grafana/k6 · error

%w

Error message

%w

What it means

While spawning the browser process, k6 calls cmd.StdoutPipe(); if the OS refuses to create the pipe, the raw error is wrapped and returned. This is an environment-level failure before the browser even starts, almost always file-descriptor exhaustion (EMFILE) or a fork/resource limit.

Source

Thrown at internal/js/modules/k6/browser/common/browser_process.go:166

	return p.meta.Cleanup() //nolint:wrapcheck
}

type command struct {
	*exec.Cmd
	done           chan struct{}
	stdout, stderr io.Reader
}

func execute(
	ctx context.Context, path string, args []string,
	dataDir *storage.Dir, logger *log.Logger,
) (command, error) {
	cmd := exec.CommandContext(ctx, path, args...) //nolint:gosec
	killAfterParent(cmd)

	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return command{}, fmt.Errorf("%w", err)
	}
	stderr, err := cmd.StderrPipe()
	if err != nil {
		return command{}, fmt.Errorf("%w", err)
	}

	// We must start the cmd before calling cmd.Wait, as otherwise the two
	// can run into a data race.
	err = cmd.Start()
	if os.IsNotExist(err) { //nolint:forbidigo
		return command{}, fmt.Errorf("file does not exist: %s", path)
	}
	if err != nil {
		return command{}, fmt.Errorf("%w", err)
	}
	if ctx.Err() != nil {
		return command{}, ContextErr(ctx)
	}

View on GitHub (pinned to 93accf6570)

Solutions

1. Check the limit with ulimit -n and raise it (e.g. ulimit -n 4096 or the systemd/containers equivalent).
2. Kill orphaned chrome/chromium processes from earlier failed runs before starting the test.
3. Reduce the number of parallel browser VUs (scenario vus/maxVUs) to lower simultaneous process spawns.

Example fix

# before
ulimit -n 1024
k6 run browser-test.js  # fails with wrapped pipe error

# after
ulimit -n 8192
k6 run browser-test.js

# fail fast if the fd limit is too low for parallel browser VUs
OPEN=$(ulimit -n)
if [ "$OPEN" -lt 2048 ]; then
  echo "open-file limit $OPEN is too low for browser scenarios" >&2; exit 1
fi

When it happens

Trigger: Launching a browser when the k6 process has hit its open-file limit (ulimit -n) — typically with many concurrent VUs each spawning a browser and websocket connections.

Common situations: High concurrency browser scenarios on CI runners with low default ulimits; leaking chrome processes from previous crashed runs; long soak tests accumulating descriptors.

Related errors

• browser process ended unexpectedly
• influxdb's ConcurrentWrites must be a positive number
• 105
• context is done before all '%s' events were processed
• launching browser: %w

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/0eafc413f9abd9db. Report an issue: GitHub.