docker/cli · error

writing config to tar file for config copy

Error message

writing config to tar file for config copy: %w

What it means

Returned by copyDockerConfigIntoContainer (create.go:420) when io.Copy fails writing the serialized docker config bytes into the in-memory tar.Writer. Because the destination is an in-memory bytes.Buffer, this path is only reached under abnormal conditions (the wrapped %w describes the cause).

Solutions

1. Free memory / reduce concurrent container-create load and retry.
2. Upgrade the Docker client to a current version in case of a tar writer bug.
3. Capture the wrapped underlying error to identify the real cause; this message alone is a symptom.

// No meaningful pre-check; both buffers are in-memory. Monitor memory:
import "runtime"
var m runtime.MemStats
runtime.ReadMemStats(&m)
if m.Alloc > maxMem { return errors.New("insufficient memory for config copy") }

// Treat as transient/internal; retry once after freeing resources:
if err := createRun(); err != nil && strings.Contains(err.Error(), "writing config to tar") {
    runtime.GC(); time.Sleep(backoff); /* retry once */
}

When it happens

Trigger: An internal failure while copying the config buffer into the tar writer during config-copy into the container. Given both source and sink are in-memory buffers, a genuine I/O error here is extremely rare and usually points to a tar.Writer state bug or memory pressure.

Common situations: Rare; typically a symptom of memory exhaustion or a corrupted tar writer state rather than user input. May surface under heavy concurrent create operations or OOM conditions.

Related errors

• closing tar for config copy failed
• saving creds
• copying config.json into container failed
• invalid value: . Valid memory swappiness range is 0-100
• container ID file found, make sure the other container…

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/6fe5874b8a728151. Report an issue: GitHub.

Appendix: source

Thrown at cli/command/container/create.go:420

// /root/.docker/config.json.
func copyDockerConfigIntoContainer(ctx context.Context, apiClient client.APIClient, containerID string, configPath string, config *configfile.ConfigFile) error {
	var configBuf bytes.Buffer
	if err := config.SaveToWriter(&configBuf); err != nil {
		return fmt.Errorf("saving creds: %w", err)
	}

	// We don't need to get super fancy with the tar creation.
	var tarBuf bytes.Buffer
	tarWriter := tar.NewWriter(&tarBuf)
	_ = tarWriter.WriteHeader(&tar.Header{
		Name: configPath,
		Size: int64(configBuf.Len()),
		Mode: 0o600,
	})

	if _, err := io.Copy(tarWriter, &configBuf); err != nil {
		_ = tarWriter.Close()
		return fmt.Errorf("writing config to tar file for config copy: %w", err)
	}

	if err := tarWriter.Close(); err != nil {
		return fmt.Errorf("closing tar for config copy failed: %w", err)
	}

	_, err := apiClient.CopyToContainer(ctx, containerID, client.CopyToContainerOptions{
		DestinationPath: "/",
		Content:         &tarBuf,
	})
	if err != nil {
		return fmt.Errorf("copying config.json into container failed: %w", err)
	}

	return nil
}

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