What causes "creating temporary log file %s: %w" in cloudflare/cloudflared?

Check disk space and write permissions on os.TempDir() (TMPDIR)

creating temporary log file %s: %w

Error message

creating temporary log file %s: %w

What it means

Wrapped os.Create failure in diagnostic CopyFilesFromDirectory: the temporary merged log file (cloudflared.log in the OS temp dir) that individual rolling logs are concatenated into could not be created, so log collection aborts.

Source

Thrown at diagnostic/log_collector_utils.go:83

	return NewLogInformation(outputHandle.Name(), true, false), nil
}

func CopyFilesFromDirectory(path string) (string, error) {
	const defaultLogFilename = "cloudflared.log"

	// rolling logs have as suffix the current date thus
	// when iterating the path files they are already in
	// chronological order
	files, err := os.ReadDir(path)
	if err != nil {
		return "", fmt.Errorf("error reading directory %s: %w", path, err)
	}

	// nolint: gosec
	outputHandle, err := os.Create(filepath.Join(os.TempDir(), logFilename))
	if err != nil {
		return "", fmt.Errorf("creating temporary log file %s: %w", logFilename, err)
	}
	defer func() { _ = outputHandle.Close() }()

	for _, file := range files {
		// nolint: gosec
		logHandle, err := os.Open(filepath.Join(path, file.Name()))
		if err != nil {
			return "", fmt.Errorf("error opening file %s: %w", file.Name(), err)
		}
		_, err = io.Copy(outputHandle, logHandle)
		_ = logHandle.Close()
		if err != nil {
			return "", fmt.Errorf("error copying file %s: %w", file.Name(), err)
		}
	}

	// nolint: gosec
	logHandle, err := os.Open(filepath.Join(path, defaultLogFilename))

View on GitHub (pinned to 2253eeeb25)

Solutions

Check disk space and write permissions on os.TempDir() (TMPDIR)
Free space or point TMPDIR at a writable location
Re-run the diagnostic collection

Example fix

// before
TMPDIR=/ro/mount cloudflared diagnostic collect
// after
TMPDIR=/var/tmp cloudflared diagnostic collect

Defensive patterns

Strategy: try-catch

Validate before calling

if err := checkWritable(os.TempDir()); err != nil { os.Setenv("TMPDIR", "/var/tmp") }

Try / catch

out, err := diagnostic.CopyFilesFromDirectory(ctx, fs, logDir, logFilename)
if err != nil && strings.Contains(err.Error(), "creating temporary log file") {
	return fmt.Errorf("cannot write temp logs: %w — check disk space/TMPDIR", err)
}

Prevention

Ensure /tmp (or TMPDIR) is writable and has headroom
Avoid read-only tmpfs in containers used for diagnostics
Clean stale temp files periodically

When it happens

Trigger: os.Create(filepath.Join(os.TempDir(), logFilename)) fails: /tmp is full, read-only, or the process lacks write permission; TMPDIR points somewhere invalid.

Common situations: Containers with read-only /tmp or tiny tmpfs; restrictive TMPDIR; disk-quota exhaustion during diagnostics.

Understand the failure class

Background: "Permission denied" / "Failed to write" file errors: why a library can't write its files to disk (EACCES, EPERM, ENOSPC) and how to fix them — this error's family across 43 libraries.

Related errors

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/72698687bf544ac3. Report an issue: GitHub.