Refactor machine decompress.go

Added some tests to verify that files extractions works
with different compression format.

Created a decompressor interface with 2 main methods:
  reader(): returns an io.Reader for the specific compression algorithm
  copy(): extracts the compressed file into the file provided as param

Created 5 decompressor types:
- gzip: extract gzip files
- xz: extract xz files
- zip: extract zip files
- generic: extract any other file using github.com/containers/image/v5/pkg/compression
- uncompressed: only do a copy of the file

Minor fix to the progress bar instances: added a call to bar.Abort(false)
that happens before Progress.Wait() to avoid that it hangs when a bar is
not set as completed although extraction is done.

Signed-off-by: Mario Loriedo <mario.loriedo@gmail.com>

pkg/machine/compression/compression_test.go

@@ -1,6 +1,11 @@
 package compression
 
-import "testing"
+import (
+	"os"
+	"testing"
+
+	"github.com/containers/podman/v5/pkg/machine/define"
+)
 
 func Test_compressionFromFile(t *testing.T) {
 	type args struct {
@@ -89,3 +94,102 @@ func TestImageCompression_String(t *testing.T) {
 		})
 	}
 }
+
+func Test_Decompress(t *testing.T) {
+	type args struct {
+		src string
+		dst string
+	}
+
+	type want struct {
+		content string
+	}
+
+	tests := []struct {
+		name string
+		args args
+		want want
+	}{
+		{
+			name: "zip",
+			args: args{
+				src: "./testfiles/sample.zip",
+				dst: "./testfiles/hellozip",
+			},
+			want: want{
+				content: "zip\n",
+			},
+		},
+		{
+			name: "xz",
+			args: args{
+				src: "./testfiles/sample.xz",
+				dst: "./testfiles/helloxz",
+			},
+			want: want{
+				content: "xz\n",
+			},
+		},
+		{
+			name: "gzip",
+			args: args{
+				src: "./testfiles/sample.gz",
+				dst: "./testfiles/hellogz",
+			},
+			want: want{
+				content: "gzip\n",
+			},
+		},
+		{
+			name: "bzip2",
+			args: args{
+				src: "./testfiles/sample.bz2",
+				dst: "./testfiles/hellobz2",
+			},
+			want: want{
+				content: "bzip2\n",
+			},
+		},
+		{
+			name: "zstd",
+			args: args{
+				src: "./testfiles/sample.zst",
+				dst: "./testfiles/hellozstd",
+			},
+			want: want{
+				content: "zstd\n",
+			},
+		},
+		{
+			name: "uncompressed",
+			args: args{
+				src: "./testfiles/sample.uncompressed",
+				dst: "./testfiles/hellozuncompressed",
+			},
+			want: want{
+				content: "uncompressed\n",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			srcVMFile := &define.VMFile{Path: tt.args.src}
+			dstFilePath := tt.args.dst
+
+			defer os.Remove(dstFilePath)
+
+			if err := Decompress(srcVMFile, dstFilePath); err != nil {
+				t.Fatalf("decompress() error = %v", err)
+			}
+
+			data, err := os.ReadFile(dstFilePath)
+			if err != nil {
+				t.Fatalf("ReadFile() error = %v", err)
+			}
+
+			if got := string(data); got != tt.want.content {
+				t.Fatalf("content = %v, want %v", got, tt.want.content)
+			}
+		})
+	}
+}

pkg/machine/compression/decompress.go

@@ -1,313 +1,114 @@
 package compression
 
 import (
-	"archive/zip"
-	"bufio"
-	"compress/gzip"
-	"errors"
-	"fmt"
 	"io"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
 
-	"github.com/containers/image/v5/pkg/compression"
 	"github.com/containers/podman/v5/pkg/machine/define"
 	"github.com/containers/podman/v5/utils"
 	"github.com/containers/storage/pkg/archive"
-	crcOs "github.com/crc-org/crc/v2/pkg/os"
-	"github.com/klauspost/compress/zstd"
 	"github.com/sirupsen/logrus"
-	"github.com/ulikunitz/xz"
 )
 
