fix: re-use embedded union reader if possible (#2814)

* fix: re-use embedded union reader if possible

Previously, because file.LocationReadCloser embeds a ReadCloser that
might be a UnionReader, but doesn't implement the interface itself, the
type assertion would fall and Syft would fall back to io.ReadAll to
enable seeking on the underlying reader, resulting in a potentially
large extra allocation.

Instead, check whether the passed ReadCloser is a
file.LocationReadCloser, and if so, try to use the embedded ReadCloser
as a UnionReader.

Signed-off-by: Will Murphy <will.murphy@anchore.com>

* lint fix

Signed-off-by: Will Murphy <will.murphy@anchore.com>

* Assert that underlying reader is returned

Signed-off-by: Will Murphy <will.murphy@anchore.com>

---------

Signed-off-by: Will Murphy <will.murphy@anchore.com>

syft/internal/unionreader/union_reader.go

@@ -7,6 +7,7 @@ import (
 
 	macho "github.com/anchore/go-macholibre"
 	"github.com/anchore/syft/internal/log"
+	"github.com/anchore/syft/syft/file"
 )
 
 // UnionReader is a single interface with all reading functions needed by multi-arch binary catalogers
@@ -44,6 +45,17 @@ func GetUnionReader(readerCloser io.ReadCloser) (UnionReader, error) {
 		return reader, nil
 	}
 
+	// file.LocationReadCloser embeds a ReadCloser, which is likely
+	// to implement UnionReader. Check whether the embedded read closer
+	// implements UnionReader, and just return that if so.
+	r, ok := readerCloser.(file.LocationReadCloser)
+	if ok {
+		ur, ok := r.ReadCloser.(UnionReader)
+		if ok {
+			return ur, nil
+		}
+	}
+
 	b, err := io.ReadAll(readerCloser)
 	if err != nil {
 		return nil, fmt.Errorf("unable to read contents from binary: %w", err)

syft/internal/unionreader/union_reader_test.go

@@ -7,6 +7,8 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	"github.com/anchore/syft/syft/file"
 )
 
 func Test_getUnionReader_notUnionReader(t *testing.T) {
@@ -28,3 +30,34 @@ func Test_getUnionReader_notUnionReader(t *testing.T) {
 
 	assert.Equal(t, expectedContents, string(b))
 }
+
+type panickingUnionReader struct{}
+
+func (p2 *panickingUnionReader) ReadAt(p []byte, off int64) (n int, err error) {
+	panic("don't call this in your unit test!")
+}
+
+func (p2 *panickingUnionReader) Seek(offset int64, whence int) (int64, error) {
+	panic("don't call this in your unit test!")
+}
+
+func (p2 *panickingUnionReader) Read(p []byte) (n int, err error) {
+	panic("don't call this in your unit test!")
+}
+
+func (p2 *panickingUnionReader) Close() error {
+	panic("don't call this in your unit test!")
+}
+
+var _ UnionReader = (*panickingUnionReader)(nil)
+
+func Test_getUnionReader_fileLocationReadCloser(t *testing.T) {
+	// panickingUnionReader is a UnionReader
+	p := &panickingUnionReader{}
+	embedsUnionReader := file.NewLocationReadCloser(file.Location{}, p)
+
+	// embedded union reader is returned without "ReadAll" invocation
+	ur, err := GetUnionReader(embedsUnionReader)
+	require.NoError(t, err)
+	require.Equal(t, p, ur)
+}