Function GetDigest

docker/docker_image.go:141–186

GetDigest returns the image's digest Use this to optimize and avoid use of an ImageSource based on the returned digest; if you are going to use an ImageSource anyway, it’s more efficient to create it first and compute the digest from the value returned by GetManifest. NOTE: Implemented to avoid Dock

(ctx context.Context, sys *types.SystemContext, ref types.ImageReference)

// NOTE: Implemented to avoid Docker Hub API limits, and mirror configuration may be
// ignored (but may be implemented in the future)
func GetDigest(ctx context.Context, sys *types.SystemContext, ref types.ImageReference) (digest.Digest, error) {
	dr, ok := ref.(dockerReference)
	if !ok {
		return "", errors.New("ref must be a dockerReference")
	}
	if dr.isUnknownDigest {
		return "", fmt.Errorf("docker: reference %q is for unknown digest case; cannot get digest", dr.StringWithinTransport())
	}

	tagOrDigest, err := dr.tagOrDigest()
	if err != nil {
		return "", err
	}

	registryConfig, err := loadRegistryConfiguration(sys)
	if err != nil {
		return "", err
	}
	client, err := newDockerClientFromRef(sys, dr, registryConfig, false, "pull")
	if err != nil {
		return "", fmt.Errorf("failed to create client: %w", err)
	}
	defer client.Close()

	path := fmt.Sprintf(manifestPath, reference.Path(dr.ref), tagOrDigest)
	headers := map[string][]string{
		"Accept": manifest.DefaultRequestedManifestMIMETypes,
	}

	res, err := client.makeRequest(ctx, http.MethodHead, path, headers, nil, v2Auth, nil)
	if err != nil {
		return "", err
	}

	defer res.Body.Close()
	if res.StatusCode != http.StatusOK {
		return "", fmt.Errorf("reading digest %s in %s: %w", tagOrDigest, dr.ref.Name(), registryHTTPResponseToError(res))
	}

	dig, err := digest.Parse(res.Header.Get("Docker-Content-Digest"))
	if err != nil {
		return "", err
	}

	return dig, nil
}