squashfs: passthrough squashfs compressed layers

squashfs blobs are internally compressed, so bypass
compression/decompression

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

pkg/compression/compression.go

@@ -35,13 +35,17 @@ var (
 	// Zstd:chunked compression.
 	ZstdChunked = internal.NewAlgorithm(types.ZstdChunkedAlgorithmName, types.ZstdAlgorithmName, /* Note: InternalUnstableUndocumentedMIMEQuestionMark is not ZstdChunkedAlgorithmName */
 		nil, ZstdDecompressor, compressor.ZstdCompressor)
+	// Squashfs compression.
+	Squashfs = internal.NewAlgorithm(types.SquashfsAlgorithmName, types.SquashfsAlgorithmName,
+		[]byte{0x68, 0x73, 0x71, 0x73}, SquashfsDecompressor, nil)
 
 	compressionAlgorithms = map[string]Algorithm{
 		Gzip.Name():        Gzip,
 		Bzip2.Name():       Bzip2,
 		Xz.Name():          Xz,
 		Zstd.Name():        Zstd,
 		ZstdChunked.Name(): ZstdChunked,
+		Squashfs.Name():    Squashfs,
 	}
 )
 
@@ -77,6 +81,11 @@ func XzDecompressor(r io.Reader) (io.ReadCloser, error) {
 	return ioutil.NopCloser(r), nil
 }
 
+// SquashfsDecompressor is a DecompressorFunc for the squashfs compression algorithm.
+func SquashfsDecompressor(r io.Reader) (io.ReadCloser, error) {
+	return ioutil.NopCloser(r), nil
+}
+
 // gzipCompressor is a CompressorFunc for the gzip compression algorithm.
 func gzipCompressor(r io.Writer, metadata map[string]string, level *int) (io.WriteCloser, error) {
 	if level != nil {

pkg/compression/types/types.go

@@ -38,4 +38,9 @@ const (
 	// will actually be available. (In fact it is intended for this types package not to depend
 	// on any of the implementations.)
 	ZstdChunkedAlgorithmName = "zstd:chunked"
+	// SquashfsAlgorithName is the name used by pkg/compression.Squashfs
+	// NOTE: Importing only this /types package does not inherently guarantee a Squashfs algorithm
+	// will actually be available. (In fact it is intended for this types package not to depend
+	// on any of the implementations.)
+	SquashfsAlgorithmName = "squashfs"
 )