-// Decompress is a generic wrapper for various decompression algos
-// TODO this needs some love.  in the various decompression functions that are
-// called, the same uncompressed path is being opened multiple times.
-func Decompress(localPath *define.VMFile, uncompressedPath string) error {
-	var isZip bool
-	uncompressedFileWriter, err := os.OpenFile(uncompressedPath, os.O_CREATE|os.O_RDWR, 0600)
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := uncompressedFileWriter.Close(); err != nil && !errors.Is(err, os.ErrClosed) {
-			logrus.Warnf("unable to close decompressed file %s: %q", uncompressedPath, err)
-		}
-	}()
-	sourceFile, err := localPath.Read()
-	if err != nil {
-		return err
-	}
-	if strings.HasSuffix(localPath.GetPath(), ".zip") {
-		isZip = true
-	}
-	compressionType := archive.DetectCompression(sourceFile)
+const (
+	zipExt            = ".zip"
+	progressBarPrefix = "Extracting compressed file"
+	macOs             = "darwin"
+)
 
-	prefix := "Extracting compressed file"
-	prefix += ": " + filepath.Base(uncompressedPath)
-	switch compressionType {
-	case archive.Xz:
-		return decompressXZ(prefix, localPath.GetPath(), uncompressedFileWriter)
-	case archive.Uncompressed:
-		if isZip && runtime.GOOS == "windows" {
-			return decompressZip(prefix, localPath.GetPath(), uncompressedFileWriter)
-		}
-		// here we should just do a copy
-		dstFile, err := os.Open(localPath.GetPath())
-		if err != nil {
-			return err
-		}
-		// darwin really struggles with sparse files. being diligent here
-		fmt.Printf("Copying uncompressed file %q to %q/n", localPath.GetPath(), dstFile.Name())
-
-		// Keeping CRC implementation for now, but ideally this could be pruned and
-		// sparsewriter could be used.  in that case, this area needs rework or
-		// sparsewriter be made to honor the *file interface
-		_, err = crcOs.CopySparse(uncompressedFileWriter, dstFile)
-		return err
-	case archive.Gzip:
-		if runtime.GOOS == "darwin" {
-			return decompressGzWithSparse(prefix, localPath, uncompressedFileWriter)
-		}
-		fallthrough
-	case archive.Zstd:
-		if runtime.GOOS == "darwin" {
-			return decompressZstdWithSparse(prefix, localPath, uncompressedFileWriter)
-		}
-		fallthrough
-	default:
-		return decompressEverythingElse(prefix, localPath.GetPath(), uncompressedFileWriter)
-	}
-
-	// if compressionType != archive.Uncompressed || isZip {
-	// 	prefix = "Extracting compressed file"
-	// }
-	// prefix += ": " + filepath.Base(uncompressedPath)
-	// if compressionType == archive.Xz {
-	// 	return decompressXZ(prefix, localPath.GetPath(), uncompressedFileWriter)
-	// }
-	// if isZip && runtime.GOOS == "windows" {
-	// 	return decompressZip(prefix, localPath.GetPath(), uncompressedFileWriter)
-	// }
-
-	//  Unfortunately GZ is not sparse capable.  Lets handle it differently
-	// if compressionType == archive.Gzip && runtime.GOOS == "darwin" {
-	// 	return decompressGzWithSparse(prefix, localPath, uncompressedPath)
-	// }
-	// return decompressEverythingElse(prefix, localPath.GetPath(), uncompressedFileWriter)
+type decompressor interface {
+	srcFilePath() string
+	reader() (io.Reader, error)
+	copy(w *os.File, r io.Reader) error
+	close()
 }
 
-// Will error out if file without .Xz already exists
-// Maybe extracting then renaming is a good idea here..
-// depends on Xz: not pre-installed on mac, so it becomes a brew dependency
-func decompressXZ(prefix string, src string, output io.WriteCloser) error {
-	var read io.Reader
-	var cmd *exec.Cmd
+func newDecompressor(compressedFilePath string, compressedFileContent []byte) decompressor {
+	compressionType := archive.DetectCompression(compressedFileContent)
+	os := runtime.GOOS
+	hasZipSuffix := strings.HasSuffix(compressedFilePath, zipExt)
 
