Remove extraneous code from CraneEngine

Turns out that we were making an extra call to io.Copy that is
completely unnecessary. We can use the io.Reader directly from crane's
mutate.Extract call.

Signed-off-by: Brad P. Crochet <brad@redhat.com>

internal/engine/engine.go

@@ -38,7 +38,6 @@ import (
 
 	"github.com/google/go-containerregistry/pkg/crane"
 	"github.com/google/go-containerregistry/pkg/name"
-	cranev1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/cache"
 	"github.com/google/go-containerregistry/pkg/v1/mutate"
 )
@@ -96,12 +95,6 @@ type craneEngine struct {
 	results  certification.Results
 }
 
-func export(img cranev1.Image, w io.Writer) error {
-	fs := mutate.Extract(img)
-	_, err := io.Copy(w, fs)
-	return err
-}
-
 func (c *craneEngine) CranePlatform() string {
 	return c.platform
 }
@@ -152,28 +145,27 @@ func (c *craneEngine) ExecuteChecks(ctx context.Context) error {
 		return fmt.Errorf("failed to create container expansion directory: %s: %v", containerFSPath, err)
 	}
 
-	// export/flatten, and extract
-	logger.V(log.DBG).Info("exporting and flattening image")
-	r, w := io.Pipe()
-	go func() {
-		logger.V(log.DBG).Info("writing container filesystem", "outputDirectory", containerFSPath)
+	// Wrap this critical section in a closure to that we can close the
+	// mutate.Extract reader sooner than the end of the checks
+	if err := func() error {
+		// export/flatten, and extract
+		logger.V(log.DBG).Info("exporting and flattening image")
+		fs := mutate.Extract(img)
+		defer fs.Close()
 
-		// Close the writer with any errors encountered during
-		// extraction. These errors will be returned by the reader end
-		// on subsequent reads. If err == nil, the reader will return
-		// EOF.
-		w.CloseWithError(export(img, w))
-	}()
-
-	logger.V(log.DBG).Info("extracting container filesystem", "path", containerFSPath)
-	if err := untar(ctx, containerFSPath, r); err != nil {
-		return fmt.Errorf("failed to extract tarball: %v", err)
-	}
+		logger.V(log.DBG).Info("extracting container filesystem", "path", containerFSPath)
+		if err := untar(ctx, containerFSPath, fs); err != nil {
+			return fmt.Errorf("failed to extract tarball: %v", err)
+		}
 
-	// explicitly discarding from the reader for cases where there is data in the reader after it sends an EOF
-	_, err = io.Copy(io.Discard, r)
-	if err != nil {
-		return fmt.Errorf("failed to drain io reader: %v", err)
+		// explicitly discarding from the reader for cases where there is data in the reader after it sends an EOF
+		_, err = io.Copy(io.Discard, fs)
+		if err != nil {
+			return fmt.Errorf("failed to drain io reader: %v", err)
+		}
+		return nil
+	}(); err != nil {
+		return err
 	}
 
 	reference, err := name.ParseReference(c.image)