dropbox: fix retries of multipart uploads with incorrect_offset error

Before this fix, rclone retries chunks of multipart uploads. However
if they had been partially received dropbox would reply with an
incorrect_offset error which rclone was ignoring.

This patch parses the new offset from the error response and uses it
to adjust the data that rclone sends so it is the same as what dropbox
is expecting.

See: https://forum.rclone.org/t/dropbox-rate-limiting-for-upload/29779

backend/dropbox/dropbox.go

@@ -1650,13 +1650,37 @@ func (o *Object) uploadChunked(ctx context.Context, in0 io.Reader, commitInfo *f
 		}
 
 		chunk := readers.NewRepeatableLimitReaderBuffer(in, buf, chunkSize)
+		skip := int64(0)
 		err = o.fs.pacer.Call(func() (bool, error) {
 			// seek to the start in case this is a retry
-			if _, err = chunk.Seek(0, io.SeekStart); err != nil {
+			if _, err = chunk.Seek(skip, io.SeekStart); err != nil {
-				return false, nil
+				return false, err
 			}
 			err = o.fs.srv.UploadSessionAppendV2(&appendArg, chunk)
 			// after session is started, we retry everything
+			if err != nil {
+				// Check for incorrect offset error and retry with new offset
+				if uErr, ok := err.(files.UploadSessionAppendV2APIError); ok {
+					if uErr.EndpointError != nil && uErr.EndpointError.IncorrectOffset != nil {
+						correctOffset := uErr.EndpointError.IncorrectOffset.CorrectOffset
+						delta := int64(correctOffset) - int64(cursor.Offset)
+						skip += delta
+						what := fmt.Sprintf("incorrect offset error receved: sent %d, need %d, skip %d", cursor.Offset, correctOffset, skip)
+						if skip < 0 {
+							return false, fmt.Errorf("can't seek backwards to correct offset: %s", what)
+						} else if skip == chunkSize {
+							fs.Debugf(o, "%s: chunk received OK - continuing", what)
+							return false, nil
+						} else if skip > chunkSize {
+							// This error should never happen
+							return false, fmt.Errorf("can't seek forwards by more than a chunk to correct offset: %s", what)
+						}
+						// Skip the sent data on next retry
+						cursor.Offset = uint64(int64(cursor.Offset) + delta)
+						fs.Debugf(o, "%s: skipping bytes on retry to fix offset", what)
+					}
+				}
+			}
 			return err != nil, err
 		})
 		if err != nil {