-	stat, err := os.Stat(src)
+	switch {
+	case compressionType == archive.Xz:
+		return newXzDecompressor(compressedFilePath)
+	case compressionType == archive.Uncompressed && hasZipSuffix:
+		return newZipDecompressor(compressedFilePath)
+	case compressionType == archive.Uncompressed:
+		return newUncompressedDecompressor(compressedFilePath)
+	case compressionType == archive.Gzip && os == macOs:
+		return newGzipDecompressor(compressedFilePath)
+	default:
+		return newGenericDecompressor(compressedFilePath)
+	}
+}
+
+func Decompress(srcVMFile *define.VMFile, dstFilePath string) error {
+	srcFilePath := srcVMFile.GetPath()
+	// Are we reading full image file?
+	// Only few bytes are read to detect
+	// the compression type
+	srcFileContent, err := srcVMFile.Read()
 	if err != nil {
 		return err
 	}
-	file, err := os.Open(src)
+
+	d := newDecompressor(srcFilePath, srcFileContent)
+	return runDecompression(d, dstFilePath)
+}
+
+func runDecompression(d decompressor, dstFilePath string) error {
+	decompressorReader, err := d.reader()
 	if err != nil {
 		return err
 	}
-	defer file.Close()
+	defer d.close()
 
-	p, bar := utils.ProgressBar(prefix, stat.Size(), prefix+": done")
-	proxyReader := bar.ProxyReader(file)
+	stat, err := os.Stat(d.srcFilePath())
+	if err != nil {
+		return err
+	}
+
+	initMsg := progressBarPrefix + ": " + filepath.Base(dstFilePath)
+	finalMsg := initMsg + ": done"
+
+	// We are getting the compressed file size but
+	// the progress bar needs the full size of the
+	// decompressed file.
+	// As a result the progress bar shows 100%
+	// before the decompression completes.
+	// A workaround is to set the size to -1 but the
+	// side effect is that we won't see any advancment in
+	// the bar.
+	// An update in utils.ProgressBar to handle is needed
+	// to improve the case of size=-1 (i.e. unkwonw size).
+	p, bar := utils.ProgressBar(initMsg, stat.Size(), finalMsg)
+	// Wait for bars to complete and then shut down the bars container
+	defer p.Wait()
+
+	readProxy := bar.ProxyReader(decompressorReader)
+	// Interrupts the bar goroutine. It's important that
+	// bar.Abort(false) is called before p.Wait(), otherwise
+	// can hang.
+	defer bar.Abort(false)
+
+	dstFileWriter, err := os.OpenFile(dstFilePath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, stat.Mode())
+	if err != nil {
+		logrus.Errorf("Unable to open destination file %s for writing: %q", dstFilePath, err)
+		return err
+	}
 	defer func() {
-		if err := proxyReader.Close(); err != nil {
-			logrus.Error(err)
+		if err := dstFileWriter.Close(); err != nil {
+			logrus.Errorf("Unable to to close destination file %s: %q", dstFilePath, err)
 		}
 	}()
 
