cloudflare/cloudflared · warning

Error starting tracing

Error message

Error starting tracing

What it means

StartServer optionally enables Go runtime execution tracing when the --trace-output flag points at a file; if trace.Start(tmpTraceFile) fails the error is wrapped as 'Error starting tracing'. trace.Start fails mainly when tracing is already active in this process or the runtime build doesn't support it, so cloudflared aborts startup rather than running without the requested trace.

Source

Thrown at cmd/cloudflared/tunnel/cmd.go:367

			traceOutputFilepath := c.String(cfdflags.TraceOutput)
			//nolint:gosec // File path is safe because it is explicitly provided by the user via the --trace-output flag
			if err := os.Rename(tmpTraceFile.Name(), traceOutputFilepath); err != nil {
				traceLog.
					Err(err).
					Str(LogFieldTraceOutputFilepath, traceOutputFilepath).
					Msg("Failed to rename temporary trace output file")
			} else {
				//nolint:gosec // File path is safe, since it is created by os.CreateTemp
				err := os.Remove(tmpTraceFile.Name())
				if err != nil {
					traceLog.Err(err).Msg("Failed to remove the temporary trace file")
				}
			}
		}()

		if err := trace.Start(tmpTraceFile); err != nil {
			traceLog.Err(err).Msg("Failed to start trace")
			return errors.Wrap(err, "Error starting tracing")
		}
		defer trace.Stop()
	}

	info.Log(log)
	logClientOptions(c, log)

	// this context drives the server, when it's canceled tunnel and all other components (origins, dns, etc...) should stop
	ctx, cancel := context.WithCancel(c.Context)
	defer cancel()

	go waitForSignal(graceShutdownC, log)

	connectedSignal := signal.New(make(chan struct{}))
	go notifySystemd(connectedSignal)
	if c.IsSet("pidfile") {
		go writePidFile(connectedSignal, c.String("pidfile"), log)
	}

View on GitHub (pinned to 2253eeeb25)

Solutions

  1. Remove the --trace-output flag if you don't need an execution trace — this code path only runs when it is set.
  2. Ensure only one tracing session runs in the process; don't nest cloudflared inside a tool that enabled runtime tracing.
  3. Retry without other tracing activators (e.g. GODEBUG settings) competing for the tracer.
  4. If it persists, check the wrapped inner error for the runtime's specific reason and consider upgrading Go/cloudflared.

Example fix

// before
cloudflared tunnel run --name mytunnel --trace-output /tmp/trace.out

// after (tracing disabled)
cloudflared tunnel run --name mytunnel
Defensive patterns

Strategy: try-catch

Validate before calling

// only pass the flag when tracing is truly needed
if traceRequested && tracingAlreadyActive() { flags = remove(flags, "--trace-output") }

Try / catch

if err := startServer(); err != nil && strings.Contains(err.Error(), "Error starting tracing") {
    log.Warnf("tracing unavailable (%v); retrying without --trace-output", err)
    err = startServerWithoutTrace()
}

Prevention

When it happens

Trigger: Running cloudflared with --trace-output=<file> when the Go runtime rejects trace.Start: execution tracing already enabled for the process, or trace.Start returning an error for the opened file (runtime/trace unsupported state). Note the file open itself is handled earlier — this error is specifically from trace.Start.

Common situations: Nested cloudflared invocations or wrappers that already started tracing; supplying --trace-output in an environment where runtime tracing was already activated; rare Go runtime/version edge cases around trace startup.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/58cf944acb7bc8e6. Report an issue: GitHub.