Fix 'file already closed' error for compressed images (#137)

When unpacking compressed images that are not compressed as zip archives
(e.g. gzip, xz) the image would unpack nearly all of the way and then
would fail with an error like: 'read |0: file already closed' as is
documented in GitHub issue
[61](#61).

As the decompression is offloaded to an external tool for these
compression types and the data is streamed through a reader and writer
in order to allow reporting on progress, we ensure that the exec.Cmd
instance for the external decompression tool is not torn down until the
associated reader is closed. The
[docs](https://pkg.go.dev/os/exec#Cmd.StdoutPipe) for Cmd.StdoutPipe
explain this in more detail.

pkg/image/image.go

@@ -125,7 +125,7 @@ func (s *imageOpener) openzip(f *os.File) (Image, error) {
 	}
 
 	//transfer ownership
-	mc := &multiCloser{zippedfileReader, []io.Closer{zippedfileReader, f}, zippedfile.UncompressedSize64}
+	mc := &multiCloser{zippedfileReader, []io.Closer{zippedfileReader, f}, nil, zippedfile.UncompressedSize64}
 	f = nil
 
 	return mc, nil
@@ -167,7 +167,7 @@ func uncompress(f *os.File, fastcmd string, slowNewReader func(r io.Reader) (io.
 	}
 
 	//transfer ownership
-	mc := &multiCloser{r, []io.Closer{f}, 0}
+	mc := &multiCloser{r, []io.Closer{f}, nil, 0}
 	f = nil
 
 	return mc, nil
@@ -188,12 +188,8 @@ func xzFastlane(cmd string, f *os.File) (Image, error) {
 		return nil, err
 	}
 
-	go func() {
-		xzcat.Wait()
-	}()
-
 	// use mc for size estimate
-	mc := &multiCloser{r, []io.Closer{r}, 0}
+	mc := &multiCloser{r, []io.Closer{r}, xzcat, 0}
 
 	return mc, nil
 
@@ -202,6 +198,12 @@ func xzFastlane(cmd string, f *os.File) (Image, error) {
 type multiCloser struct {
 	io.Reader
 	c []io.Closer
+	// As the reader that is stored in c is the StdoutPipe of an exec.Cmd
+	// instance, we must ensure the command isn't closed prior to the reader
+	// being closed otherwise we end up with and error like:
+	// 'read |0: file already closed'.
+	// See also https://pkg.go.dev/os/exec#Cmd.StdoutPipe
+	command *exec.Cmd
 
 	sizeEstimate uint64
 }
@@ -210,7 +212,14 @@ func (n *multiCloser) Close() error {
 	for _, c := range n.c {
 		c.Close()
 	}
-	return nil
+
+	var err error
+	// Wait for the readers (including StdoutPipe) to be closed
+	// and then initiate the command shutdown if we have a command.
+	if n.command != nil {
+		err = n.command.Wait()
+	}
+	return err
 }
 
 func (f *multiCloser) SizeEstimate() uint64 { return f.sizeEstimate }