-	// Prefer Xz utils for fastest performance, fallback to go xi2 impl
-	if _, err := exec.LookPath("xz"); err == nil {
-		cmd = exec.Command("xz", "-d", "-c")
-		cmd.Stdin = proxyReader
-		read, err = cmd.StdoutPipe()
-		if err != nil {
-			return err
-		}
-		cmd.Stderr = os.Stderr
-	} else {
-		// This XZ implementation is reliant on buffering. It is also 3x+ slower than XZ utils.
-		// Consider replacing with a faster implementation (e.g. xi2) if podman machine is
-		// updated with a larger image for the distribution base.
-		buf := bufio.NewReader(proxyReader)
-		read, err = xz.NewReader(buf)
-		if err != nil {
-			return err
-		}
+	err = d.copy(dstFileWriter, readProxy)
+	if err != nil {
+		logrus.Errorf("Error extracting compressed file: %q", err)
+		return err
 	}
-
-	done := make(chan bool)
-	go func() {
-		if _, err := io.Copy(output, read); err != nil {
-			logrus.Error(err)
-		}
-		output.Close()
-		done <- true
-	}()
-
-	if cmd != nil {
-		err := cmd.Start()
-		if err != nil {
-			return err
-		}
-		p.Wait()
-		return cmd.Wait()
-	}
-	<-done
-	p.Wait()
 	return nil
 }
-
-func decompressEverythingElse(prefix string, src string, output io.WriteCloser) error {
-	stat, err := os.Stat(src)
-	if err != nil {
-		return err
-	}
-	f, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	p, bar := utils.ProgressBar(prefix, stat.Size(), prefix+": done")
-	proxyReader := bar.ProxyReader(f)
-	defer func() {
-		if err := proxyReader.Close(); err != nil {
-			logrus.Error(err)
-		}
-	}()
-	uncompressStream, _, err := compression.AutoDecompress(proxyReader)
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := uncompressStream.Close(); err != nil {
-			logrus.Error(err)
-		}
-		if err := output.Close(); err != nil {
-			logrus.Error(err)
-		}
-	}()
-
-	_, err = io.Copy(output, uncompressStream)
-	p.Wait()
-	return err
-}
-
-func decompressZip(prefix string, src string, output io.WriteCloser) error {
-	zipReader, err := zip.OpenReader(src)
-	if err != nil {
-		return err
-	}
-	if len(zipReader.File) != 1 {
-		return errors.New("machine image files should consist of a single compressed file")
-	}
-	f, err := zipReader.File[0].Open()
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := f.Close(); err != nil {
-			logrus.Error(err)
-		}
-	}()
-	defer func() {
-		if err := output.Close(); err != nil {
-			logrus.Error(err)
-		}
-	}()
-	size := int64(zipReader.File[0].CompressedSize64)
-	p, bar := utils.ProgressBar(prefix, size, prefix+": done")
-	proxyReader := bar.ProxyReader(f)
-	defer func() {
-		if err := proxyReader.Close(); err != nil {
-			logrus.Error(err)
-		}
-	}()
-	_, err = io.Copy(output, proxyReader)
-	p.Wait()
-	return err
-}
-
-func decompressWithSparse(prefix string, compressedReader io.Reader, uncompressedFile *os.File) error {
-	dstFile := NewSparseWriter(uncompressedFile)
-	defer func() {
-		if err := dstFile.Close(); err != nil {
-			logrus.Errorf("unable to close uncompressed file %s: %q", uncompressedFile.Name(), err)
-		}
-	}()
-
-	// TODO remove the following line when progress bars work
-	_ = prefix
-	// p, bar := utils.ProgressBar(prefix, stat.Size(), prefix+": done")
-	// proxyReader := bar.ProxyReader(f)
-	// defer func() {
-	// 	if err := proxyReader.Close(); err != nil {
-	// 		logrus.Error(err)
-	// 	}
-	// }()
-
-	// p.Wait()
-	_, err := io.Copy(dstFile, compressedReader)
-	return err
-}
-
-func decompressGzWithSparse(prefix string, compressedPath *define.VMFile, uncompressedFileWriter *os.File) error {
-	logrus.Debugf("decompressing %s", compressedPath.GetPath())
-	f, err := os.Open(compressedPath.GetPath())
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := f.Close(); err != nil {
-			logrus.Errorf("unable to close on compressed file %s: %q", compressedPath.GetPath(), err)
-		}
-	}()
-
-	gzReader, err := gzip.NewReader(f)
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := gzReader.Close(); err != nil {
-			logrus.Errorf("unable to close gzreader: %q", err)
-		}
-	}()
-	// This way we get something to look at in debug mode
-	defer func() {
-		logrus.Debug("decompression complete")
-	}()
-	return decompressWithSparse(prefix, gzReader, uncompressedFileWriter)
-}
-
-func decompressZstdWithSparse(prefix string, compressedPath *define.VMFile, uncompressedFileWriter *os.File) error {
-	logrus.Debugf("decompressing %s", compressedPath.GetPath())
-	f, err := os.Open(compressedPath.GetPath())
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := f.Close(); err != nil {
-			logrus.Errorf("unable to close on compressed file %s: %q", compressedPath.GetPath(), err)
-		}
-	}()
-
-	zstdReader, err := zstd.NewReader(f)
-	if err != nil {
-		return err
-	}
-	defer zstdReader.Close()
-
-	// This way we get something to look at in debug mode
-	defer func() {
-		logrus.Debug("decompression complete")
-	}()
-	return decompressWithSparse(prefix, zstdReader, uncompressedFileWriter)
-}

