box: reuse multipart upload part buffers via the global pool

Each part of a multipart upload allocated a fresh part-sized buffer
(the size is chosen by Box, typically 8-32 MiB) with up to --transfers
parts in flight, so large uploads churned allocations and GC.

Buffer parts with multipart.NewRW instead so the memory comes from
rclone's global pool, is reused across parts and files, and is part of
rclone's central memory management.

The pool.RW is seekable so the retry closure seeks back to the start
before each attempt instead of rebuilding a bytes.Reader, and the
per-part SHA1 digest is computed by reading the buffer and seeking
back. The body goes through lib/rest which already stops the transport
from closing it; the uploading goroutine owns and closes the buffer.
The whole-file SHA1 used for the commit is unchanged.

The FsPutRetry integration test covers the retry of a failed upload
request and checks the buffers are returned to the pool.

backend/box/upload.go

@@ -3,7 +3,6 @@
 package box
 
 import (
-	"bytes"
 	"context"
 	"crypto/sha1"
 	"encoding/base64"
@@ -20,6 +19,7 @@ import (
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/lib/atexit"
+	"github.com/rclone/rclone/lib/multipart"
 	"github.com/rclone/rclone/lib/rest"
 )
 
@@ -55,9 +55,15 @@ func sha1Digest(digest []byte) string {
 }
 
 // uploadPart uploads a part in an upload session
-func (o *Object) uploadPart(ctx context.Context, SessionID string, offset, totalSize int64, chunk []byte, wrap accounting.WrapFn, options ...fs.OpenOption) (response *api.UploadPartResponse, err error) {
-	chunkSize := int64(len(chunk))
-	sha1sum := sha1.Sum(chunk)
+//
+// chunk must contain exactly the part's data and is read from the start
+// on each attempt.
+func (o *Object) uploadPart(ctx context.Context, SessionID string, offset, totalSize int64, chunk io.ReadSeeker, wrap accounting.WrapFn, options ...fs.OpenOption) (response *api.UploadPartResponse, err error) {
+	hasher := sha1.New()
+	chunkSize, err := io.Copy(hasher, chunk)
+	if err != nil {
+		return nil, err
+	}
 	opts := rest.Opts{
 		Method:        "PUT",
 		Path:          "/files/upload_sessions/" + SessionID,
@@ -67,12 +73,16 @@ func (o *Object) uploadPart(ctx context.Context, SessionID string, offset, total
 		ContentRange:  fmt.Sprintf("bytes %d-%d/%d", offset, offset+chunkSize-1, totalSize),
 		Options:       options,
 		ExtraHeaders: map[string]string{
-			"Digest": sha1Digest(sha1sum[:]),
+			"Digest": sha1Digest(hasher.Sum(nil)),
 		},
 	}
 	var resp *http.Response
 	err = o.fs.pacer.Call(func() (bool, error) {
-		opts.Body = wrap(bytes.NewReader(chunk))
+		_, err = chunk.Seek(0, io.SeekStart)
+		if err != nil {
+			return false, err
+		}
+		opts.Body = wrap(chunk)
 		resp, err = o.fs.srv.CallJSON(ctx, &opts, nil, &response)
 		return shouldRetry(ctx, resp, err)
 	})
@@ -213,27 +223,25 @@ outer:
 
 		reqSize := min(remaining, chunkSize)
 
-		// Make a block of memory
-		buf := make([]byte, reqSize)
-
-		// Read the chunk
-		_, err = io.ReadFull(in, buf)
+		// Read the chunk into memory from the global pool, hashing
+		// it for the whole file hash (must be done sequentially)
+		rw := multipart.NewRW().Reserve(reqSize)
+		_, err = io.CopyN(rw, io.TeeReader(in, hash), reqSize)
 		if err != nil {
+			_ = rw.Close()
 			err = fmt.Errorf("multipart upload failed to read source: %w", err)
 			break outer
 		}
 
-		// Make the global hash (must be done sequentially)
-		_, _ = hash.Write(buf)
-
 		// Transfer the chunk
 		wg.Add(1)
 		o.fs.uploadToken.Get()
 		go func(part int, position int64) {
 			defer wg.Done()
 			defer o.fs.uploadToken.Put()
+			defer func() { _ = rw.Close() }()
 			fs.Debugf(o, "Uploading part %d/%d offset %v/%v part size %v", part+1, session.TotalParts, fs.SizeSuffix(position), fs.SizeSuffix(size), fs.SizeSuffix(chunkSize))
-			partResponse, err := o.uploadPart(ctx, session.ID, position, size, buf, wrap, options...)
+			partResponse, err := o.uploadPart(ctx, session.ID, position, size, rw, wrap, options...)
 			if err != nil {
 				err = fmt.Errorf("multipart upload failed to upload part: %w", err)
 				select {