grafana/k6 · error

draining upload response body: %w

Error message

draining upload response body: %w

What it means

Raised in RemoteFilePersister.Persist when io.Copy(io.Discard, resp.Body) fails while draining the upload response body. Draining enables connection reuse; failure here means the response body could not be fully read — connection reset mid-body or a read timeout — even though the server may have accepted the upload. The drain error is treated as fatal for the persist call.

Source

Thrown at internal/js/modules/k6/browser/storage/file_persister.go:113

func (r *RemoteFilePersister) Persist(ctx context.Context, path string, data io.Reader) (err error) {
	psResp, err := r.requestPresignedURL(ctx, path)
	if err != nil {
		return fmt.Errorf("requesting presigned url: %w", err)
	}

	req, err := newFileUploadRequest(ctx, psResp, data)
	if err != nil {
		return fmt.Errorf("creating upload request: %w", err)
	}

	resp, err := r.httpClient.Do(req) //nolint:gosec
	if err != nil {
		return fmt.Errorf("performing upload request: %w", err)
	}
	defer resp.Body.Close() //nolint:errcheck

	if _, err := io.Copy(io.Discard, resp.Body); err != nil {
		return fmt.Errorf("draining upload response body: %w", err)
	}

	if err := checkStatusCode(resp); err != nil {
		return fmt.Errorf("uploading: %w", err)
	}

	return nil
}

// requestPresignedURL will request a new presigned URL from the remote server
// and returns a [PresignedURLResponse] that contains the presigned URL details.
func (r *RemoteFilePersister) requestPresignedURL(ctx context.Context, path string) (PresignedURLResponse, error) {
	b, err := buildPresignedRequestBody(r.basePath, path)
	if err != nil {
		return PresignedURLResponse{}, fmt.Errorf("building request body: %w", err)
	}

	req, err := http.NewRequestWithContext(

View on GitHub (pinned to 01ffac6f24)

Solutions

1. Retry the persist; the artifact may or may not have been uploaded
2. Check for proxies/load balancers that truncate response bodies
3. Verify network stability between k6 and the storage endpoint
4. If persistent, disable cloud artifact upload and use local storage

When it happens

Trigger: Thrown at internal/js/modules/k6/browser/storage/file_persister.go:113 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/77f656605c324a88. Report an issue: GitHub.