pkg/machine/compression/generic.go

@@ -0,0 +1,54 @@
+package compression
+
+import (
+	"io"
+	"os"
+
+	"github.com/containers/image/v5/pkg/compression"
+	"github.com/sirupsen/logrus"
+)
+
+type genericDecompressor struct {
+	compressedFilePath string
+	compressedFile     *os.File
+	uncompressStream   io.ReadCloser
+}
+
+func newGenericDecompressor(compressedFilePath string) decompressor {
+	return &genericDecompressor{
+		compressedFilePath: compressedFilePath,
+	}
+}
+
+func (d *genericDecompressor) srcFilePath() string {
+	return d.compressedFilePath
+}
+
+func (d *genericDecompressor) reader() (io.Reader, error) {
+	srcFile, err := os.Open(d.compressedFilePath)
+	if err != nil {
+		return nil, err
+	}
+	d.compressedFile = srcFile
+	return srcFile, nil
+}
+
+func (d *genericDecompressor) copy(w *os.File, r io.Reader) error {
+	uncompressStream, _, err := compression.AutoDecompress(r)
+	if err != nil {
+		return err
+	}
+	d.uncompressStream = uncompressStream
+
+	_, err = io.Copy(w, uncompressStream)
+	return err
+}
+
+func (d *genericDecompressor) close() {
+	if err := d.compressedFile.Close(); err != nil {
+		logrus.Errorf("Unable to close compressed file: %q", err)
+	}
+	if err := d.uncompressStream.Close(); err != nil {
+		logrus.Errorf("Unable to close uncompressed stream: %q", err)
+	}
+}

pkg/machine/compression/gzip.go

@@ -0,0 +1,56 @@
+package compression
+
+import (
+	"compress/gzip"
+	"io"
+	"os"
+
+	crcOs "github.com/crc-org/crc/v2/pkg/os"
+	"github.com/sirupsen/logrus"
+)
+
+type gzDecompressor struct {
+	compressedFilePath string
+	compressedFile     *os.File
+	gzReader           *gzip.Reader
+}
+
+func newGzipDecompressor(compressedFilePath string) decompressor {
+	return &gzDecompressor{
+		compressedFilePath: compressedFilePath,
+	}
+}
+
+func (d *gzDecompressor) srcFilePath() string {
+	return d.compressedFilePath
+}
+
+func (d *gzDecompressor) reader() (io.Reader, error) {
+	srcFile, err := os.Open(d.compressedFilePath)
+	if err != nil {
+		return nil, err
+	}
+	d.compressedFile = srcFile
+
+	gzReader, err := gzip.NewReader(srcFile)
+	if err != nil {
+		return gzReader, err
+	}
+	d.gzReader = gzReader
+
+	return gzReader, nil
+}
+
+func (*gzDecompressor) copy(w *os.File, r io.Reader) error {
+	_, err := crcOs.CopySparse(w, r)
+	return err
+}
+
+func (d *gzDecompressor) close() {
+	if err := d.compressedFile.Close(); err != nil {
+		logrus.Errorf("Unable to close gz file: %q", err)
+	}
+	if err := d.gzReader.Close(); err != nil {
+		logrus.Errorf("Unable to close gz file: %q", err)
+	}
+}

pkg/machine/compression/testfiles/sample.uncompressed

@@ -0,0 +1 @@
+uncompressed

pkg/machine/compression/uncompressed.go

@@ -0,0 +1,45 @@
+package compression
+
+import (
+	"io"
+	"os"
+
+	crcOs "github.com/crc-org/crc/v2/pkg/os"
+	"github.com/sirupsen/logrus"
+)
+
+type uncompressedDecompressor struct {
+	compressedFilePath string
+	compressedFile     *os.File
+}
+
+func newUncompressedDecompressor(compressedFilePath string) decompressor {
+	return &uncompressedDecompressor{
+		compressedFilePath: compressedFilePath,
+	}
+}
+
+func (d *uncompressedDecompressor) srcFilePath() string {
+	return d.compressedFilePath
+}
+
+func (d *uncompressedDecompressor) reader() (io.Reader, error) {
+	srcFile, err := os.Open(d.compressedFilePath)
+	if err != nil {
+		return nil, err
+	}
+	d.compressedFile = srcFile
+
+	return srcFile, nil
+}
+
+func (*uncompressedDecompressor) copy(w *os.File, r io.Reader) error {
+	_, err := crcOs.CopySparse(w, r)
+	return err
+}
+
+func (d *uncompressedDecompressor) close() {
+	if err := d.compressedFile.Close(); err != nil {
+		logrus.Errorf("Unable to close gz file: %q", err)
+	}
+}

