Function convertSIFToElements

sif/load.go:150–216

convertSIFToElements processes sifImage and creates/returns the relevant elements for constructing an OCI-like image: - A path to a tar file containing a root filesystem, - A command to run. The returned tar file path is inside tempDir, which can be assumed to be empty at start, and is exclusively u

(ctx context.Context, sifImage *sif.FileImage, tempDir string)

// at start, and is exclusively used by the current process (i.e. it is safe
// to use hard-coded relative paths within it).
func convertSIFToElements(ctx context.Context, sifImage *sif.FileImage, tempDir string) (string, []string, error) {
	// We could allocate unique names for all of these using os.{CreateTemp,MkdirTemp}, but tempDir is exclusive,
	// so we can just hard-code a set of unique values here.
	// We create and/or manage cleanup of these two paths.
	squashFSPath := filepath.Join(tempDir, "rootfs.squashfs")
	tarPath := filepath.Join(tempDir, "rootfs.tar")
	// We only allocate these paths, the user is responsible for cleaning them up.
	extractedRootPath := filepath.Join(tempDir, "rootfs")
	scriptPath := filepath.Join(tempDir, "script")

	succeeded := false
	// It's safe for the Remove calls to happen even before we create the files, because tempDir is exclusive
	// for our use.
	// Ideally we would remove squashFSPath immediately after creating extractedRootPath, but we need
	// to run both creation and consumption of extractedRootPath in the same fakeroot context.
	// So, overall, this process requires at least 2 compressed copies (SIF and squashFSPath) and 2
	// uncompressed copies (extractedRootPath and tarPath) of the data, all using up space at the same time.
	// That's rather unsatisfactory, ideally we would be streaming the data directly from a squashfs parser
	// reading from the SIF file to a tarball, for 1 compressed and 1 uncompressed copy.
	defer os.Remove(squashFSPath)
	defer func() {
		if !succeeded {
			os.Remove(tarPath)
		}
	}()

	command, injectedScript, err := processDefFile(sifImage)
	if err != nil {
		return "", nil, err
	}

	rootFS, err := sifImage.GetDescriptor(sif.WithPartitionType(sif.PartPrimSys))
	if err != nil {
		return "", nil, fmt.Errorf("looking up rootfs from SIF file: %w", err)
	}
	// TODO: We'd prefer not to make a full copy of the file here; unsquashfs ≥ 4.4
	// has an -o option that allows extracting a squashfs from the SIF file directly,
	// but that version is not currently available in RHEL 8.
	logrus.Debugf("Creating a temporary squashfs image %s ...", squashFSPath)
	if err := func() (retErr error) { // A scope for defer
		f, err := os.Create(squashFSPath)
		if err != nil {
			return err
		}
		// since we are writing to this file, make sure we handle err on Close()
		defer func() {
			closeErr := f.Close()
			if retErr == nil {
				retErr = closeErr
			}
		}()
		// TODO: This can take quite some time, and should ideally be cancellable using ctx.Done().
		if _, err := io.CopyN(f, rootFS.GetReader(), rootFS.Size()); err != nil {
			return err
		}
		return nil
	}(); err != nil {
		return "", nil, err