*: Remove multipart object re-slicing

Closes #843.

Before, multipartComplete read all parts of Big object to the memory, combine them and generate final Big object.
These step consume time and memory, eventually any system will fail to load all parts in mem or timeout during
the process.
After, object slicing process works from the first uploaded part. Calculating each part hash and whole object
hash during whole process. Storing object hash state to each part metadata in tree service.

Signed-off-by: Evgenii Baidakov <evgenii@nspcc.io>

api/data/tree.go

@@ -90,6 +90,13 @@ type PartInfo struct {
 	Created time.Time
 	// Server creation time.
 	ServerCreated time.Time
+
+	// MultipartHash contains internal state of the [hash.Hash] to calculate whole object payload hash.
+	MultipartHash []byte
+	// HomoHash contains internal state of the [hash.Hash] to calculate whole object homomorphic payload hash.
+	HomoHash []byte
+	// Elements contain [oid.ID] object list for the current part.
+	Elements []oid.ID
 }
 
 // ToHeaderString form short part representation to use in S3-Completed-Parts header.

api/layer/layer.go

@@ -9,6 +9,7 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/nats-io/nats.go"
@@ -50,6 +51,7 @@ type (
 		ncontroller EventListener
 		cache       *Cache
 		treeService TreeService
+		buffers     *sync.Pool
 	}
 
 	Config struct {
@@ -266,13 +268,20 @@ func (f MsgHandlerFunc) HandleMessage(ctx context.Context, msg *nats.Msg) error
 // NewLayer creates an instance of a layer. It checks credentials
 // and establishes gRPC connection with the node.
 func NewLayer(log *zap.Logger, neoFS NeoFS, config *Config) Client {
+	buffers := sync.Pool{}
+	buffers.New = func() any {
+		b := make([]byte, neoFS.MaxObjectSize())
+		return &b
+	}
+
 	return &layer{
 		neoFS:       neoFS,
 		log:         log,
 		anonymous:   config.Anonymous,
 		resolver:    config.Resolver,
 		cache:       NewCache(config.Caches),
 		treeService: config.TreeService,
+		buffers:     &buffers,
 	}
 }