pkg/machine/compression/xz.go

@@ -0,0 +1,87 @@
+package compression
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"os/exec"
+
+	"github.com/sirupsen/logrus"
+	"github.com/ulikunitz/xz"
+)
+
+type xzDecompressor struct {
+	compressedFilePath string
+	compressedFile     *os.File
+}
+
+func newXzDecompressor(compressedFilePath string) decompressor {
+	return &xzDecompressor{
+		compressedFilePath: compressedFilePath,
+	}
+}
+
+func (d *xzDecompressor) srcFilePath() string {
+	return d.compressedFilePath
+}
+
+func (d *xzDecompressor) reader() (io.Reader, error) {
+	srcFile, err := os.Open(d.compressedFilePath)
+	if err != nil {
+		return nil, err
+	}
+	d.compressedFile = srcFile
+	return srcFile, nil
+}
+
+// Will error out if file without .Xz already exists
+// Maybe extracting then renaming is a good idea here..
+// depends on Xz: not pre-installed on mac, so it becomes a brew dependency
+func (*xzDecompressor) copy(w *os.File, r io.Reader) error {
+	var cmd *exec.Cmd
+	var read io.Reader
+
+	// Prefer Xz utils for fastest performance, fallback to go xi2 impl
+	if _, err := exec.LookPath("xz"); err == nil {
+		cmd = exec.Command("xz", "-d", "-c")
+		cmd.Stdin = r
+		read, err = cmd.StdoutPipe()
+		if err != nil {
+			return err
+		}
+		cmd.Stderr = os.Stderr
+	} else {
+		// This XZ implementation is reliant on buffering. It is also 3x+ slower than XZ utils.
+		// Consider replacing with a faster implementation (e.g. xi2) if podman machine is
+		// updated with a larger image for the distribution base.
+		buf := bufio.NewReader(r)
+		read, err = xz.NewReader(buf)
+		if err != nil {
+			return err
+		}
+	}
+
+	done := make(chan bool)
+	go func() {
+		if _, err := io.Copy(w, read); err != nil {
+			logrus.Error(err)
+		}
+		done <- true
+	}()
+
+	if cmd != nil {
+		err := cmd.Start()
+		if err != nil {
+			return err
+		}
+		return cmd.Wait()
+	}
+	<-done
+	return nil
+}
+
+func (d *xzDecompressor) close() {
+	if err := d.compressedFile.Close(); err != nil {
+		logrus.Errorf("Unable to close xz file: %q", err)
+	}
+}

pkg/machine/compression/zip.go

@@ -0,0 +1,57 @@
+package compression
+
+import (
+	"archive/zip"
+	"errors"
+	"io"
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+type zipDecompressor struct {
+	compressedFilePath string
+	zipReader          *zip.ReadCloser
+	fileReader         io.ReadCloser
+}
+
+func newZipDecompressor(compressedFilePath string) decompressor {
+	return &zipDecompressor{
+		compressedFilePath: compressedFilePath,
+	}
+}
+
+func (d *zipDecompressor) srcFilePath() string {
+	return d.compressedFilePath
+}
+
+func (d *zipDecompressor) reader() (io.Reader, error) {
+	zipReader, err := zip.OpenReader(d.compressedFilePath)
+	if err != nil {
+		return nil, err
+	}
+	d.zipReader = zipReader
+	if len(zipReader.File) != 1 {
+		return nil, errors.New("machine image files should consist of a single compressed file")
+	}
+	z, err := zipReader.File[0].Open()
+	if err != nil {
+		return nil, err
+	}
+	d.fileReader = z
+	return z, nil
+}
+
+func (*zipDecompressor) copy(w *os.File, r io.Reader) error {
+	_, err := io.Copy(w, r)
+	return err
+}
+
+func (d *zipDecompressor) close() {
+	if err := d.zipReader.Close(); err != nil {
+		logrus.Errorf("Unable to close zip file: %q", err)
+	}
+	if err := d.fileReader.Close(); err != nil {
+		logrus.Errorf("Unable to close zip file: %q", err)
+	}
+}