len(partPayload))
+	}
+
+	if p.Off+p.Ln > uint64(len(partPayload)) {
+		return nil, fmt.Errorf("invalid range: %d-%d/%d", p.Off, p.Off+p.Ln, len(partPayload))
+	}
+
+	return bytes.NewReader(partPayload[p.Off : p.Off+p.Ln]), nil
+}
+
+func prepareDataReader() ([]byte, []PartObj, *readerInitiatorMock) {
+	mockInitReader := &readerInitiatorMock{
+		parts: map[oid.ID][]byte{
+			oidtest.ID(): []byte("first part 1"),
+			oidtest.ID(): []byte("second part 2"),
+			oidtest.ID(): []byte("third part 3"),
+		},
+	}
+
+	var fullPayload []byte
+	parts := make([]PartObj, 0, len(mockInitReader.parts))
+	for id, payload := range mockInitReader.parts {
+		parts = append(parts, PartObj{OID: id, Size: uint64(len(payload))})
+		fullPayload = append(fullPayload, payload...)
+	}
+
+	return fullPayload, parts, mockInitReader
+}
+
+func TestMultiReader(t *testing.T) {
+	ctx := context.Background()
+
+	fullPayload, parts, mockInitReader := prepareDataReader()
+
+	for _, tc := range []struct {
+		name string
+		off  uint64
+		ln   uint64
+		err  error
+	}{
+		{
+			name: "simple read all",
+		},
+		{
+			name: "simple read with length",
+			ln:   uint64(len(fullPayload)),
+		},
+		{
+			name: "middle of parts",
+			off:  parts[0].Size + 2,
+			ln:   4,
+		},
+		{
+			name: "first and second",
+			off:  parts[0].Size - 4,
+			ln:   8,
+		},
+		{
+			name: "first and third",
+			off:  parts[0].Size - 4,
+			ln:   parts[1].Size + 8,
+		},
+		{
+			name: "second part",
+			off:  parts[0].Size,
+			ln:   parts[1].Size,
+		},
+		{
+			name: "second and third",
+			off:  parts[0].Size,
+			ln:   parts[1].Size + parts[2].Size,
+		},
+		{
+			name: "offset out of range",
+			off:  uint64(len(fullPayload) + 1),
+			ln:   1,
+			err:  errOffsetIsOutOfRange,
+		},
+		{
+			name: "zero length",
+			off:  parts[1].Size + 1,
+			ln:   0,
+			err:  errorZeroRangeLength,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			multiReader, err := NewMultiObjectReader(ctx, MultiObjectReaderConfig{
+				Initiator: mockInitReader,
+				Parts:     parts,
+				Off:       tc.off,
+				Ln:        tc.ln,
+			})
+			require.ErrorIs(t, err, tc.err)
+
+			if tc.err == nil {
+				off := tc.off
+				ln := tc.ln
+				if off+ln == 0 {
+					ln = uint64(len(fullPayload))
+				}
+				data, err := io.ReadAll(multiReader)
+				require.NoError(t, err)
+				require.Equal(t, fullPayload[off:off+ln], data)
+			}
+		})
+	}
+}

internal/service/frostfs/pool_wrapper.go

@@ -1,4 +1,4 @@
-package services
+package frostfs
 
 import (
 	"context"

tree/tree_test.go

@@ -21,11 +21,11 @@ func (m nodeMeta) GetValue() []byte {
 
 type nodeResponse struct {
 	meta      []nodeMeta
-	timestamp uint64
+	timestamp []uint64
 }
 
 func (n nodeResponse) GetTimestamp() []uint64 {
-	return []uint64{n.timestamp}
+	return n.timestamp
 }
 
 func (n nodeResponse) GetMeta() []Meta {
@@ -59,7 +59,7 @@ func TestGetLatestNode(t *testing.T) {
 			name: "one node of the object version",
 			nodes: []NodeResponse{
 				nodeResponse{
-					timestamp: 1,
+					timestamp: []uint64{1},
 					meta: []nodeMeta{
 						{
 							key:   oidKV,
@@ -74,11 +74,11 @@ func TestGetLatestNode(t *testing.T) {
 			name: "one node of the object version and one node of the secondary object",
 			nodes: []NodeResponse{
 				nodeResponse{
-					timestamp: 3,
+					timestamp: []uint64{3},
 					meta:      []nodeMeta{},
 				},
 				nodeResponse{
-					timestamp: 1,
+					timestamp: []uint64{1},
 					meta: []nodeMeta{
 						{
 							key:   oidKV,
@@ -93,11 +93,11 @@ func TestGetLatestNode(t *testing.T) {
 			name: "all nodes represent a secondary object",
 			nodes: []NodeResponse{
 				nodeResponse{
-					timestamp: 3,
+					timestamp: []uint64{3},
 					meta:      []nodeMeta{},
 				},
 				nodeResponse{
-					timestamp: 5,
+					timestamp: []uint64{5},
 					meta:      []nodeMeta{},
 				},
 			},
@@ -107,7 +107,7 @@ func TestGetLatestNode(t *testing.T) {
 			name: "several nodes of different types and with different timestamp",
 			nodes: []NodeResponse{
 				nodeResponse{
-					timestamp: 1,
+					timestamp: []uint64{1},
 					meta: []nodeMeta{
 						{
 							key:   oidKV,
@@ -116,11 +116,11 @@ func TestGetLatestNode(t *testing.T) {
 					},
 				},
 				nodeResponse{
-					timestamp: 3,
+					timestamp: []uint64{3},
 					meta:      []nodeMeta{},
 				},
 				nodeResponse{
-					timestamp: 4,
+					timestamp: []uint64{4},
 					meta: []nodeMeta{
 						{
 							key:   oidKV,
@@ -129,7 +129,7 @@ func TestGetLatestNode(t *testing.T) {
 					},
 				},
 				nodeResponse{
-					timestamp: 6,
+					timestamp: []uint64{6},
 					meta:      []nodeMeta{},
 				},
 			},